Why does the calculator refuse very low payments?

If the payment is less than the monthly interest, the balance grows every month and the debt never clears. The maths returns no solution, so the tool tells you instead.

Should I pay the highest rate or the smallest balance first?

Paying the highest rate first costs least mathematically. Paying the smallest balance first gives quicker wins and helps some people stay motivated — both are defensible.

Does the calculator handle extra one-off payments?

Not directly. You can approximate one by reducing the amount owed by the lump sum before calculating.

Payment Calculator

Two questions come up constantly with debt: what will the payment be, and how long will it take if I pay a set amount each month? This calculator answers either one.

Fill in the fields above and select Calculate to see your result and the full working.

Please note: Financial calculators produce estimates using the figures you enter. They exclude fees, taxes, insurance and rate changes unless a field asks for them. They are not financial advice — confirm any significant decision with a qualified adviser or your lender.

Formula used

Payment = P·r ÷ (1 − (1 + r)⁻ⁿ). Rearranged for time: n = −log(1 − P·r ÷ payment) ÷ log(1 + r)

How to use this calculator

Choose whether you know the term or the payment.
Enter the balance owed and the annual interest rate.
Fill in either the term in months or the monthly payment you can afford.
In term mode, try adding 10% to the payment and watch how much interest disappears.

Example calculation

$12,000 at 12% with a $400 monthly payment:

Monthly interest at the start = 12,000 × 0.01 = $120

n = −log(1 − 12,000 × 0.01 ÷ 400) ÷ log(1.01) ≈ 35 months

Total repaid ≈ $13,900, so about $1,900 in interest.

What does this result mean?

The payoff-time mode exposes how minimum payments work. Because a minimum is usually only slightly above the interest charge, almost none of it reduces the balance. Increasing a payment by a modest amount often cuts years off the term, since every extra unit goes entirely against principal.

Frequently asked questions

Why does the calculator refuse very low payments?: If the payment is less than the monthly interest, the balance grows every month and the debt never clears. The maths returns no solution, so the tool tells you instead.
Should I pay the highest rate or the smallest balance first?: Paying the highest rate first costs least mathematically. Paying the smallest balance first gives quicker wins and helps some people stay motivated — both are defensible.
Does the calculator handle extra one-off payments?: Not directly. You can approximate one by reducing the amount owed by the lump sum before calculating.

Related calculators

More in Finance & Money

Browse the Finance & Money category or the full calculator directory.