A loan offer usually leads with one number: the monthly payment. It is the figure that decides whether the loan fits your budget, so it is the one lenders put in large type. But the payment on its own tells you surprisingly little about what the loan actually costs, and two loans with similar payments can differ by thousands in total interest. This guide explains exactly how a loan payment is worked out, what each part of it pays for, and how to use that knowledge to borrow more cheaply.
What makes up a loan payment
Almost every personal loan, car loan and mortgage in the world is an amortising loan. You borrow a fixed amount, the principal, and repay it in equal instalments over a fixed term. Each instalment does two jobs at once:
- it pays the interest that has built up on the balance since the last payment, and
- whatever is left over reduces the principal — the amount you still owe.
The payment stays the same every month, but the split between those two parts changes. At the start the balance is large, so most of the payment goes on interest. As the balance falls, the interest portion shrinks and more of each payment goes towards the debt itself. That gradual shift is what “amortisation” means.
The loan payment formula
The monthly payment that exactly repays a loan over its term comes from one standard formula:
- M is the monthly payment.
- P is the principal — the amount borrowed.
- r is the monthly interest rate: the annual rate divided by 12, as a decimal.
- n is the number of monthly payments: years × 12.
The formula looks intimidating, but it only says one thing: find the payment whose stream of instalments, discounted at the loan’s interest rate, is worth exactly what you borrowed today. You rarely need to work it by hand — the loan calculator does it instantly — but seeing each step makes clear why the rate and the term matter so much.
Step-by-step example
Take a 20,000 loan at 7% a year, repaid monthly over 5 years.
- Monthly rate: r = 7% ÷ 12 = 0.07 ÷ 12 = 0.0058333.
- Number of payments: n = 5 × 12 = 60.
- Growth factor: (1 + 0.0058333)60 ≈ 1.4176, so (1 + r)−60 ≈ 0.7054.
- Payment: M = 20,000 × 0.0058333 ÷ (1 − 0.7054) ≈ 116.67 ÷ 0.2946 ≈ 396.02.
The monthly payment is about 396.02, and the total interest over the five years comes to about 3,761.
Over the life of the loan you pay back roughly 23,761 for the 20,000 you borrowed. The extra 3,761 — almost 19% of the amount borrowed — is the real price of the loan, and it is a number the monthly payment alone hides.
Why the term matters more than it looks
Stretching a loan over more years lowers the monthly payment, which is why longer terms are so often offered. But because interest is charged on the outstanding balance every month, keeping the balance high for longer means paying interest for longer. Here is the same 20,000 at 7% over three different terms:
| Term | Monthly payment | Total interest |
|---|---|---|
| 3 years | 617.54 | 2,231.50 |
| 5 years | 396.02 | 3,761.48 |
| 7 years | 301.85 | 5,355.80 |
Going from five years to seven saves about 94 a month but costs nearly 1,600 more in interest. Going the other way, from five years to three, costs 221 more a month and saves over 1,500. Neither choice is automatically right — a lower payment that you can comfortably keep up is better than a higher one that strains your budget — but it should be a deliberate choice, made with the total cost in view.
How the balance falls: the amortisation schedule
An amortisation schedule lists every payment with its interest and principal parts and the balance left afterwards. On our 20,000 loan, the first payment is 116.67 interest and 279.35 principal. By the final year, the balance is small, so the interest part of each payment is only a few units and almost all of it goes to the principal.
Two practical consequences follow. First, in the early years you build up very little equity: after a year of payments on a long mortgage, the balance may have barely moved. Second, anything you pay early has the biggest effect, because it removes principal that would otherwise have been charging interest for the longest time. The amortisation calculator shows the full schedule year by year.
Overpaying: the cheapest way to cut the cost
Most loans let you pay more than the required amount, and every extra payment goes straight to the principal. Add 100 a month to our example loan and the result changes considerably:
Before overpaying, check two things. Some lenders charge an early repayment fee, particularly on fixed-rate mortgages, so read the terms. And if you have debts at a higher rate — a credit card, for instance — overpaying those first saves more.
Interest rate, APR and the real cost
The rate in the formula is the loan’s nominal annual rate. The figure lenders are usually required to show, the APR (annual percentage rate), also folds in compulsory fees such as arrangement charges, so it is the better number for comparing offers. A loan at 6.5% with a large arrangement fee can cost more than one at 6.9% with none. When two loans have the same term, the one with the lower APR is cheaper overall; when the terms differ, compare the total amount repayable too.
Small differences in rate add up. On 20,000 over five years, every extra percentage point of interest adds roughly 560 to 570 to the total cost. On a 25-year mortgage of 250,000, a single point can add well over 40,000. That is why shopping around, improving a credit score before applying, or putting down a bigger deposit can be worth far more than the effort they take.
Common loan types, same formula
The same arithmetic sits behind most borrowing you will come across:
- Personal loans — usually unsecured, 1 to 7 years. See the personal loan calculator.
- Car loans — secured on the vehicle, often with a deposit or trade-in. The auto loan calculator handles those extras.
- Mortgages — the longest and largest loans most people take on, usually with property tax and insurance added to the monthly bill. The mortgage calculator includes them.
Credit cards are the main exception: they have no fixed term, so the payment is a minimum percentage of the balance instead. Paying only the minimum can stretch a small balance over many years, which is why card debt is usually the first to clear.
Common mistakes
- Using the annual rate as the monthly rate. 7% a year is about 0.583% a month. Forgetting to divide by 12 makes the payment look enormous.
- Comparing payments instead of total cost. A lower monthly payment on a longer term often costs more overall.
- Ignoring fees. Compare APRs, not headline rates, and check for early repayment charges.
- Borrowing to the limit of the payment you can afford. Leave room for rate rises on variable loans and for changes in your income.
- Forgetting that overpayments are most valuable early. The same extra payment saves more interest in year one than in year five.
Frequently asked questions
How is the monthly payment on a loan calculated?
Divide the annual rate by 12 to get the monthly rate r, multiply the years by 12 to get the number of payments n, and use M = P × r ÷ (1 − (1 + r)−n). On 20,000 at 7% over 5 years, that gives about 396.02 a month.
Why is so much of my early payment interest?
Interest is charged on the balance you still owe, and at the start the balance is at its highest. As you repay the principal, the interest part of each payment falls and the principal part grows, even though the payment itself stays the same.
Is a shorter loan always better?
A shorter term always costs less in total interest, but it needs a higher monthly payment. The best term is the shortest one whose payment you can keep up comfortably, with room to spare for unexpected costs.
Final thoughts
The monthly payment is where a loan decision starts, not where it ends. Once you know how the payment is built — interest on the balance, with the rest reducing the debt — the other numbers make sense: why longer terms cost more, why early overpayments are so effective, and why a small difference in rate matters. Run your own figures through the loan calculator, look at the total interest as well as the payment, and you will be comparing loans on the number that really counts.