Loan Interest Calculator – Calculate Loan Interest & Total Cost

💰 Loan Interest Calculator

Type in your loan details and this Loan Interest Calculator shows the whole picture — not just the monthly payment. You’ll see total interest, total repayment, the interest-to-principal split, and a full year-by-year breakdown of what your loan actually costs you.

What you’re borrowing before any interest. Any currency works — just use the same one throughout.

The yearly rate you’ve been quoted. This one drives the total interest figure more than anything else.

How long the loan runs. Longer terms lower the monthly figure but push total interest up.

Amortized is how mortgages, auto loans, and most installment loans work. Simple interest charges interest only on the original amount — common on some short-term and informal loans.

Payment Amount
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Total Interest
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Total Principal
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Total Repayment
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Interest as % of Total Cost
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Year 1 Interest
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Total Interest You’ll Pay
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Set your inputs to calculate
Principal: — Interest: —

That bar shows how much of every unit you repay is the loan itself versus the cost of borrowing it. Even small changes to the rate or the term move it noticeably.

Detail Value

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🌍 Put every amount in the same currency. The calculator doesn’t convert between currencies — use USD the whole way through, or EUR, or GBP, or whatever you work in.

How to Use This Loan Interest Calculator

Loan Amount. Enter what you’re borrowing before interest is added. This is the principal, and it’s the starting point for every other number the calculator produces.

Interest Rate. Your annual rate as a percentage. Rate is the biggest driver of total interest, so use the number you’ve actually been quoted, not a rough guess.

Loan Term. Pick months or years. Longer terms bring the monthly payment down, but they push total interest up — usually by more than people expect.

Payment Frequency. Monthly is the default for most loans, but you can also run biweekly, weekly, or yearly. Under the calculation model used here, more frequent payments generally reduce total interest because the balance is reduced more often. Real-world conventions can vary, so check how your lender accrues interest.

Interest Type. Choose “Amortized Loan” for mortgages, auto loans, and most installment credit. Choose “Simple Interest” for loans where interest is charged only on the original amount.

The Formula Behind Every Loan Interest Calculation

For an amortized loan, the payment comes from the standard reducing-balance formula:

Payment = P × r × (1 + r)n ÷ ((1 + r)n − 1)

P is your loan amount, r is the rate per period (annual rate ÷ payment frequency ÷ 100), and n is the total number of payments across the term.

Every payment is split into two parts: interest on the remaining balance, and repayment of principal. Early on, the balance is big, so most of the payment goes to interest. As the balance falls, that flips — interest shrinks and principal grows. The total payment stays the same; only the split changes.

Simple interest works differently. Under the model used by this calculator, total interest is P × annual rate × years, added to the principal and then divided evenly across the payments. Interest is calculated on the original principal for the full loan term, so the total interest does not decrease as the balance is paid down.

What Each Result Actually Tells You

The calculator gives you several numbers, and each one answers a different question:

  • Payment Amount. What you pay every period — monthly, biweekly, weekly, or yearly depending on the frequency you selected. This is the one that has to fit your cash flow.
  • Total Interest. Every interest charge added up across the life of the loan. This is the real price of borrowing, and the number people miss when they only look at the payment.
  • Total Principal. The amount you borrowed — before any interest is added.
  • Total Repayment. Principal plus interest. What the loan itself costs you across the whole term.
  • Interest as % of Total Cost. A quick read on how much of your total outgo is interest rather than principal. Short, low-rate loans might show 10%. Long, higher-rate ones can push past 40%.
  • Year 1 Interest. How much of your first year’s payments goes to interest. On amortized loans, this is often the highest single year because the balance is at its peak.
  • Principal vs. Interest split. A bar showing how the two stack up. Seeing them side by side makes the cost difference obvious at a glance.

Year-by-Year Interest Breakdown

One of the most useful outputs is the year-by-year view. It shows how much interest you pay each year, how much principal you retire, and where the balance sits at year-end.

On an amortized loan, the pattern is very consistent. Interest peaks in year one and steadily falls. Principal grows slowly at first, then accelerates. Around the midpoint of the term, the split usually gets closer to fifty-fifty, and by the final years almost everything goes to principal.

