A child support order is not fixed for the life of the child. Every state lets a parent ask for a new amount, but each decides what counts as a big enough change, and all follow one federal rule about timing that catches many parents out.
The federal rule: no going back before you ask
Federal law requires every state to treat each support payment, from its due date, as a judgment by operation of law that is "not subject to retroactive modification" (42 U.S.C. §666(a)(9)). The only exception a state may allow is the period while a petition for modification is pending, and only from the date the other parent was given notice of it.
So a lost job or a move does not change what is owed until a request is on file. Payments due before then stay owed at the old amount, even if a court later agrees the order was too high.
Agency review every three years
Parents whose case is handled by the state child support agency (a "Title IV-D" case) have a second route. Under 42 U.S.C. §666(a)(10) and 45 C.F.R. §303.8, the state must review the order every 3 years, or on a shorter cycle it chooses, when either parent asks, or automatically when support rights are assigned to the state because the family receives public assistance. The state may recompute the amount under its guidelines, apply a cost-of-living adjustment, or use automated comparisons with wage or tax data.
- A review within the cycle needs no proof of a change in circumstances (§666(a)(10)(A)(iii)).
- A request outside the cycle must show a substantial change in circumstances (§666(a)(10)(B)).
- The need to provide for the child's health care in the order, through insurance or other means, must be enough to start an adjustment, even if the amount does not change (45 C.F.R. §303.8(d)).
- The state must tell both parents of their right to a review at least once every 3 years (§666(a)(10)(C)).
- A cost-of-living or automated adjustment can be contested within 30 days of the notice.
- The state must finish the review, and either adjust the order or decide not to, within 180 calendar days of the request or of locating the other parent, whichever is later (45 C.F.R. §303.8(e)).
States may set a dollar or percentage gap that justifies an adjustment (§303.8(c)), which is why the agency thresholds below differ.
Incarceration
Federal rules require every state's guidelines to provide that incarceration may not be treated as voluntary unemployment when orders are set or modified (45 C.F.R. §302.56(c)(3)). When the agency learns that a paying parent will be incarcerated for more than 180 calendar days, the state must either start a review on its own or, within 15 business days, tell both parents that they can request one. Neither the notice nor the review is required if state law already changes the obligation automatically on incarceration (§303.8(b)(2) and (b)(7)(ii)).
What counts as a change, state by state
The court test applies to a motion or petition; the agency test to reviews by the state agency in its own cases. "Effective from" is the earliest date a new amount usually reaches back to.
| State | Court test | Agency review | Effective from |
|---|---|---|---|
| Texas | Material and substantial change, or 3 years since the order and the guideline amount differs by 20% or $100 | Not covered here | The earlier of service of citation or an appearance in the suit |
| California | May be modified at any time the court finds necessary; no numeric test in the statute | Not covered here | Filing of the motion or order to show cause, or a later date |
| New York | Substantial change; or 3 years since the order; or a 15% change in either parent's gross income | Not covered here | Arrears before the application cannot be reduced |
| New Jersey | Changed circumstances since the order; new guidelines alone are not enough | Review every 3 years without showing a change; cost-of-living adjustment every 2 years | Not covered here |
| Colorado | Substantial and continuing change; under 10% is deemed not substantial | Request granted if 36 months since the last review | Filing of the motion, never earlier (one exception) |
| Pennsylvania | Substantial change in circumstances | Reviewed at least every 3 years without proof of a change | Filing of the petition, unless the order says otherwise; earlier only in limited cases |
| Virginia | Change in the parents' circumstances or the children's benefit | Every 3 years; change if at least 10% and at least $25 a month | Notice of the petition to the other parent |
| Arizona | Substantial and continuing change; a 15% variance is evidence of one | Every 3 years without showing a change | First day of the month after notice, not before filing |
| Florida | Change in circumstances; a guideline change counts only if at least 15% or $50, whichever is greater | At least every 3 years; change if at least 10% and at least $25 | Filing of the modification action |
Texas
The three-year, 20% or $100 test does not apply when the parents agreed to an amount different from the guidelines; then only a material and substantial change will do. Incarceration for more than 180 days is a material and substantial change, and so is release if support was reduced or suspended during the incarceration (Tex. Fam. Code §156.401).
