Enter a loan's principal, rate, and term to calculate the total interest you'll pay over a standard monthly amortization schedule.
How to Use the Loan Interest Calculator
Set the loan principal, the annual interest rate, and the term in years. Payments are fixed at monthly — twelve a year — so the three result cards update the moment any field changes, with total interest given the top billing.
The monthly payment is the number people check first when shopping for a loan, and it's also the number that hides the real cost best. Two loans with nearly identical monthly payments can differ by thousands of dollars in total interest once the term length changes. This tool leads with that figure on purpose.
The Amortization Formula
Every fixed-rate, monthly-pay loan reduces to one equation:
M = P × [r(1+r)ⁿ] ÷ [(1+r)ⁿ − 1]P is the principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments — years × 12. Multiply the resulting payment M by n for total paid, then subtract P for total interest.
Run the defaults and the shape becomes obvious: a $20,000 loan at 7% over five years lands at a monthly payment just under $400, but total interest comes out above $3,700 — nearly a fifth of what was borrowed, on a rate that doesn't sound aggressive at all.
Interest Is Front-Loaded
Because interest accrues on the balance still outstanding, early payments carry the heaviest interest share, and the split shifts toward principal only as the balance shrinks. Stretch the same loan across a longer term and the monthly payment drops — but the total interest climbs, since you're paying for the money longer at the same rate.
Take that same $20,000 loan again: the very first payment sends roughly $117 toward interest and about $279 toward principal. By the sixtieth and final payment, interest has shrunk to a few cents and almost the entire $396 wipes out what's left of the balance. The payment amount never moves across those five years — only the split underneath it does.
Term length moves the interest total more sharply than most people expect. Cut a five-year loan to three years at an unchanged rate and total interest typically falls by close to half; the tradeoff is a monthly payment that's noticeably higher.
No Default Rate, on Purpose
This calculator doesn't ship with a preset interest rate. Rates depend on lender, credit profile, loan type, and country — a single "typical" number would be a guess dressed up as data. Use whatever your own loan offer actually quotes, and try a couple of rate scenarios side by side to see how much a point of difference moves the interest total.
A single point of rate matters more than it sounds. Move that same loan from 7% to 8% and total interest climbs by roughly $570, even though the monthly payment itself moves only about ten dollars — small rate gaps between competing offers compound into real money over a multi-year term.
Need something other than a monthly schedule — biweekly or weekly payments, say? The Loan Payment Calculator covers that. This one stays deliberately narrow so the interest figure is easy to reason about.