When a lender tells you a loan costs 6% per year, most people nod and sign. But very few people understand what that 6% actually does to their money over time — and the answer is surprising enough that it's worth understanding before you borrow.
The Basic Idea: Amortization
Most personal loans and mortgages are amortizing loans. This means every monthly payment covers two things: interest on the remaining balance, and a chunk of the principal. Over time, as the balance falls, the interest portion shrinks and the principal portion grows — until the final payment, which is almost entirely principal.
This is why, on a 30-year mortgage, you can be paying for 10 years and still owe most of the original amount. In the early years, most of each payment is interest.
The Formula
The standard monthly payment formula looks like this:
Where: M = monthly payment, P = principal (amount borrowed), r = monthly interest rate (annual rate ÷ 12), n = total number of payments (years × 12).
You don't need to memorize this — that's what our Loan Calculator is for. But understanding that the exponent n grows exponentially explains why longer loan terms feel affordable monthly but cost dramatically more in total.
A Real Example
Say you borrow $20,000 at 7% annual interest:
- 3-year term: ~$618/month, total paid ~$22,235 (interest: ~$2,235)
- 5-year term: ~$396/month, total paid ~$23,760 (interest: ~$3,760)
- 7-year term: ~$302/month, total paid ~$25,368 (interest: ~$5,368)
Stretching from 3 to 7 years cuts your monthly payment in half — but you pay more than double the interest over the life of the loan. That's the trade-off every borrower faces.
APR vs. Interest Rate
The interest rate is the cost of borrowing the principal. The APR (Annual Percentage Rate) includes the interest rate plus fees — origination fees, broker fees, closing costs — expressed as a yearly rate. APR is always higher than the interest rate and gives you a truer picture of total borrowing cost. Always compare APRs, not just rates, when shopping for loans.
Fixed vs. Variable Rates
A fixed rate stays the same for the life of the loan. Your payment is predictable. A variable rate starts lower but adjusts periodically based on a benchmark rate (like the federal funds rate). Variable rates suit borrowers who plan to repay quickly; fixed rates suit anyone who needs certainty over a long term.
The Most Powerful Move: Extra Payments
Because early payments are mostly interest, making even one extra principal payment per year can dramatically reduce both the term and total cost of a mortgage. On a 30-year $300,000 mortgage at 6.5%, one extra monthly payment per year saves roughly 5 years and tens of thousands in interest.
Use our Loan Calculator to model different scenarios before you commit to any loan. A few minutes of planning can save you years of payments.