UK Debt Payoff Calculator (Snowball vs Avalanche)

When you are juggling more than one debt, the order you pay them off in makes a real difference to how much interest you pay and how quickly you become debt-free. This calculator compares the two most popular strategies, the debt snowball and the debt avalanche, side by side using your own balances, interest rates and minimum payments.

How to use the debt payoff calculator

Add each debt you want to include, with its current balance, annual interest rate (APR) and minimum monthly payment. Then enter any extra amount you can put towards your debts each month, on top of all your minimum payments combined. The calculator simulates both the snowball and avalanche methods and shows how many months each would take, and how much total interest each would cost.

How the calculation works

Every month, the calculator adds interest to each remaining debt based on its APR, then applies the minimum payment to every debt. Any money left over, your extra payment plus the minimum payments freed up from debts you have already cleared, is then put entirely towards one target debt. The snowball method always targets whichever remaining debt has the smallest balance. The avalanche method always targets whichever remaining debt has the highest interest rate. Once a debt reaches zero, its minimum payment is added to the pool used to attack the next target debt, which is why both methods tend to speed up as debts are cleared.

Worked example

Take a £500 store card at 5% APR with a £20 minimum payment, and a £3,000 credit card at 24% APR with a £90 minimum payment, with £150 extra to put towards debt each month. Using the snowball method, the calculator clears the smaller store card first, in month 3, then the credit card by month 16, paying around £598 in total interest. Using the avalanche method, the calculator targets the higher-rate credit card first, clearing it by month 15 and the store card the month after, also finishing by month 16, but paying only around £511 in total interest, a saving of roughly £87 simply from changing the order the debts are attacked in.

In this example both methods take the same overall time, since the total monthly payment is identical either way, but the avalanche method still comes out cheaper because it prioritises the most expensive debt. Try entering your own debts and extra payment amount in the calculator above to see the difference for your situation.

Common mistakes to avoid

A common mistake is only ever paying minimum payments, which can mean a high-interest debt like a credit card takes many years to clear and costs far more in interest than the original balance. Even a modest extra payment each month, directed strategically, makes a significant difference over time.

Another mistake is ignoring interest rates entirely and always paying off whichever debt feels most urgent or annoying, rather than following a consistent method. It is also worth checking whether a balance transfer credit card or debt consolidation loan could reduce your average interest rate before committing to a long payoff plan, since a lower rate on the same balance reduces the total interest under either method.

Related calculators

If you are focused on a single credit card, our credit card payoff calculator goes into more detail on that specific balance, and our loan repayment calculator covers a standard personal loan. Once your debts are cleared, our savings goal calculator can help you redirect that same monthly payment towards building an emergency fund or savings goal instead. To understand why interest rate matters so much to the avalanche method, see our guide to compound interest explained.