Tracks simple interest on a sum payable (e.g. a costs order) where one or more payments on account are made before the balance is settled. Enter the total payable, the interest rate (default 8% — the Judgments Act 1838 rate), and the date interest starts. Add each payment on account with its date. The calculator shows the interest that had accrued by the date of each payment and the total interest outstanding to the calculation date. Payments are applied to lets you choose the appropriation basis: principal only (each payment reduces the principal; accrued interest is preserved and stays outstanding — the usual model for payments on account of a costs order), or interest first, then principal (each payment discharges accrued interest before reducing principal — the conventional appropriation for part-payments of a judgment debt carrying interest under the Judgments Act 1838, s.17). Interest is simple in both cases and never compounds.