How to Calculate Interest on a Loan
Understanding how loan interest is calculated is the difference between borrowing confidently and being surprised every month. This guide walks you through simple interest, amortized interest (used by almost every real loan), and how to figure out the total interest you'll pay — with worked examples for each.
The two types of loan interest
All consumer loans use one of two methods to calculate interest:
- Simple interest — calculated only on the original principal. Common for short-term personal loans, some auto loans, and informal loans between people.
- Amortized interest — calculated on the remaining balance each period. This is what mortgages, most auto loans, and most personal loans use.
The same loan amount and rate can produce very different total interest depending on which method is used and how long the term is. Let's go through both.
Simple interest: the easy formula
The simple interest formula is one of the most useful equations in personal finance:
Where:
- Principal (P) — the amount borrowed.
- Rate (R) — the annual interest rate as a decimal (6% = 0.06).
- Time (T) — the loan length in years.
Worked example: $5,000 loan at 8% for 2 years
Interest = $5,000 × 0.08 × 2 = $800.
Total amount you'll repay = $5,000 + $800 = $5,800. Spread over 24 months, that's about $241.67/month.
Amortized interest: how real loans work
Most loans you'll actually take out (mortgages, car loans, personal loans) use amortization. Each month, interest is calculated on the remaining balance, and the rest of your payment reduces the principal. As the balance shrinks, the interest portion shrinks and the principal portion grows.
Step 1: Calculate monthly interest
On a $20,000 loan at 7%, the first month's interest is: $20,000 × (0.07 ÷ 12) = $116.67.
Step 2: Calculate the monthly payment
The standard amortization formula gives a constant monthly payment that pays the loan off exactly on schedule:
Where r = monthly rate (annual ÷ 12), n = total number of payments. Don't worry — you don't need to do this by hand. Our loan calculator handles it instantly.
Step 3: Split each payment into principal and interest
For our $20,000 / 7% / 5-year loan, the calculated monthly payment is about $396.02. Here's how the first three months break down:
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | $396.02 | $116.67 | $279.35 | $19,720.65 |
| 2 | $396.02 | $115.04 | $280.98 | $19,439.67 |
| 3 | $396.02 | $113.40 | $282.62 | $19,157.05 |
Notice how the interest portion drops slightly each month and the principal portion grows. By the final months, almost the entire payment is principal.
Calculating total interest paid
For any amortized loan, total interest is straightforward:
For our $20,000 / 7% / 5-year loan: ($396.02 × 60) − $20,000 = $23,761 − $20,000 = $3,761 in total interest.
How term length changes total interest
| Term | Monthly payment | Total interest |
|---|---|---|
| 3 years | ~$617.54 | ~$2,231 |
| 5 years | ~$396.02 | ~$3,761 |
| 7 years | ~$301.85 | ~$5,355 |
Same $20,000 loan, same 7% rate — but stretching from 3 to 7 years more than doubles the total interest. That's the cost of a smaller monthly payment.
Interest rate vs APR
When comparing loan offers, look at APR rather than the headline interest rate. APR includes most fees (origination, points, some closing costs) converted into an annualized rate, so two loans with the same interest rate but different fees will show up as different APRs. The higher APR is the more expensive loan, even if the interest rates look identical.
Three ways to pay less interest
- Borrow less. Sounds obvious, but a bigger down payment or a smaller car directly cuts every interest payment.
- Choose a shorter term. Higher monthly payment, but dramatically less total interest.
- Pay extra principal early. Every extra dollar paid in the first half of the loan saves multiple dollars of future interest.
For the math behind extra payments, see how to pay off a loan faster.
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Open Loan CalculatorFrequently Asked Questions
How do I calculate interest on a loan?
For simple interest: Interest = Principal × Rate × Time. For example, $10,000 borrowed at 6% for 3 years = $10,000 × 0.06 × 3 = $1,800 in interest. Most real loans use amortized interest, where you pay interest on the remaining balance each month — that requires a slightly more involved formula or a calculator.
What's the difference between simple and amortized interest?
Simple interest is calculated only on the original principal — the interest amount stays the same each period. Amortized interest is calculated on the remaining balance, which shrinks as you make payments. Almost every car loan, mortgage, and personal loan uses amortized interest, which is why early payments are mostly interest and later payments are mostly principal.
How is monthly interest calculated?
On an amortized loan, divide the annual rate by 12 to get the monthly rate, then multiply by the current balance. On a $20,000 loan at 7%, the first month's interest is $20,000 × (0.07/12) = about $116.67. Whatever's left of your payment goes to principal.
What is APR vs interest rate?
The interest rate is what the lender charges on the principal. APR (annual percentage rate) includes the interest rate plus most fees (origination, points, some closing costs), so it reflects the true cost of the loan. APR is usually slightly higher than the rate and is the better number for comparing offers.
How can I pay less interest on a loan?
Three levers: borrow less, choose a shorter term, or get a lower rate. Even small extra payments early in the loan have outsized impact because they reduce the balance interest is calculated on. Refinancing to a lower rate can also work if the savings beat the closing costs.
Why is most of my early payment going to interest?
On amortized loans, interest is charged on the remaining balance — and the balance is biggest at the start. So the first payments are mostly interest with a little principal. As the balance shrinks, the interest portion shrinks too and more of your payment goes to principal. By the final years, almost the entire payment is principal.
How do I calculate total interest paid over the life of a loan?
Total interest = (monthly payment × number of payments) − loan amount. On a $20,000 loan at 7% over 5 years, the monthly payment is about $396 and you make 60 payments — total paid is $23,760, so total interest is $3,760. Our loan calculator does this automatically.