Debt Interest Calculator UK — See What Your Debts Really Cost

A debt interest calculator shows how much of what you pay each month goes on interest rather than reducing what you owe. For each debt, monthly interest is roughly the balance × APR ÷ 12: £5,000 at 18% APR costs about £75 a month. Enter your debts below to see your total interest this month, over a year, and over the next 1, 3 and 5 years at your current payments.

Your debts (up to 5)

Take-home pay a month (£, optional)

Add this to see interest as a share of your income.

Enter each debt's balance, APR and what you pay each month, then press Calculate to see what your debts cost in interest.

Interest this month

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Interest over 1 year

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Interest over 3 years

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Interest over 5 years

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Debt	Interest this month	Share of payment	Clears in	Total interest to clear

Cut the interest: see a highest-APR-first payoff plan

How interest on your debt is calculated

Lenders quote an APR, an annual rate. To estimate a month's interest, divide the APR by 12 and multiply by your balance: monthly interest ≈ balance × APR ÷ 12. A £3,000 credit card at 24.9% APR costs about £62 a month; a £5,000 balance at 18% about £75.

Interest is taken first, so only the part of your payment above the interest reduces the balance. That is why the 1, 3 and 5-year figures matter more than this month's number: they show what your debts cost over time at the payments you make now.

Credit cards usually work out interest daily, and loans have their interest built into a fixed repayment, so your statement can differ by a few pounds. The calculator is designed to show the size of the cost, not to match a statement to the penny.

How much interest am I paying? A worked example

Take a £5,000 balance at 18% APR. The first month's interest is £75. What it costs in total depends on what you pay:

Monthly payment	Clears in	Total interest
£150	47 months	£1,984
£250	24 months	£989
£75	Never: the payment only covers the interest	£900 every year

An extra £100 a month roughly halves the interest and clears the debt nearly two years sooner. Paying only the interest keeps the balance exactly where it is. The credit card minimum payment calculator shows how this plays out when your payment shrinks as the balance falls.

How to pay less interest on your debts

Overpay the highest APR first. Every extra pound on your most expensive debt saves the most interest. That is the debt avalanche; the debt snowball clears the smallest balance first for quicker wins.
Move card balances to 0%. A 0% balance transfer stops interest for the promotional period, usually for a one-off fee. It works if you clear the balance before the deal ends.
Pay more than the minimum. Minimum payments are set so that a large share goes on interest. Fixing your payment at today's amount, rather than letting it fall, cuts years off a card.
Talk to your lender if you are struggling. If you are in persistent debt or can't keep up, lenders must work with you, and free advice services such as StepChange and National Debtline can ask creditors to freeze interest through a debt management plan.

If interest is taking a large share of your income, the debt-to-income ratio calculator shows how stretched your repayments are overall.

Build a free payoff plan with the debt snowball calculator

Frequently asked questions

How do I calculate the interest on my debt?

Multiply the balance by the APR, then divide by 12 for a rough monthly figure. £5,000 at 18% APR is £5,000 × 0.18 ÷ 12 = £75 of interest a month. Credit cards usually work out interest daily, so the exact amount on your statement can differ slightly, but this gets you close.

How much interest will I pay on £5,000 of debt?

At 18% APR it is about £75 in the first month. If you pay £150 a month it takes 47 months to clear and costs about £1,984 in interest. Paying £250 a month clears it in 24 months for about £989, so an extra £100 a month roughly halves the interest.

Why does my balance barely go down?

Because interest is taken first. If £75 of a £150 payment goes on interest, only £75 reduces what you owe. Paying only the minimum keeps that share high for years. Even a small overpayment goes straight onto the balance, which cuts next month's interest too.

Is APR the same as a monthly interest rate?

No. APR is an annual rate that also includes some standard fees. A rough monthly rate is the APR divided by 12, so 24% APR is about 2% a month. This calculator uses that monthly rate, which is how most people estimate their interest.

Which debt should I pay off first to save interest?

Paying extra on the debt with the highest APR saves the most interest. That is the debt avalanche method. Clearing the smallest balance first (the debt snowball) costs a little more but gives quicker wins. Either way, keep paying the minimum on everything else.

Disclaimer: This calculator is for information only and isn't financial advice. Figures are estimates based on the APR and payments you enter, using a monthly interest calculation; your lender's figures may differ. If you're struggling with debt, contact a free UK debt advice service such as StepChange (stepchange.org) or National Debtline. For personal advice, consult an adviser regulated by the Financial Conduct Authority (FCA).

How this calculator works

The method

For each debt, this month's interest is the balance multiplied by the APR divided by 12. The 1, 3 and 5-year figures run each debt forward month by month: interest is added, then your monthly payment is taken off, until the debt is cleared or the period ends. Payments are never more than the balance plus that month's interest, and debts are treated separately, so money freed up when one debt clears is not moved onto another.

Assumptions & limitations

The APR you enter stays the same, and you make the same payment every month with no new spending, fees or charges.
Interest is worked out monthly. Credit cards usually charge interest daily, so your statement may differ by a few pounds.
If a payment does not cover the month's interest, the debt is shown as never clearing at that payment.

Data sources

Every calculation runs in your browser on the balances and rates you enter, so the results reflect your own figures. The approach follows standard UK personal-finance conventions; for reference figures and guidance we point to MoneyHelper and the Bank of England. Figures and rules change over time — always check the original source.

Built and reviewed by a UK Chartered Mechanical Engineer. More on our methodology.

Methodology & trust

Written and reviewed by Peter Barclay, a UK Chartered Mechanical Engineer — who builds and maintains these tools, pairing engineering-mathematics training with a focus on the mechanics of debt repayment. The calculators use standard amortization formulas and fixed repayment orders. Read our methodology or more about the author.

DebtSnowball is not a financial adviser and is not authorised or regulated by the FCA (Financial Conduct Authority). These tools are for education and information only, not financial advice. If you are struggling with debt, get free, impartial help from StepChange, National Debtline or Citizens Advice.

Last reviewed: November 2026 Spotted an error? Report it and we will fix it.