California
The court cannot change amounts that accrued before the motion was filed (Fam. Code §3651(c)). When the change is ordered because a parent lost a job, it reaches back to the later of the date the motion was served or the date of unemployment, unless the court finds good cause not to (§3653(b)).
New York
The three-year and 15% grounds apply unless the parents opted out in a valid written agreement. A drop in income counts only if it was involuntary and the parent has looked diligently for suitable work. Incarceration is not voluntary unemployment and does not bar a finding of substantial change. An increase based on newly discovered evidence can reach back to the date of the first application (Family Ct. Act §451; Dom. Rel. Law §236(B)(9)(b) for divorce cases).
New Jersey
A parent must file a motion and show a change in circumstances other than the adoption of revised guidelines (Appendix IX-A, para. 23). Separately, orders that include child support entered, modified or enforced on or after September 1, 1998 provide for a cost-of-living adjustment every two years, and older orders are subject to the same adjustment going forward. The parties get notice and 30 days to contest it; a paying parent can object if their income has not risen at least as fast as inflation, or if the order already provides its own periodic adjustment. The adjustment does not stop either parent from asking the court for a modification or the agency for a three-year review (R. 5:6B).
Colorado
The 10% test is measured by applying the guidelines to the parents' circumstances on the day the motion is filed. The new amount normally starts on the filing date unless that would cause undue hardship or substantial injustice; an earlier date is possible when physical care has changed: a court-ordered, voluntary or mutually agreed change of physical care lets the change run from that date, but generally not for any time more than five years before the motion (C.R.S. §14-10-122(1)(d), (5)). A motion can also be based on an order that has no medical support provision (§14-10-122(1)(a)). Sooner than 36 months, an agency review request must give a reason that could change the amount (C.R.S. §26-13-121).
Pennsylvania
A support order runs from the date the complaint or petition for modification is filed, unless the order says otherwise (Pa.R.Civ.P. 1910.17(a)). Incarceration, other than for nonpayment of support, is a material and substantial change that may justify modifying or ending the order when the parent has no verifiable income or assets to collect from (23 Pa.C.S. §4352(a.2)). A change can reach back before the petition only if the parent was kept from filing by a significant disability, the other party's misrepresentation or another compelling reason, and filed promptly once able (§4352(e)).
Virginia
Incarceration for 180 or more consecutive days is a material change in circumstances and is not voluntary unemployment (Va. Code §20-108.1). In an agency review, the modified order runs from the date notice of the review was served, and either parent may ask for a hearing within 30 days of receipt of notice by the requesting party. That shortcut does not apply when the order deviated from the guidelines when it was entered; the agency then schedules a court hearing (§63.2-1921(A)-(B)).
Arizona
Adding health insurance, or a change in its availability, can be a substantial and continuing change (A.R.S. §25-503). The 15% figure comes from the state guidelines that apply to orders entered after January 1, 2022; a revised draft, dated October 21, 2026, would apply to orders entered after January 1, 2027 if adopted.
Florida
The court may reduce support even if the parent asking has not paid what is already owed (Fla. Stat. §61.14). A parent's failure to regularly use the time-sharing schedule that reduced the support amount, when the failure was not caused by the other parent, is a substantial change of circumstances, and the change then reaches back to the date that parent stopped using the schedule (§61.30(11)(c)).
Court motion or agency review
A court motion can be filed at any time, but the parent asking usually must prove a change; adding health coverage to an order is an exception in some states (Fla. Stat. §409.2564(11)(e) for agency cases; C.R.S. §14-10-122(1)(a)). An agency review needs no proof inside the three-year cycle. Either way the new figure comes from the state's current guidelines. A calculator can show a hypothetical amount under those guidelines, but the comparison does not decide whether an order can be modified: some grounds are not about the amount at all, numeric tests apply only to particular grounds (in Florida, for example, the 15% or $50 test applies only when the guideline difference is the change relied on, Fla. Stat. §61.30(1)(b)), and agreements and exceptions can change the answer.
This guide is general information, not advice on a particular order. A family law attorney or the state child support agency can explain how the rules apply to a specific case.