Missouri Child Support Arrears Interest 2026: How 12% Is Calculated

12% Per Year: Missouri's Rate Under RSMo § 454.520 Explained

Missouri Revised Statutes § 454.520 sets the interest rate on unpaid child support at 1% per month, which compounds to an annual equivalent of 12%. The statute uses simple interest — not compound — so each month's calculation is 1% of the outstanding arrearage at the end of that month. The 12% annual figure is a shorthand for planning purposes, not a single end-of-year calculation.

This rate has been in force since September 1982. It is statutory, not discretionary: a court cannot order a lower interest rate on child support arrears under Missouri law, and a paying parent cannot negotiate it away. The 1% per month accrues automatically from the day a payment becomes overdue, regardless of whether the custodial parent has requested interest or whether the Family Support Division has issued any formal notice.

Why 12% matters in real dollars. A parent behind by $800 per month for 12 months has principal arrears of $9,600. At 12% per year on that principal alone, the annual interest is approximately $1,152 — but because Missouri calculates at month-end on the running total, the actual interest is slightly higher as each new missed payment enlarges the base before the 1% is applied. The total after one year without any payment is closer to $10,224 in combined principal and interest.

How Missouri compares to other high-rate states. California charges 10% per year under Cal. Fam. Code § 17433.5. Illinois charges 9% under 735 ILCS 5/2-1303. Washington charges 12% under RCW 4.56.110, matching Missouri's rate. Missouri and Washington sit at the top of the national range for child support arrears interest. States like Texas charge no statutory arrears interest unless the court orders it separately. For a custodial parent comparing enforcement strategies across state lines — for example, when a paying parent moves — Missouri's 12% is among the strongest levers available.

The 12% applies to child support arrears specifically. It does not apply to child support that is current. A parent making all payments on time faces zero interest accrual. Interest begins on the day a specific installment goes unpaid and continues until that installment, plus all accrued interest, is paid in full or otherwise discharged.

For planning purposes, the rate is the starting point. The calculation method — applied month by month against the running arrearage — is where the specifics matter, and that is covered in the next section.

How Missouri Calculates Arrears Interest: The Month-End Formula

Missouri's interest calculation method is more precise than a simple annual percentage applied once per year. Under RSMo § 454.520, interest accrues at the close of business on the last day of each month. The calculation for any given month is: (total arrearage at end of that day, minus the current month's installment) × 1%.

Breaking down the formula. "Total arrearage at the end of the last day of the month" includes all prior unpaid installments plus all prior interest that has already accrued. It does not include the installment that was due in the current month — that installment is subtracted before applying the 1%. This subtraction prevents the current month's payment from generating interest in the same month it was supposed to have been paid.

Example — Month 1 without payment. A paying parent owes $800 per month and makes no payment in January. At the close of January 31, the arrearage from prior months is $0 (nothing was previously owed). The current month installment is $800. Interest for January: ($0 − $0 subtracted for current month, then plus the now-past-due $800 as of month end) × 1%. In practice, at the end of January, $800 is now overdue. Interest: $800 × 1% = $8.00.

Example — Month 2 without payment. At the end of February, the total arrearage is: prior principal ($800) + prior interest ($8) = $808. The current month installment ($800 for February) is subtracted. So the base is $808 − $800... Wait — let me state this correctly as Missouri courts apply it: the formula multiplies the outstanding arrearage (prior months) by 1%, without subtracting the current month's installment from prior months' principal. The current month's own installment generates its first interest at the close of the month it falls due. So: end of Month 2, the prior principal is $800, the Month 2 principal ($800) also becomes overdue at end of Month 2. The interest base for Month 2 is ($800 existing + $800 new) − the current installment ($800 due this month) = $800. Interest for Month 2: $800 × 1% = $8. Then the total from Month 1 interest ($8) also carries forward.

Running total after 2 months with no payment: Principal $1,600 + Interest ($8 + $8) = $1,616.

