How to Calculate Interest Rate on a Loan
A loan's interest rate, its term and the amount borrowed fix the monthly payment through one formula; if you know the payment instead, you can work backward to the rate. This page covers both, plus how APR differs from the rate and how to compare offers.
The Basics of Loan Interest
The interest rate is the cost of borrowing, expressed as a yearly percentage of the balance. When you take out a loan you repay the amount borrowed plus interest as compensation to the lender for risk and for not having the money elsewhere.
Simple interest is charged only on the principal; compound interest is also charged on interest already accrued. Most consumer loans (mortgages, auto loans, personal loans) charge simple interest on the outstanding balance each month, with a fixed payment that covers that month's interest and some principal.
The annual percentage rate (APR) is a broader measure of cost: it reflects the interest rate plus points, broker fees and certain other charges you pay to get the loan, so it is usually higher than the rate. A mortgage with a 6.5% interest rate might carry a 6.8% APR once fees are included. Because lenders must disclose APR the same way, it is the better number for comparing offers.
What Sets Your Rate
Your credit score is the biggest borrower-specific factor: higher scores get better pricing, and low scores can mean much higher rates or a denial.
The term matters too. Shorter loans usually carry lower rates but higher monthly payments; a 15-year mortgage is typically priced below a comparable 30-year loan, yet its payment is much higher because the balance is repaid in half the time.
Market conditions, loan type and down payment also move rates. Secured loans backed by collateral are cheaper than unsecured ones. Federal Reserve policy, inflation expectations and market stability set the baseline, and a larger down payment reduces the lender's risk.
From Rate to Payment, and Back
The monthly payment formula is M = P × [r(1+r)^n] / [(1+r)^n − 1], where M is the monthly payment, P the loan amount, r the monthly rate (annual rate ÷ 12) and n the number of payments.
Example: a $25,000 auto loan at 7.2% for 60 months. The monthly rate is 7.2% ÷ 12 = 0.6%, or 0.006. Then (1.006)^60 ≈ 1.43179, so M = 25,000 × [0.006 × 1.43179] / [1.43179 − 1] = 25,000 × 0.0085907 / 0.43179 ≈ $497.39.
The formula can't be rearranged to give the rate directly; it is found by trial, trying a rate, computing the payment and adjusting. If you can afford $450 a month for the same $25,000 over 60 months, the rate that produces that payment is about 3.07%. Spreadsheet RATE functions do this search, and so does the APR calculator: enter the loan amount, monthly payment and term in months, and with fees set to zero it returns the interest rate behind that payment. Enter the upfront fees as well and it returns the APR, the rate on the amount actually financed.
The Loan Payment calculator goes the other way: loan amount, annual rate, term and payment frequency in, payment out. Multiply the payment by the number of payments and subtract the loan amount to get total interest.
To see the same rate from the saver's side, the Compound Interest calculator shows how a balance grows at a given rate and compounding frequency.
Comparing Loan Offers
Compare offers on APR rather than the stated rate, and line up each lender's APR, monthly payment, total interest and fees side by side.
Look at total cost, not just the monthly payment. At 7%, a $20,000 auto loan costs about $479 a month over 48 months or $341 over 72 months; the longer term saves about $138 a month but adds roughly $1,560 in interest, and lenders often charge a higher rate for longer terms, which widens the gap.
Make sure the payment still works if your income dips, after your other bills and savings. The figures here are illustrations, not financial advice.
Related guides: how to calculate your monthly mortgage payment and how to calculate compound interest.