Income Change Checker
The CMS will only reassess maintenance if the paying parent's income has changed by 25% or more. Check whether the threshold has been met and see your estimated new maintenance amount.
Check the 25% income change threshold
The CMS will only reassess maintenance if the paying parent's income has changed by 25% or more since the last assessment. Enter both figures to check.
The income figure the CMS currently uses
The paying parent's current actual income
Children the paying parent supports in their home
The 25% rule, in plain terms
The CMS does not adjust a case every time earnings move. Between annual reviews it will only change the figure if gross income has shifted by 25% or more against the income already held on the case. That test sits in the Child Support Maintenance Calculation Regulations 2012, at regulation 17 and regulation 34. Anything smaller and the original figure stands until the review date comes round.
The rule cuts both ways. A paying parent who loses overtime worth 15% of their pay carries on paying the old amount. A receiving parent whose ex takes a 20% pay rise sees nothing extra until the review. One rule, applied to both households, and it is the single most common reason people are told the CMS cannot do anything.
Working the threshold out yourself
Take the gross annual income used in your current calculation. It is printed on your calculation letter. Multiply it by 0.75 to get the drop threshold and by 1.25 to get the rise threshold. If the new income falls outside that window, the case can be superseded. If it lands inside it, it cannot.
| Income on the case | A drop qualifies at or below | A rise qualifies at or above |
|---|---|---|
| £20,000 | £15,000 | £25,000 |
| £35,000 | £26,250 | £43,750 |
| £52,000 | £39,000 | £65,000 |
| £80,000 | £60,000 | £100,000 |
The checker above runs that arithmetic and then puts the new figure through the rate bands, so you can see whether the threshold is met and roughly what the payment becomes. For a full calculation including shared care and other children, use the main CMS calculator.
The figure the CMS uses is not your latest payslip
This catches out more people than the threshold itself. The CMS takes gross taxable income reported to HMRC for the latest available tax year. It is historic on purpose. Someone made redundant in March can spend months being assessed on the salary they earned before it, because that is what HMRC holds.
Two things follow from that. A large pay rise may not surface for a long time. And when it does surface at the annual review, it arrives as a step rather than a drift, which is why review letters so often land as a shock. The annual review predictor is built to take that surprise away before the letter arrives.
How the new payment is worked out
Once a change clears 25%, the CMS reruns the standard formula on the new gross weekly figure. There are five income bands, and which one you land in matters more than the exact pound amount.
| Band | Gross weekly income | How it is charged |
|---|---|---|
| Nil rate | Under £7 | Nothing payable |
| Flat rate | £7 to £100 | £7 a week, whatever the number of children |
| Reduced rate | £100.01 to £199.99 | £7 plus a percentage of the income above £100 |
| Basic rate | £200 to £800 | 12% for 1 child, 16% for 2, 19% for 3 or more |
| Basic plus | £800.01 to £3,000 | Basic rate on the first £800, then 9%, 12% or 15% on the excess |
Gross weekly income above £3,000 is ignored completely. A parent earning £2,000 a week and a parent earning £5,000 a week are treated identically by the formula. Section 8 of the Child Support Act 1991 also removes the courts from most child maintenance disputes, so the ceiling is not easily worked around.
Deductions applied before the percentages
Other children living with the paying parent come off the gross income first, before any percentage is applied:
- 1 other child: gross income reduced by 11%
- 2 other children: reduced by 14%
- 3 or more: reduced by 16%
The child maintenance percentages are then applied to what is left, not to the original figure. If a new child has joined the paying parent's household since the last assessment, that alone can change the outcome without any change in earnings at all.
Shared care comes off the total
Nights the child spends with the paying parent reduce the weekly figure in fixed bands, based on nights per year:
| Nights per year | Reduction |
|---|---|
| 52 to 103 | 1/7 |
| 104 to 155 | 2/7 |
| 156 to 174 | 3/7 |
| 175 or more | 1/2, plus a further £7 per child per week |
The bands are blunt. One night a year either side of a boundary can move the payment by a seventh. The CMS works from the pattern it decides applies, which is not always the pattern that actually happens, and disputes over nights are among the hardest parts of a case to resolve because the evidence is usually one parent's word against the other's.
