Credit Card Payoff Calculator

Where the first payment goes

On a £5,000 balance at 19.99%, the first month's interest is £83.29. Pay £150 and only £66.71 reduces the debt — under half of what you handed over.

That ratio improves every month as the balance falls, which is why progress feels glacial at first and then accelerates. The final payments are almost entirely principal.

The case that never clears

If the payment is at or below the monthly interest, the balance grows regardless of how long you keep paying. The formula returns the logarithm of a negative number, and the honest answer is not a number at all.

This page reports that explicitly rather than showing an error, because it is the single most important thing a payoff calculator can tell someone: at £80 a month against £83.29 of interest, the debt is growing.

Anything above the interest makes progress, though the first increments are slow. Paying £150 instead of £100 on that balance cuts the term from over eight years to about four.

Why paying more helps more than proportionally

Raising the payment by half — from £150 to £225 — does not cut the interest by a third. It cuts it by well over half.

The reason is compounding working in reverse: every pound of extra payment removes principal that would otherwise have generated interest for the remaining term. The saving compounds the same way the debt does.