Child support arrears calculator.

How are child support arrears calculated?

Arrears are support that came due under an order or agreement and was not paid. The basic arithmetic is the total of the payments due up to a date, less the payments made. This tool counts one payment on the same day of each month as the first due date (a payment due on the 31st falls on the last day of shorter months) and subtracts the payments you list.

Interest. The Ontario Family Responsibility Office says interest on unpaid support can be claimed only if the interest rate is stated in the order or, where the order has no interest provision, under the Courts of Justice Act and the Family Law Rules for orders that meet the listed date requirements. This tool does not look up a rate. Enter the rate your order states. It then adds simple interest on the unpaid balance for the days it was outstanding. Payments reduce the balance; interest is not compounded and no payment is applied to interest first. That is an assumption, not the FRO method.

Worked example. $500 a month from 1 January 2026, calculated up to 1 June 2026, with payments of $500 on 1 February and 1 April. Six payments are due ($3,000), $1,000 is paid, so $2,000.00 is unpaid. With a 5% yearly rate the simple interest estimate is $18.77.