If you are already receiving tax credits you may be subject to an adjustment from previous years. This means that HM Revenue & Customs may be reducing your payments to compensate for overpayments in previous years. This affects about one third of tax credit recipients. The calculator does not take into account overpayments and underpayments - it works out your entitlement on the basis that previous tax credit awards were correct.
You should get more information if you have been told that you have a tax credit overpayment that you have to pay back, or even if you notice a worrying drop in the amount of payments you are getting. For further help and information please see the overpayments guide on the the Low Income Tax Reform Group's website.
To find out more about how the tax credit system works, including the difference between an ‘initial’ and a ‘final’ award, go to how tax credits work. More information is also available on telling the tax credits office about changes of circumstance.