How Loan Interest Really Works

Each monthly loan payment splits into two parts: interest on the remaining balance plus principal reduction. Early in the loan, most of your payment is interest; later, most of it reduces principal. This schedule is called amortization, and it is why the last years of a mortgage feel so different from the first.

The standard amortising-loan formula is M = P × r(1+r)^n / ((1+r)^n − 1), where P is the principal, r the monthly interest rate and n the total number of payments. RovoTools applies exactly this formula in the Loan Calculator, so the numbers you see match what a bank amortisation table would show.

A worked example

Borrow $300,000 at 6% annual interest for 30 years. The monthly rate is 0.5% (0.005) and n = 360. Plugging into the formula gives a monthly payment of about $1,799. In month one, roughly $1,500 of that is interest and only about $300 reduces principal. By the final years, almost the entire payment is principal.

That front-loading is not a trick — it is compound interest running in the lender's favour on a declining balance. Every early dollar of principal you reduce stops interest accruing on it for the rest of the term.

Extra payments compound your savings

Because interest accrues on the outstanding balance, extra principal payments compound your savings. Even one extra payment a year (for example, biweekly half-payments) can cut years off a mortgage and tens of thousands in interest.

Try this experiment: enter your loan in the Loan Calculator, note the total interest, then reduce the term by two years and compare. The monthly rise is usually smaller than people expect, while the interest saving is large.

APR, fixed and variable rates

Watch the annual percentage rate (APR), not just the headline rate — APR includes certain fees and reflects a closer picture of the true yearly cost. Origination fees, points and closing costs can make a low rate more expensive than a slightly higher no-fee rate.

Fixed rates give certainty for the full term. Variable (or adjustable) rates start cheaper but can reset according to an index plus a margin. Model the worst plausible reset, not just today's teaser, before choosing variable.

Private modelling

All RovoTools finance calculators run locally in your browser, so you can model scenarios privately without signing up or sharing financial details. Compare home vs personal vs car loan structures, test early payoff, and keep the spreadsheet closed.