A worked example, start to finish
Take a paying parent assessed on gross income of £45,000 a year. They change jobs and the new gross income is £33,000. That is a fall of £12,000, or 26.7% of the original figure, so it clears the threshold and the case can be superseded. Two qualifying children, one other child living in the paying parent's household, and 104 nights of shared care a year.
| Step | On £45,000 | On £33,000 |
|---|---|---|
| Gross weekly income | £865.38 | £634.62 |
| Less 11% for 1 other child | £770.19 | £564.81 |
| Basic rate at 16% for 2 children | £123.23 | £90.37 |
| Less 2/7 for 104 nights of shared care | £88.02 | £64.55 |
The weekly payment falls by £23.47, which is about £1,220 across a year. Notice how much work the two adjustments do. Before the other child and shared care are taken into account the difference looks like £32.86 a week. The order matters as well: the 11% comes off the gross income, the 16% is applied to what is left, and the shared care fraction comes off the resulting maintenance figure. Get the order wrong and you will not match the CMS letter.
One more detail sits in the first row. On £45,000 the parent starts in the basic plus band at £865.38 a week, but the deduction for the other child pulls them back under £800 and into basic rate. Band boundaries move for reasons that have nothing to do with earnings.
Where the official position is contested
The threshold is not a fairness test. It is an administrative filter that exists to keep caseloads manageable. A household can lose a quarter of its income in real terms across two smaller changes and still fail the test, because each change is measured against the figure on the case rather than added together. That is the rule as written, and it is worth knowing the reason rather than assuming your case has been handled badly.
The other contested point is verification. The CMS treats the HMRC figure as correct unless challenged. For employed earners that is usually fair. For company directors and the self-employed it can miss income entirely, which is what the variation rules exist to deal with. Check the variation grounds checker if the figure on the case looks nothing like the lifestyle behind it.
Reporting a change, step by step
- Work out the percentage difference first, using the checker above.
- Gather evidence: payslips, a P45, an award letter, or accounts.
- Report it to the CMS and note the date. Follow up in writing so there is a record of when you told them.
- Ask what effective date has been applied and get it confirmed in writing.
- Check the recalculation against the bands above rather than accepting it on trust.
If the change creates a balance in either direction, the arrears calculator will show what has built up, and the backdate calculator shows how effective dates translate into pounds.
If the CMS gets the decision wrong
A refusal to supersede is a decision, and decisions can be challenged. You must ask for a mandatory reconsideration within one calendar month of the decision before you can appeal to the First-tier Tribunal. Miss that window and the route closes. Our mandatory reconsideration tool sets out what to put in the request and what evidence carries weight.
Keep the two things separate in your head. A wrong figure is a challenge. A changed figure is a supersession. Mixing them up is the fastest way to get a letter back saying the threshold has not been met, when the real point was that the income on the case was never right.
Common questions
What counts as a 25% income change for the CMS?
The comparison is between the gross annual income used in your current calculation and the new gross annual income. Multiply the figure on your calculation letter by 0.75 and by 1.25. If the new income sits outside that window, the case can be changed mid-year under regulations 17 and 34 of the Child Support Maintenance Calculation Regulations 2012. If it sits inside the window, the existing figure stands until the annual review.
Will the CMS reduce my payments if I lose overtime?
Only if the loss is big enough. Overtime is part of gross taxable income, so losing it counts, but the drop has to reach 25% of the income held on the case before the CMS will act between reviews. A worker on a £30,000 basic with £6,000 of overtime who loses all the overtime has dropped by around 17%, so the payment does not change until the annual review picks up the lower HMRC figure.
Does the CMS use my current salary or last year's?
It uses gross taxable income reported to HMRC for the latest available tax year. The figure is historic by design, so a pay rise or a redundancy can take months to show up. That is also why the annual review can feel like a sudden jump rather than a gradual change: the CMS is catching up with something that happened in the past rather than tracking your pay week by week.
Do pension contributions change the figure the CMS uses?
Salary sacrifice contributions reduce gross taxable pay before it reaches HMRC, so they reduce the figure the CMS sees. Personal pension contributions paid out of net pay do not, because the gross income reported to HMRC is unchanged. That difference is not obvious from any CMS letter, and it is one of the main reasons two people on identical salaries can end up with different assessments.
How far back does a change apply once the CMS accepts it?
The CMS sets an effective date, and it is rarely the day your income actually changed. It is usually tied to when the change was reported and verified. Your decision letter should state the date in plain terms. If it does not, ask for it in writing, because the effective date decides how much is owed or credited and it can be challenged in the same way as any other decision.
What can I do if the change is under 25%?
Three things. Check that the income figure on the case is correct in the first place, because an error is not the same as a change. Check whether the shared care nights or the number of other children living with the paying parent are right. Then check whether a variation ground applies, since unearned income and special expenses sit outside the 25% test entirely.