Home › Finance › Loan Payment Calculator

Loan Payment Calculator

Computes the level monthly payment of a fully amortizing fixed-rate loan from principal, annual rate and term, plus the total paid and total interest over the life of the loan.

When to use

You need the monthly payment or total interest cost of an auto, personal, student or other fixed-rate installment loan given its amount, APR and term.

Do not use when: You need property tax, insurance and HOA in the payment (use mortgage-payment), or you know the payment and want the payoff time (use debt-payoff).

Formula

i = annual_rate_percent / 1200; n = 12·term_years + term_months; monthly_payment = principal × i / (1 − (1 + i)^−n); if i = 0, monthly_payment = principal / n. total_payment = monthly_payment × n; total_interest = total_payment − principal.

Standard annuity (amortization) formula with monthly compounding and payments at the end of each month, as used for US fixed-rate consumer loans. Lenders round the payment to cents and adjust the final payment, so real totals can differ by a few cents.

Inputs

Parameter	Type	Unit	Required	Description
`principal`	number		yes	Amount borrowed. Range: > 0, ≤ 1000000000000
`annual_rate_percent`	number	%	yes	Nominal annual rate in percent; the monthly rate is this / 12. 0 is allowed for interest-free loans. Range: ≥ 0, ≤ 100
`term_years`	number	years	no	Loan term in years; fractional values allowed. Added to term_months when both are given. Range: ≥ 0, ≤ 100
`term_months`	integer	months	default 0	Additional months of term (e.g. term_months=60 alone for a 5-year auto loan). Range: ≥ 0, ≤ 1200

Outputs

Output	Type	Description
`monthly_payment`	number	Level payment due each month (principal + interest).
`number_of_payments`	integer	Total months = 12 × term_years + term_months.
`total_payment`	number	monthly_payment × number_of_payments (unrounded payment).
`total_interest`	number	total_payment − principal.
`payoff_summary`	string	Plain-language summary of the schedule.

Example

200,000 at 6 % for 30 years: {"principal":200000,"annual_rate_percent":6,"term_years":30} → {"monthly_payment":1199.1,"number_of_payments":360,"total_payment":431676.38,"total_interest":231676.38}

25,000 auto loan at 7 % for 60 months: {"principal":25000,"annual_rate_percent":7,"term_months":60} → {"monthly_payment":495.03,"number_of_payments":60,"total_payment":29701.8,"total_interest":4701.8}

GET https://tttkmbb.com/api/v1/calculate/loan-payment?principal=200000&annual_rate_percent=6&term_years=30

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/loan-payment (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/loan-payment · Markdown: https://tttkmbb.com/finance/loan-payment.md · JSON definition: https://tttkmbb.com/finance/loan-payment.json
MCP: server https://tttkmbb.com/mcp, tool calculate_loan_payment
OpenAPI operationId: calculate_loan_payment
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Amortization calculator (annuity payment formula) (reference)
Investopedia – Amortization: Definition, Formula and Example (reference)

FAQ

Is the APR the same as the interest rate used here?

The formula uses the note rate divided by 12. A lender's disclosed APR also folds in fees, so if you enter the APR the payment will be slightly overstated when fees are large.

What if the rate is 0 %?

The annuity formula divides by zero, so the calculator returns principal / number_of_payments instead (e.g. 12,000 over 12 months = 1,000 per month).

How do I model extra payments?

This calculator assumes the fixed schedule only. Use debt-payoff with the higher payment to see the shortened payoff time and interest saved.

Related calculators

Mortgage Payment Calculator — Same amortization plus taxes, insurance and HOA for a home purchase.
Debt Payoff Calculator — Given a payment, find how long a balance takes to clear.
APY Calculator — Effective annual cost of a nominal rate with monthly compounding.