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Loan Interest Calculator
A $20,000 personal loan at 9% over 5 years generates $4,910 in interest – nearly a quarter of the original amount. Knowing that number before you sign lets you compare offers, negotiate better terms, and plan repayments. A loan interest calculator gives you that clarity in seconds.
How to Calculate Loan Interest
The calculator above computes total interest, monthly payment, and a full repayment breakdown based on three inputs: loan amount (principal), annual interest rate, and loan term. It handles both simple and compound interest methods.
Simple Interest Formula
Most installment loans – auto, personal, and standard mortgages – use simple interest:
I = P × r × t
- I = total interest
- P = principal (loan amount)
- r = annual interest rate (as a decimal: 9% = 0.09)
- t = time in years
Example: a $15,000 loan at 7% for 4 years:
I = 15,000 × 0.07 × 4 = $4,200
Total repayment: 15,000 + 4,200 = $19,200
Monthly payment: 19,200 ÷ 48 = $400
Compound Interest Formula
Credit cards and some student loans compound interest – meaning you pay interest on interest:
A = P × (1 + r/n)^(n × t)
- A = total amount owed
- n = number of compounding periods per year (12 for monthly)
Total interest = A − P
Example: a $10,000 balance at 18% APR compounded monthly for 3 years:
A = 10,000 × (1 + 0.18/12)^(12 × 3) = 10,000 × 1.7159 = $17,159
Total interest = 17,159 − 10,000 = $7,159
Compound interest costs $2,959 more than the same loan at simple interest ($4,200 equivalent rate over the same period). The calculator shows both methods side by side so you can compare.
How Much Will Your Loan Really Cost?
The monthly payment is only part of the picture. Two loans with the same payment can have very different total costs. The table below shows how interest accumulates on a $25,000 personal loan at various rates and terms.
| Rate | 3-Year Term | 5-Year Term | 7-Year Term |
|---|---|---|---|
| 6% | $2,372 interest / $761 mo | $4,000 / $483 mo | $5,667 / $367 mo |
| 9% | $3,614 / $789 mo | $6,121 / $519 mo | $8,708 / $402 mo |
| 12% | $4,885 / $833 mo | $8,367 / $556 mo | $12,022 / $441 mo |
| 15% | $6,186 / $877 mo | $10,790 / $597 mo | $15,753 / $485 mo |
Longer terms lower the monthly payment but dramatically increase total interest. At 12%, extending from 3 to 7 years nearly triples the interest paid – from $4,885 to $12,022.
What Affects the Interest You Pay?
Five factors determine your total interest cost:
Principal. The more you borrow, the more interest accrues. A $30,000 loan at 8% for 5 years costs $6,498 in interest; a $20,000 loan under the same terms costs $4,332.
Interest rate. Even a 1% difference matters. On a $25,000 loan over 5 years, 7% generates $4,702 in interest while 8% generates $5,416 – a $714 gap.
Loan term. Shorter terms mean higher payments but less interest overall because the principal is repaid faster.
Interest type. Compound interest costs more than simple interest at the same nominal rate, especially over longer periods.
Payment frequency. Biweekly payments (26 per year) effectively make one extra monthly payment annually, reducing principal faster and cutting total interest.
Simple vs Compound Interest: Which Costs More?
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Formula | P × r × t | P × (1 + r/n)^(n×t) − P |
| Interest base | Original principal only | Principal + accumulated interest |
| Common in | Auto, personal, mortgage loans | Credit cards, some student loans |
| Cost over time | Linear growth | Exponential growth |
| Early payoff savings | Moderate | Significant |
With simple interest, the interest charge each period depends only on the remaining principal – making extra payments directly reduces future interest. With compound interest, the effect of extra payments is even more powerful because it breaks the compounding cycle.
What Is Amortization and Why Does It Matter?
Amortization is the process of splitting each payment between interest and principal. In the early months, most of your payment covers interest. Over time, the balance shifts toward principal.
For a $200,000 mortgage at 7% over 30 years:
- Month 1: $1,164 goes to interest, $185 to principal
- Year 5: $1,040 to interest, $309 to principal
- Year 20: $562 to interest, $787 to principal
- Year 30: $13 to interest, $1,336 to principal
This front-loading of interest means that refinancing or selling early in the loan term returns less equity than many borrowers expect. The calculator above generates a full amortization schedule so you can see exactly where each dollar goes.
How to Reduce Your Loan Interest
- Improve your credit score. Borrowers with scores above 760 typically qualify for rates 2–4 percentage points lower than those below 620. On a $30,000 auto loan over 5 years, that difference can save over $3,500.
- Shorten the term. A 3-year term instead of 5 at the same rate always reduces total interest, though monthly payments increase.
- Make extra payments. Even $50/month extra on a $20,000 loan at 8% over 5 years saves roughly $440 in interest and pays off 5 months early.
- Shop multiple lenders. Rates for the same borrower can vary by 1–3% across lenders. Always compare at least three offers.
- Negotiate the rate, not the monthly payment. Dealers and brokers can stretch the term to lower your payment while increasing total cost. Focus on the APR.
Common Loan Types and Their Typical Rates (2026)
| Loan Type | Typical APR Range | Interest Type | Term |
|---|---|---|---|
| Mortgage (30-year fixed) | 5.5% – 7.5% | Simple (amortized) | 15–30 years |
| Auto loan (new) | 4.5% – 9.0% | Simple | 3–7 years |
| Personal loan (unsecured) | 6.0% – 24.0% | Simple | 1–7 years |
| Credit card | 18.0% – 29.99% | Compound (daily) | Revolving |
| Student loan (federal) | 5.5% – 8.5% | Simple | 10–25 years |
Rates vary by credit profile, lender, and market conditions. Check current averages from Federal Reserve or Bankrate before applying.
This calculator provides estimates for informational purposes only. Actual loan costs depend on lender-specific terms, fees, and your creditworthiness. Consult your lender for official figures.
Frequently Asked Questions
How is loan interest calculated?
Lenders typically use the simple interest formula I = P × r × t for most consumer loans, where P is principal, r is the annual rate, and t is time in years. Mortgages and credit cards often use compound interest, where interest accrues on previously accumulated interest.
What is the difference between APR and interest rate?
The interest rate is the cost of borrowing the principal alone. APR (Annual Percentage Rate) includes the interest rate plus fees and other charges, giving a truer picture of the total cost of the loan. APR is always equal to or higher than the nominal interest rate.
Does paying extra on a loan reduce total interest?
Yes. Extra payments go directly toward reducing the principal balance. Since interest is calculated on the remaining principal, a lower balance means less interest accrues each period, which can save thousands over the life of the loan.
Is simple or compound interest more common for loans?
Simple interest is standard for auto loans, personal loans, and most mortgages. Compound interest applies to credit cards, student loans in some cases, and investment products. Always check your loan agreement to confirm which method is used.
How does loan term length affect total interest paid?
A longer term reduces each monthly payment but increases total interest because the principal stays outstanding longer. For example, a $20,000 loan at 8% over 5 years costs $4,332 in interest, while the same loan over 7 years costs $6,175.
Can I deduct loan interest on my taxes?
Mortgage interest on a primary residence is often deductible if you itemize. Student loan interest may be deductible up to $2,500. Personal loan and credit card interest is generally not deductible. Consult a tax professional for your situation.