How Loan Interest Works

Interest is the rent you pay for borrowed money — but two loans with the same rate can end up costing wildly different amounts, because what matters is not just the rate but how the interest is computed and collected. Almost every car loan, mortgage, and personal loan uses amortized interest, which quietly front-loads the interest into the early years.

Simple interest: the flat-rate version

With simple interest, the charge is computed once against the original principal and never recomputed as the balance changes:

I = P × r × t

Borrow $20,000 at 6% for 5 years and the interest is 20,000 × 0.06 × 5 = $6,000 — a straight line, $1,200 per year, whether the balance is $20,000 or $2,000. Simple interest is rare in long-term lending precisely because it ignores the fact that borrowers pay their balance down. It shows up in some short-term loans and bond coupons; consumer installment loans almost never use it.

Amortized interest: how real loans work

An amortized loan is built around a constant payment that covers both interest and principal, with the balance shrinking to exactly zero at the end of the term. Each month the lender first takes the interest earned on the current balance, and whatever is left of your payment reduces the principal. The payment that makes this come out even is:

M = P × r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

where:

• M — the fixed monthly payment
• P — the principal, the amount borrowed
• r — the monthly interest rate (annual rate ÷ 12)
• n — the total number of monthly payments

This is the standard amortization formula, and it is the same math whether the loan is a 60-month car loan or a 360-month mortgage. (For the special case of paying extra against a mortgage, see how mortgage payoff works.)

A worked example

Take a $20,000 car loan at 6% APR for 60 months. The monthly rate is r = 0.06 / 12 = 0.005, so (1.005)⁶⁰ ≈ 1.34885 and:

M = 20,000 × 0.005 × 1.34885 / (1.34885 − 1) = $386.66

Sixty payments of $386.66 total $23,199.60 — so the interest is about $3,200, not the $6,000 simple interest would charge. The saving comes from the shrinking balance: you are not renting the full $20,000 for five years, only whatever part you have not yet repaid.

The first payment shows the mechanics: interest that month is 20,000 × 0.005 = $100.00, so $100.00 of the $386.66 is interest and $286.66 is principal. By the final payment the roles have flipped — the balance is down to roughly $387, so only about $1.90 is interest and the rest retires the principal.

What "front-loaded" interest means

Because the interest charge each month is proportional to the remaining balance, the interest share of each payment is largest at the start and smallest at the end. On a 30-year mortgage the crossover — where principal finally overtakes interest inside the payment — does not happen until well past the halfway point of the term. This is why amortization feels front-loaded: the expensive interest years come first.

A common myth says the lender "collects all the interest up front" and therefore paying early is wasted. The opposite is true: there is no pre-collected interest pool. Interest accrues on the live balance every month, so every dollar of extra principal permanently removes the interest that dollar would have generated for the rest of the term. Front-loading just means early extra payments have the most life left in them to save.

Where the monthly rate comes from

The r in the formula is the annual rate divided by 12, which is the convention for US installment loans: a 6% APR means 0.5% per month. Note that this is a nominal annual rate split into months, not a compounded effective rate — the payment formula charges simple interest within each period. The gap between nominal and effective annual rates is exactly what APR vs APY explains.

Interest-only and other structures

Not every loan amortizes. On an interest-only loan, the payment covers only the interest accrual, so the principal never falls and the full balance comes due at the end of the term (often as a balloon payment or refinance). Credit cards work on a revolving version of the same idea: pay less than the full balance and interest keeps accruing on what remains. These structures are cheaper per month but riskier, because nothing is being paid down unless you choose to.

Try it yourself

The Loan Calculator computes the monthly payment and total interest for any amount, rate, and term using the formula above. The Amortization Schedule Generator shows the month-by-month split between interest and principal so you can watch the front-loading happen. The Loan Payoff Calculator works out how long a balance takes to clear at a given payment, and the Interest-Only Loan Calculator contrasts the interest-only structure with full amortization.