After 12 months without payment at $800/month: - Principal: $800 × 12 = $9,600 - Interest (approximate): each month adds 1% of the running arrearage - Month 1: $8; Month 2: $8; Month 3: $16 (on $1,600 prior); Month 4: $24; rising as the base grows - Total interest after 12 months: approximately $624 - Grand total: approximately $10,224

The cumulative effect. As the arrearage grows — because each missed month adds both principal and prior interest — the monthly interest charge rises. After 24 months with no $800 payment: principal approximately $19,200, interest approximately $2,496, grand total approximately $21,696. This is meaningfully different from a simple $19,200 × 12% = $2,304 calculation because the interest compounds month over month on the running balance.

What this means for paying parents. Every month of delay is more expensive than the last, because the previous month's interest is now part of the base. Getting current quickly — even partially — reduces the interest base and slows future accrual in a way that a lump sum payment years later cannot fully replicate.

Step-by-Step: Calculating Your Missouri Arrears Interest

Calculating Missouri child support arrears interest requires tracking each month's end-of-month balance and applying 1% to the prior arrearage. Here is a method that follows the RSMo § 454.520 framework:

Step 1 — Gather the court order and payment history. You need the ordered monthly amount, the due date, and a complete chronological record of what was paid and when. Use Missouri Child Support Enforcement Unit (CSEU) records or State Disbursement Unit (SDU) records rather than informal ledgers whenever possible.

Step 2 — List each month's starting arrearage. For month one without payment, the prior arrearage is $0. For month two, it is the prior month's principal plus prior interest. For each subsequent month, carry forward all unpaid principal and all unpaid interest.

Step 3 — Apply 1% at month end. At the close of each month, multiply the prior arrearage (the amount owed from months before the current one) by 1%. Record that interest. Add the current month's missed installment to principal.

Step 4 — Sum all monthly interest charges. The total interest is the sum of each month's 1% calculation. The total balance is principal plus total interest.

Step 5 — Account for any partial payments. If a partial payment was made, it is typically applied first to current support, then to arrears principal, then to interest — depending on the court order or CSEU allocation rules. Reduce the balance accordingly before computing the next month's interest.

Quick-reference calculation for common scenarios: - $500/month, 12 months behind: principal $6,000, interest ~$390, total ~$6,390 - $800/month, 12 months behind: principal $9,600, interest ~$624, total ~$10,224 - $1,200/month, 12 months behind: principal $14,400, interest ~$936, total ~$15,336 - $800/month, 24 months behind: principal $19,200, interest ~$2,496, total ~$21,696

The day-by-day rate for planning. If you need a daily accrual rate (for example, to calculate interest to a mid-month settlement date), use: (current arrearage balance × 12%) ÷ 365. For a $10,000 balance: ($10,000 × 0.12) ÷ 365 ≈ $3.29 per day.

Use TheLegalCalc's Missouri Child Support Arrears Interest Calculator at /child-support-arrears-interest-calculator/missouri to run these calculations without manual spreadsheet work. Enter the monthly support amount, the number of months in arrears, and any partial payments received. The calculator applies the 1%-per-month Missouri method. For authoritative figures — especially before filing an affidavit or appearing in court — obtain a certified arrearage statement from the Missouri Family Support Division.

The Affidavit Requirement: Why You Must File Before Collecting Interest

Missouri's interest on child support arrears accrues automatically under RSMo § 454.520. But collecting that interest — executing on it against the paying parent's wages, bank accounts, or tax refunds — requires a procedural step that many custodial parents overlook: filing a sworn affidavit with the Circuit Clerk.

RSMo § 454.520.5 provides that before a creditor may execute on accrued interest, they must file an affidavit with the clerk of the circuit court. That affidavit must include: - A complete payment history showing each installment that was due, the amount actually paid, and the date of payment - A detailed calculation of the interest accrued on each unpaid or underpaid installment - A sworn statement by the person filing that the information is accurate

Without that affidavit on file, a creditor cannot lawfully execute against the debtor's property to collect the interest — even if the statutory interest has been accruing for years. The interest is real; the right to collect it is contingent on this procedural step.

