Unpaid child support in Montana does not shrink with time or with a later change in circumstances. A court can change support only for installments that come due after the other parent has actual notice of a motion to modify, so every payment missed before then remains owed at the ordered amount (MCA § 40-4-208(1)). The person owed has a long time to collect: 10 years after the support obligation ends (MCA § 27-2-201(3)). This page is part of our Montana Divorce Guide.
Why Past-Due Support Cannot Be Reduced Later
The statute says a decree "may be modified by a court as to maintenance or support only as to installments accruing subsequent to actual notice to the parties of the motion for modification" (MCA § 40-4-208(1)). Each monthly installment, once due, is outside the court's power to modify. The same rule applies when a district court modifies a docketed administrative support order (MCA § 40-5-227(2)).
For the parent who pays, this is the reason to seek a modification as soon as a lasting change occurs. Explaining the hardship later does not undo the balance. See modifying child support.
How Long a Parent Has to Collect
Montana's limitation statute has a provision specific to child support. An action to collect past-due support that accrued after October 1, 1993, under a court or administrative order, must be commenced "within 10 years of the termination of support obligation or within 10 years from entry of a lump-sum judgment or order for support arrears, whichever is later" (MCA § 27-2-201(3)).
Two points follow. The period does not begin running with each missed payment. It is measured from the end of the obligation, so arrears from a child's early years can still be collectible after the child is grown. And reducing the arrears to a lump-sum judgment starts a new 10-year period from that judgment. Support debts set through the Division's administrative process are subject to the same limitation statutes (MCA § 40-5-254). For when the obligation ends, see when child support ends.
Contempt is on a shorter clock. A contempt petition for nonpayment must be brought within 3 years of the date of the last failure to comply with the order (MCA § 40-5-601(12)). The debt can outlast the contempt remedy.
What the Statutes Say About Interest
| Setting | What the statute provides | Source |
|---|---|---|
| Judgments generally | Interest on a judgment recovered in Montana courts runs at the bank prime loan rate plus 3%, set as of January 1 each year, and may not be compounded. | MCA § 25-9-205(1) |
| Support debt owed to the Department | The Department may collect interest at the statutory judgment rate. It is not required to keep interest balance accounts and may waive interest. | MCA § 40-5-252 |
| Support lien | The Department may charge interest on a support lien at 1% per month and may waive it if that would help collect the debt. | MCA § 40-5-248(9) |
The family law chapters we reviewed do not set a single interest rate that attaches automatically to every missed child support payment between two parents. In Division cases, the statutes say the Department is not required to keep interest balance accounts and may waive interest "if waiver would facilitate the collection of the debt." In a court case, whether interest is claimed and awarded on arrears is decided in that case. If interest matters to your situation, as the parent owed or the parent paying, ask a lawyer to review the order and the payment record before relying on a figure.
Credits, Direct Payments, and Side Agreements
A dispute over arrears can turn on what counts as a payment. The guidelines are strict. Support is paid in U.S. dollars. "Gifts, clothing, food, payment of expenses, etc., in lieu of dollars will not be allowed as a credit for payment of a child support obligation except by court or administrative order." And direct payments "will not be allowed as credit" where the obligation is payable through the clerk of court, the Division, or another entity named in the order (ARM 37.62.142).
One specific credit is written into the guidelines. When a child receives Social Security or veterans benefits based on a parent's earnings or service record, that parent's obligation is satisfied for a month in which the child's benefit equals or exceeds the obligation, and a lump sum representing accumulated monthly benefits is credited to the months it covers (ARM 37.62.144).
An informal agreement between parents to pay less does not change the order. The safe course for both parents is to put any new arrangement in front of the court or the Division. See can parents agree to no child support.
How Arrears Are Collected
When income is being withheld to satisfy an arrearage, the amount withheld includes the greater of the amount that would pay the arrears and any interest within 2 years, or 25% of the obligor's income, on top of current support and subject to the federal cap (MCA § 40-5-416(1)(b), (2)). The Division may set a lower amount for substantial hardship (MCA § 40-5-416(3)). Liens, license suspension, and tax refund offsets are covered in child support enforcement in Montana.
A court may also order a parent who is delinquent by an amount equal to six months of support, and who has the ability, to post a bond, give a mortgage, or provide other security (MCA § 40-4-209).
Keeping Records
Certified payment records kept by a clerk of court or an agency authorized to collect support "are prima facie evidence of the amount of support paid and any arrearages" (MCA § 40-5-601(6)). Paying through the channel named in the order creates that record automatically. If you have paid or received support another way, keep bank records and written acknowledgments, and raise the question of credit sooner, not later.
Bankruptcy raises its own questions about support debts, which are governed by federal law. See bankruptcy and divorce and consult a bankruptcy lawyer. For practical next steps when payments stop, see when your ex stops paying.
This page explains Montana law in general as of the 2025 Montana Code Annotated and the current Montana Child Support Guidelines. It is not legal advice about your specific case.