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Personal Loan Monthly Payment & Interest Estimate

Estimate a level monthly payment and total interest for a personal loan, including a zero-interest loan.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Results are estimates for planning and education. Confirm rates, taxes, fees, and legal requirements with the relevant institution or a qualified professional before making a financial decision.

Formula and method

A 12,000 loan at zero interest over twelve monthly periods requires a payment of 1,000, with no interest.

Payment = P × r / (1 − (1 + r)^−n); at r = 0, payment = P/n

Worked example

Enter these known values and leave the other values blank.

Amount borrowed
12000 USD
Term in equal monthly periods
1:0 yrs / mos
Annual nominal interest rate
0 %
Level monthly payment
1000 USD
Estimated total interest
0 USD

Assumptions and limitations

  • The annual nominal interest rate is divided by twelve. Payments occur at the end of equal monthly periods, with a constant nonnegative rate and no extra payments or fees.
  • A whole number of monthly periods describes scheduled equal payments. A fractional term is a continuous mathematical estimate; it is not a calendar payoff date or an exact final partial-payment amount.
  • Calculations retain precision internally and format money for display. Lender rounding, daily accrual, irregular dates and contractual charges can change an actual schedule.
  • The supplied interest rate is an input; this calculator does not recover an unknown rate from other values. Duration selectors use fixed conversion factors, not calendar dates.
  • Amounts borrowed and payments are positive. Total interest is payment times the modeled number of months minus principal; it excludes origination fees and other costs.

Common questions

Does zero interest need a special formula?

Yes. Divide principal by the number of payments. Substituting zero directly into the ordinary interest formula would divide zero by zero.

Is the interest rate the same as APR including fees?

Use the contractual rate consistent with monthly accrual. An APR that incorporates fees is not automatically the correct rate for this payment model.

References

Bookify handles zero interest explicitly as principal divided by the term and updates the corresponding inverse equations. Bookify requires a positive term and nonnegative interest, including a payment sufficient to repay principal.

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