Why this matters in practice. A custodial parent who has been owed back support for five years may have accumulated significant interest under the 12% annual rate. But if no affidavit has ever been filed, that interest cannot be enforced through garnishment, tax intercept, or bank levy until the affidavit is filed and the court accepts it. Delay in filing the affidavit does not eliminate the interest — it merely delays the ability to execute.

What to include in the affidavit. Work with the Missouri Family Support Division (dss.mo.gov/fsd) to obtain a certified arrearage statement, which shows the court-verified principal balance. The interest calculation can be prepared by the FSD, by a family law attorney, or by any competent calculation showing the month-by-month accrual under RSMo § 454.520's formula. The affidavit must be sworn, meaning it is signed under penalty of perjury before a notary or court clerk.

For paying parents. If an affidavit is filed against you and you dispute the calculation, you may file a response with the circuit court challenging the accuracy of the payment history or the interest calculation. Errors in the payment record — attributing cash payments to the wrong period, double-counting, or misapplying payments to principal vs. interest — are common sources of dispute. Documenting every payment made, with date and amount, is the best protection against an overstated affidavit.

The affidavit does not modify the underlying support order. It is solely a procedural prerequisite for enforcement. The interest itself is a creature of statute (RSMo § 454.520), not of the court order, and courts have consistently held that it accrues regardless of whether the custodial parent has taken any action.

The 10-Year Collection Window: RSMo § 516.350 and Revivals

Missouri child support orders do not last forever for enforcement purposes. Under RSMo § 516.350, a judgment for child support — including any judgment for arrears — has a 10-year window for collection. After 10 years from the date of each monthly installment, that installment is no longer collectible unless the judgment has been revived.

How the 10-year window works. Each monthly child support payment, once it becomes due and unpaid, can be thought of as its own mini-judgment under Missouri law. The 10-year clock starts running from the date that installment was due. A payment that was due January 1, 2015 becomes uncollectible after January 1, 2025, absent a revival.

Judgment revival. Missouri courts allow a judgment creditor to revive a child support judgment before the 10-year window closes. Reviving the judgment resets the 10-year clock and makes the balance collectible for another decade. To revive, the custodial parent must file a motion to revive the judgment with the circuit court — typically at least some time before the original judgment expires. An attorney can assist with this process.

What happens to interest when installments expire. If the underlying installment becomes uncollectible under § 516.350, the interest on that installment typically expires with it. There is no separate limitations period for the interest alone; interest is incident to the principal it was calculated on. Once the principal is barred, so is its associated interest.

Why this matters for custodial parents with old arrears. A paying parent who has not paid since 2012 may have installments from that year that are on the verge of the 10-year bar. A custodial parent holding those claims should consult a family law attorney immediately about revival options. Waiting until the 10-year period closes eliminates the claim permanently.

The Missouri Family Support Division's role. The FSD can assist in identifying which arrears installments are within the collection window and which may be approaching expiration. They can also initiate the revival process on behalf of custodial parents who are receiving FSD services. Contact dss.mo.gov/fsd for guidance.

Federal income tax refund intercepts and the 10-year rule. Federal tax intercepts through the Treasury Offset Program are not subject to the same state limitations period in all cases. The FSD can intercept federal refunds on qualifying child support arrears regardless of the state 10-year window in some circumstances. However, the state enforcement window still governs state court proceedings and state-issued writs.

Partial Payments: How They Affect the Balance and Interest

When a paying parent makes a partial payment toward child support arrears in Missouri, that payment reduces the principal balance — but it does not retroactively reduce the interest that has already accrued under RSMo § 454.520. Interest attaches automatically to the underlying support obligation, and a partial payment that does not satisfy a specific installment in full leaves the remaining balance continuing to accrue interest.

How partial payments are allocated. Missouri law and FSD practice generally follow a priority order for allocating partial payments: (1) current month's support obligation first, (2) oldest arrears next, (3) accrued interest. This means a partial payment that does not cover the current month's support may be applied entirely to current support — leaving the arrears and interest untouched — depending on the court order's specific language and the FSD's procedures.

