Understanding how loan interest is calculated can save you thousands of dollars over the life of a loan. Whether you're comparing offers from different lenders, deciding between a 3-year and 5-year term, or evaluating whether to pay off debt early — the math behind loan interest is simpler than most people think.
This guide walks through exactly how lenders calculate your monthly payment and the total interest you'll pay, with concrete examples and a free calculator to verify everything instantly.
Enter your loan amount, rate and term for instant payment and amortization schedule.
Calculate my loan →Before getting into the formula, it helps to understand that there are two fundamentally different ways interest can be calculated on a loan:
Simple interest is calculated only on the original principal, not on accumulated interest. It's used for some short-term loans and car loans. Formula: Interest = Principal × Rate × Time. If you borrow $5,000 at 8% for 3 years: Interest = $5,000 × 0.08 × 3 = $1,200.
This is what most personal loans, mortgages, and car loans use. Interest is recalculated on the remaining balance every month. As you pay down the principal, the interest portion of each payment decreases — but your total monthly payment stays the same. This is called an amortizing loan.
This formula gives you the fixed monthly payment for an amortizing loan. The same payment is made every month, but the split between principal and interest changes over time.
Step 1: Convert annual rate to monthly: r = 9% ÷ 12 = 0.75% = 0.0075
Step 2: Calculate total payments: n = 4 × 12 = 48
Step 3: Apply the formula:
M = 15,000 × [0.0075 × (1.0075)⁴⁸] / [(1.0075)⁴⁸ − 1]
M = 15,000 × [0.0075 × 1.4314] / [1.4314 − 1]
M = 15,000 × 0.010736 / 0.4314
M = $373.28/month
Total repaid: $373.28 × 48 = $17,917
Total interest: $17,917 − $15,000 = $2,917
With an amortizing loan, you pay the same amount every month — but the breakdown between principal and interest changes dramatically. In the early months, most of your payment goes to interest. By the end, most goes to principal.
Here are the first 6 months of the $15,000 loan above:
| Month | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | $373.28 | $112.50 | $260.78 | $14,739.22 |
| 2 | $373.28 | $110.54 | $262.74 | $14,476.48 |
| 3 | $373.28 | $108.57 | $264.71 | $14,211.77 |
| 4 | $373.28 | $106.59 | $266.69 | $13,945.08 |
| 5 | $373.28 | $104.59 | $268.69 | $13,676.39 |
| 6 | $373.28 | $102.57 | $270.71 | $13,405.68 |
This table is called an amortization schedule. LoanCalcHub generates a full amortization schedule for any loan — all 48 months in this case — and lets you export it to CSV.
One of the most important decisions when taking a loan is the repayment term. A longer term means lower monthly payments but dramatically more interest paid over time.
| Loan | Term | Monthly payment | Total interest |
|---|---|---|---|
| $15,000 at 9% | 2 years | $685.36 | $1,449 |
| $15,000 at 9% | 3 years | $477.00 | $2,172 |
| $15,000 at 9% | 4 years | $373.28 | $2,917 |
| $15,000 at 9% | 5 years | $311.38 | $3,683 |
Going from a 2-year to a 5-year term saves $374/month — but costs an extra $2,234 in interest. Whether that tradeoff makes sense depends on your cash flow situation.
| Rate | Monthly payment | Total interest | vs 8% rate |
|---|---|---|---|
| 6% | $290.00 | $2,399 | −$1,043 |
| 8% | $304.15 | $3,249 | baseline |
| 10% | $318.71 | $4,123 | +$874 |
| 14% | $349.13 | $5,948 | +$2,699 |
| 20% | $396.62 | $8,797 | +$5,548 |
$15,000 loan, 5-year term
A 2% difference in interest rate on a $15,000 loan over 5 years costs nearly $1,000 extra. On a $200,000 mortgage, that same 2% difference costs over $80,000 over 30 years. This is why comparing lenders matters enormously.
When comparing loans, always compare APR (Annual Percentage Rate), not just the interest rate. The interest rate only reflects the cost of borrowing the principal. APR includes:
Two loans with identical interest rates but different fees will have different APRs. The one with the lower APR is cheaper overall. In Europe, this is called RPMN (Slovak/Czech), RRSO (Polish), or THM (Hungarian).
Making extra payments directly toward the principal is one of the most effective ways to reduce total interest. On the $15,000 loan at 9% for 4 years:
| Extra payment | Paid off in | Interest saved |
|---|---|---|
| $0 extra | 48 months | — |
| $50/month extra | 41 months | $431 |
| $100/month extra | 36 months | $743 |
| $200/month extra | 28 months | $1,129 |
An extra $100/month saves $743 in interest and pays off the loan a full year earlier. Use the early payoff calculator on LoanCalcHub to model your exact scenario.
Enter your loan details and any extra monthly payment to see exactly how much you save.
Calculate early payoff savings →