Free Tool

CMS Backdated Payments Calculator

Calculate how much child maintenance may be owed for the period before the CMS notified the paying parent - from your application date to when payments began.

How CMS backdating works: When you apply to the Child Maintenance Service, your case has an effective date- usually the date your application was received. The paying parent's liability begins from that date, even if the CMS takes weeks to contact them. Any gap where no payments were made may be owed to you as backdated maintenance.

Enter your details

CMS application date *

The date you applied - this is your effective date

Date payments started *

When the paying parent first made a payment

Weekly maintenance amount *

As shown on your CMS schedule

Payments already received (optional)

Deduct anything already paid in this period

Collection method

Affects whether the 20% Collect & Pay surcharge note applies

What is the effective date?

When you apply to the CMS, your case is assigned an effective date- usually the date your application was received. The paying parent's legal liability starts from this date, even if it takes the CMS weeks or months to process the case and notify the paying parent.

Any maintenance that should have been paid between the effective date and when payments actually started can be treated as arrears and formally pursued by the CMS.

What to do with these figures

1Note your CMS effective date - it should be on your original CMS letter
2Contact the CMS (0800 171 2345) and ask them to pursue arrears from the effective date
3Quote these figures and ask for a written response within 15 working days
4If they don't respond, use our complaints letter generator to escalate

Disclaimer: This tool provides estimates for guidance only. Actual arrears figures should be confirmed with the CMS directly. This is not legal advice.