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Debt Snowball Calculator

See how fast a fixed payment clears your debt

How it is calculated

The tool treats one debt as a standard amortising loan with a fixed payment. The monthly rate is i = APR ÷ 100 ÷ 12 and the number of months is n = ⌈ −ln(1 − i × balance ÷ payment) ÷ ln(1 + i) ⌉, rounded up to a whole month. Total interest is found by running the balance month by month: add one month's interest, subtract the payment, repeat until it reaches zero. Total paid is the balance plus that interest. If the APR is 0, months is simply balance ÷ payment. If the payment does not exceed the first month's interest, the debt never shrinks and no result is shown. Interest is assumed to compound monthly with no new purchases or fees.

Worked example

A card balance is 8,000 at 19.9% APR with a fixed payment of 300 a month. Monthly rate: 19.9 ÷ 100 ÷ 12 = 0.016583. First month's interest is about 132.67, so the 300 payment does cut the balance. The formula gives about 35.5 months, which rounds up to 36 months. Running the balance down month by month gives total interest of $2,649.06, so the total paid is $10,649.06, slightly less than 36 × 300 because the last payment is smaller.

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Common mistakes

  • Entering the monthly interest rate instead of the annual one. The field is APR, and the tool divides by 12 itself.
  • Using a payment that is barely above the monthly interest. The payoff then takes decades; a payment at or below the interest never pays the debt off at all.
  • Expecting the snowball ordering of several debts. This tool handles one debt at a fixed payment; run each debt separately.
  • Forgetting that new charges on a card restart the clock. The result assumes no new spending.
  • Entering a minimum payment that falls as the balance falls. The tool assumes the same payment every month.

Sources

About this calculator

Enter a balance, interest rate, and fixed monthly payment and this calculator solves the standard loan amortization equation for time: n = −log(1 − rB/P) / log(1 + r), where r is the monthly interest rate, B the starting balance, and P the payment. Total interest paid is simply the sum of all payments minus the original balance.

This single-debt payoff math is the engine behind the popular debt snowball method (made famous by Dave Ramsey): list every debt smallest-balance-first, pay only the minimum on the rest, and throw every spare dollar at the smallest one until it's gone — then roll that entire payment into the next debt. Running each debt through this calculator shows exactly how many months of extra payment it takes to knock it out and free up cash for the next one.

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Comments (1)

  • Editorial team

    This tool follows one debt at a time, so use it for the roll-over step too. Run the debt once at its minimum payment and once with an extra amount, then compare months and interest; the gap is the real gain from paying extra. When the first debt clears, add its payment to the next one and run that debt with its balance as it will stand by then. How many debts are you juggling, and would a multi-debt view help?

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