A practical example. A paying parent owes $800/month and has $3,200 in principal arrears (4 months) plus $192 in accrued interest, for a total balance of $3,392. They make a payment of $800. Under typical allocation rules: $800 goes to current support for the month. The arrears ($3,200) and interest ($192) remain entirely unpaid, and interest continues accruing on the $3,200 arrearage at 1% per month.

If the same $800 were applied to the oldest arrear instead, it would reduce the principal by $800, and future interest would accrue on $2,400 instead of $3,200. The allocation rule determines which scenario applies.

What paying parents should do. If you are making partial payments and want to reduce the interest accruing on arrears, coordinate with the FSD or your attorney to ensure some portion is credited toward the oldest arrears principal. Written direction to the court clerk or FSD about how to apply a payment — if the court order does not specify — may be honored in some cases. Ask your attorney about whether you can direct application or whether the court order controls.

What custodial parents should verify. An incorrect allocation of a partial payment — for example, a lump sum applied entirely to interest rather than to principal — may leave a larger outstanding principal that generates more future interest. Keep your own running ledger and compare it to FSD statements periodically.

Payments that overpay current support. If a paying parent is current and makes extra payments that exceed the current month's obligation, the surplus is typically applied to arrears principal. This reduces the interest base for future months. Consistency in overpaying — even modest amounts above the monthly obligation — can meaningfully slow interest accrual over time.

Wage Garnishment for CS Arrears: Up to 55% Under RSMo § 454.505

When a paying parent falls behind on child support in Missouri, wage garnishment is one of the primary enforcement tools available. Missouri law under RSMo § 454.505 allows garnishment of up to 55% of the paying parent's disposable earnings for child support arrears — a figure higher than the federal Consumer Credit Protection Act's general 50% cap for child support obligations.

How the 55% limit applies. Federal law generally caps garnishment for child support at 50% of disposable earnings if the paying parent is supporting another spouse or child, or 60% if they are not. Missouri's RSMo § 454.505 applies up to 55% when arrears are involved. The higher 55% figure applies specifically when arrears are 12 or more weeks old. For current support without arrears, lower federal limits may apply.

When garnishment begins automatically. Under RSMo § 454.505, income withholding for child support is typically immediate — it begins at the time of the original support order, not only when arrears accumulate. If the paying parent changes jobs, the new employer is required to honor the income withholding order. The FSD enforces income withholding through the State Disbursement Unit.

The 1-month trigger for automatic income withholding related to arrears. When arrears reach the equivalent of one month's support obligation, Missouri law treats this as grounds for enhanced enforcement steps. Income withholding orders are already typically in place from the original order, but the arrearage triggers additional notification to the employer and may increase the withholding percentage.

Other enforcement tools alongside wage garnishment. Missouri's child support enforcement arsenal for arrears includes: federal and state income tax refund intercepts, suspension of driver's licenses and professional licenses under RSMo § 452.345, passport denial through the federal program, property liens, bank account levies, and credit bureau reporting. These tools operate in parallel with wage garnishment and can be used simultaneously.

How the interest relates to garnishment. Interest accrued under RSMo § 454.520 is part of the total enforceable balance. Once the custodial parent has filed the required affidavit under § 454.520.5, the interest balance is collectible through the same enforcement mechanisms as principal arrears — including wage garnishment. An income withholding order that covers "all arrears and interest" captures both.

If you are the paying parent. If garnishment is taking 55% of your disposable income and you believe the arrearage calculation includes an error — for example, payments you made that were not credited — contact the FSD immediately and document your payments. A court may order a reduction in the withheld percentage if you can demonstrate financial hardship, but you must file a motion; garnishment does not self-adjust.

Missouri vs California vs Illinois: How Arrears Interest Compares

Missouri's 12% annual rate on child support arrears is among the highest in the country. Understanding where Missouri fits in the national picture helps custodial parents assess their claims and helps paying parents understand the cost of delay — especially when interstate moves or enforcement across state lines is involved.

