Lenders advertise the rate. What they don't put in big letters is the number that actually matters: how many dollars of interest you'll pay over the life of the loan.
A $20,000 personal loan at 10% for 5 years doesn't cost $20,000. It costs $25,449. That extra $5,449 is the real price of borrowing. Here's how to calculate it before you sign — and how to pay less.
What determines your total interest
Three things control it:
- Loan amount — how much you borrow
- APR — the annual cost of the money
- Term — how many years you take to repay
The monthly payment formula is:
P = L × [r(1+r)^n] / [(1+r)^n − 1]
You don't need to do it by hand. Run your numbers in our loan calculator and it shows total interest instantly.
Real examples: the term changes everything
Same $20,000 loan at 9% APR:
| Term | Monthly payment | Total interest | Total cost | |---|---|---|---| | 3 years | $636 | $2,896 | $22,896 | | 5 years | $415 | $4,910 | $24,910 | | 7 years | $321 | $6,940 | $26,940 |
Stretching from 3 to 7 years drops the payment by $315/month but costs $4,044 more in interest. Low payments feel comfortable; short terms are cheap.
The rate matters more than you think
Same $20,000 over 5 years at different rates:
| APR | Monthly payment | Total interest | |---|---|---| | 6% | $387 | $3,199 | | 9% | $415 | $4,910 | | 12% | $444 | $6,667 | | 18% | $508 | $10,462 | | 24% | $560 | $13,606 |
From 6% to 24%, total interest more than quadruples. That's why comparing offers is worth thousands. Put two offers side by side with our loan comparison calculator before you commit.
5 ways to pay less interest
1. Shorten the term if your budget allows. The single most powerful move. Going from 60 to 36 months in the example above saves over $2,000.
2. Improve your credit before borrowing. The gap between 12% and 8% on $20,000 over 5 years is $2,300 in interest. Three months of credit cleanup can be worth thousands.
3. Make extra principal payments. Every extra dollar goes straight against principal and shrinks all future interest. Even $50/month makes a real dent. Our debt payoff calculator shows exactly how much you'd save.
4. Refinance when rates drop. Borrowed at 12% but now qualify for 8%? Refinancing could save thousands. Check whether it pays off with our refinance calculator.
5. Don't extend the term just to "lower the payment." Car dealers love 84-month loans because the payment looks small. The total interest is brutal. Read our auto loan guide before signing anything.
Simple vs compound interest on loans
Most personal and auto loans charge simple interest on the remaining balance: each month you pay interest only on what you still owe. That's why extra principal payments are so effective — they shrink the base that interest is calculated on.
Credit cards compound interest (it gets added to the balance), which makes them far more expensive. If you carry card debt, see how long it takes to pay off.
Should you pay off debt or invest instead?
If your loan charges 9% and your investments might earn 7%, the math favors killing the debt first — it's a guaranteed 9% return. Our full breakdown: pay off debt or invest.
FAQ
Is total interest the same as APR?
No. APR is the annual rate (a percentage). Total interest is the actual dollars you'll pay over the whole loan. Two loans with the same APR can have very different total interest if their terms differ.
Does paying biweekly instead of monthly save interest?
Yes, a bit. Paying half every two weeks equals 13 monthly payments a year instead of 12. That extra payment cuts principal and total interest. Same trick works on mortgages.
Is a higher rate with a shorter term ever better?
Sometimes. Compare total interest, not just the rate. A 10% loan for 3 years can cost less total interest than an 8% loan for 6 years. The comparison calculator makes it obvious.
Do extra payments have penalties?
Most US personal and auto loans have no prepayment penalty, but check your contract. Mortgages almost never do these days.
What is an amortization schedule?
The month-by-month table showing how much of each payment goes to interest vs principal. Early on it's mostly interest; near the end it's mostly principal. Our loan calculator generates one free.
Estimates for planning only, not financial advice.