Debt snowball & avalanche calculator
Enter your debts, choose how much extra you can pay each month, and instantly see your debt-free date and the total interest you’ll pay. It is for anyone juggling multiple credit cards, loans or store cards who wants a concrete payoff plan and to know which strategy costs least.
How it works
The calculator runs a month-by-month simulation of every debt:
- Each month it adds interest at the monthly rate
APR ÷ 12 ÷ 100to every balance, then applies each debt’s minimum payment. - Any cash left over — your extra payment plus the minimums of already-cleared debts — is thrown at the priority debt.
- The priority order depends on the strategy: snowball targets the smallest balance first; avalanche targets the highest interest rate first.
- When a debt hits zero, its payment rolls onto the next one, and the loop continues until every debt is clear.
A debt whose minimum payment cannot even cover its monthly interest is flagged, because it would never be repaid.
Snowball vs. avalanche — which to choose
Both strategies use the same total monthly cash. The difference is the order in which debts are eliminated:
| Feature | Snowball | Avalanche |
|---|---|---|
| Priority order | Smallest balance first | Highest interest rate first |
| Total interest paid | Usually more | Usually less |
| Speed (time to debt-free) | Usually slightly longer | Usually slightly shorter |
| Psychological effect | Quick early wins; motivating | Slower first win; requires patience |
The avalanche method is mathematically superior — it eliminates interest from the most expensive debt first, so less money is wasted overall. But research on personal finance behavior consistently shows that people who feel they are making progress stick with their plans longer. If you have struggled to pay down debt before, the snowball’s quick wins may matter more than the interest savings.
A practical middle ground: use the calculator to compare both strategies for your specific numbers. If the interest difference is small (say, less than £200 over the whole payoff period), the snowball’s motivation benefit likely outweighs it.
The rolling payment effect
The key mechanic of both strategies is that when one debt is cleared, its minimum payment does not disappear — it rolls onto the next priority debt. This is the “snowball” that the strategy is named for, though both methods share it. With each debt eliminated, the monthly amount hitting the priority debt grows, which is why the last debt is paid off much faster than the first.
Worked example
Three debts with a £150 extra monthly payment, snowball strategy:
| Debt | Balance | APR | Min / mo |
|---|---|---|---|
| Credit card | £2,500 | 19.9% | £60 |
| Car loan | £8,000 | 6.5% | £180 |
| Store card | £900 | 24.9% | £30 |
The snowball order is: store card (£900) → credit card (£2,500) → car loan (£8,000).
- Month 1: £150 extra + £30 minimum = £180 toward the store card
- After the store card clears (about 5 months), £180 + £60 + £150 = £390 rolls into the credit card
- After the credit card clears, the full combined payment attacks the car loan
Total time to debt-free under snowball: roughly 3 years 2 months. Switching to avalanche (store card first by rate at 24.9%, then credit card at 19.9%, then car loan) finishes in similar time but with noticeably less total interest paid.
Practical notes
- Enter accurate minimum payments. If the minimum payment barely covers the monthly interest charge, the debt will take a very long time to clear. This calculator flags debts where the minimum is too low to make progress.
- Every extra pound accelerates the result significantly. Adding even £25/month more than your current extra payment often shaves months off the payoff time.
- Once a debt is clear, resist lifestyle inflation. The rolling payment only works if you actually keep directing those freed-up minimums to the next debt rather than spending them.
Everything runs in your browser, so your balances and rates stay completely private.