Missouri — 12% per year (RSMo § 454.520). One percent per month, simple interest, calculated on the running arrearage at the end of each month. Among the highest statutory rates in the country. Required affidavit before execution. Ten-year collection window with revival option. Wage garnishment up to 55%.

California — 10% per year (Cal. Fam. Code § 17433.5). Simple interest from each payment's due date (not month-end). Interest accrues automatically; no affidavit required. Enforced through the LCSA. Ten-year limitation via CCP § 337.5, with revival options. Wage withholding standard from original order.

Illinois — 9% per year (750 ILCS 5/505). Simple interest, slightly lower than Missouri. Illinois does not automatically add arrears interest in all cases; the court may need to order it or the DCSS must calculate it. Collection window under Illinois limitations statutes.

Washington — 12% per year (RCW 4.56.110). Matches Missouri's rate. Washington applies its standard judgment interest rate of 12% to child support arrears in many cases, though some modifications apply. Strong enforcement infrastructure.

Texas — No statutory arrears interest in most cases. Texas does not impose a standard statutory interest rate on child support arrears under state law. Courts may order interest in specific circumstances, but it is not automatic. This makes Texas one of the most debtor-favorable states for paying parents on this dimension.

Florida — 6% per year (§ 55.03 Fla. Stat.). Florida judgment interest applies to child support arrears reduced to judgment, but the rate is substantially lower than Missouri's.

The interstate enforcement question. If a Missouri-ordered paying parent moves to Texas, Missouri can still enforce its order under the Uniform Interstate Family Support Act (UIFSA). Missouri's 12% interest rate continues to accrue, and Missouri's FSD can register the order in Texas for wage garnishment through Texas courts. However, the out-of-state enforcement process takes time, and some states limit what they will do on behalf of another state's interest calculation.

For custodial parents with paying parents in other states. The key practical point: where Missouri issued the order, Missouri law — including the 12% interest — controls the arrearage calculation. Registration in another state for enforcement does not change the Missouri interest rate. Contact the FSD for interstate enforcement assistance.

Calculate Your Missouri Arrears Interest and Resources

TheLegalCalc's Missouri Child Support Arrears Interest Calculator at /child-support-arrears-interest-calculator/missouri applies the RSMo § 454.520 formula: 1% per month on the running arrearage balance. Enter the ordered monthly amount, the number of months in arrears, and any partial payments. The calculator produces an estimated total arrearage including accrued interest — useful for understanding the scale of a claim before filing an affidavit or entering settlement discussions.

This estimate is a planning tool, not a certified balance. For enforcement purposes — filing an affidavit under § 454.520.5, registering an arrearage with the circuit court, or responding to a garnishment — obtain a certified arrearage statement from the Missouri Family Support Division.

Official Missouri resources:

Missouri Revisor of Statutes — RSMo § 454.520 (interest on child support arrears). The official text of the statute is available at revisor.mo.gov. Search for "454.520" to access the current version. This is the authoritative source for the 1% per month rate, the affidavit requirement under § 454.520.5, and related provisions.

Missouri Department of Social Services — Family Support Division. The FSD handles child support enforcement in Missouri, including arrearage calculations, affidavit processing, income withholding, and interstate enforcement. The official site is dss.mo.gov/fsd. Contact the FSD for your case-specific balance, payment history, and assistance with the affidavit process.

Missouri Courts. The Circuit Court in your county is where child support orders are entered and where affidavits under § 454.520.5 are filed. The Missouri Courts website at courts.mo.gov can help locate your circuit court.

If you are a custodial parent with substantial arrears, consider consulting a Missouri family law attorney before relying solely on a calculator estimate. The affidavit process, revival of judgments approaching the 10-year window, and strategic decisions about which enforcement tools to pursue first — wage garnishment vs. tax intercept vs. license suspension — all benefit from professional guidance specific to your order and the paying parent's financial situation.