This is exactly why paying extra early works so well. An extra payment in year one wipes out interest that would otherwise have been charged on that amount for the whole remaining term. The same extra payment near the end of the loan saves far less, because there’s less future interest left to eliminate.

Worked Examples

Example 1: A 25,000 Auto Loan Over Five Years

  • Loan amount: 25,000
  • Rate: 8% per year
  • Term: 5 years (60 months)
  • Payment frequency: Monthly
  • Interest type: Amortized

Monthly payment lands around 506.91. Total interest comes to about 5,414.59. Total repayment is roughly 30,414.59. Interest makes up around 17.8% of the total cost — a reasonable ratio for a five-year loan at 8%.

Example 2: A 10,000 Personal Loan Over Three Years

  • Loan amount: 10,000
  • Rate: 12% per year
  • Term: 3 years (36 months)
  • Payment frequency: Monthly
  • Interest type: Amortized

Monthly payment is about 332.14. Total interest is roughly 1,957.15. Total repayment comes to about 11,957.15. Interest is around 16.4% of total cost. Notice how a shorter term at a higher rate can land near the same ratio as Example 1 — the two effects partly cancel out.

Example 3: A 200,000 Mortgage Over 30 Years

  • Loan amount: 200,000
  • Rate: 6% per year
  • Term: 30 years (360 months)
  • Payment frequency: Monthly
  • Interest type: Amortized

Monthly payment is about 1,199.10. Total interest is roughly 231,676.38. Total repayment comes to about 431,676.38. Interest works out to roughly 53.7% of total cost — you end up paying more in interest than the amount you borrowed.

How to Lower the Interest You Pay

If the total interest on your quote looks high, these levers work — roughly in order of impact.

  1. Shorten the term. Shortening the term can reduce total interest substantially, although the payment amount will usually increase.
  2. Shop for a lower rate. Even half a point off the rate can save thousands on a long-term loan. Compare banks, credit unions, and online lenders before committing.
  3. Increase payment frequency. Under the calculation model used here, biweekly payments mean 26 half-payments a year, which works out to roughly one extra monthly payment annually. Less interest builds up over the life of the loan.
  4. Pay extra toward principal. Any extra goes straight to the balance, cutting interest on everything that’s left. Confirm your lender applies extras to principal, not to future instalments.
  5. Put more down where applicable. The less you borrow, the less interest builds on the starting balance.
  6. Improve your credit before applying. A stronger credit profile is one of the most reliable ways to unlock a lower rate.
  7. Skip the add-ons. Extended warranties, payment protection insurance, and bundled fees can be financed into the loan and attract interest themselves.

Common Mistakes When Calculating Loan Interest

A few things people get wrong when estimating the cost of borrowing.

Only looking at the monthly payment. A low monthly figure can hide a very expensive loan. That’s exactly why the total cost of loan calculator matters — the monthly number alone is misleading.

Comparing rates without comparing terms. A 5% rate over 30 years can cost far more in total interest than a 6% rate over 15 years. Compare loans with the same term when comparing rates.

Mixing simple and amortized loans. The two work on completely different models. Under the simple-interest model used here, interest is charged on the full principal for the whole term, so total interest doesn’t drop as the balance falls. Comparing that to an amortized loan on rate alone gives the wrong answer.

Forgetting about compounding frequency. Annual, monthly, and daily compounding all produce different effective rates. When comparing two loans, make sure you’re using the same compounding basis.

Ignoring fees. Origination fees, closing costs, and prepayment penalties all add to the real cost. Compare the total of interest plus fees, not just the headline rate.

Assuming interest rate is the same as APR. The interest rate is the cost of the principal. APR is broader and can include certain fees, so it’s often the better number for comparison shopping. Two loans with the same interest rate can have different APRs.

Additional Financial Resources

For a plain-English explanation of how amortization works on installment loans, Investopedia’s amortization page is a solid reference.

For consumer-protection guidance on installment credit and what lenders must disclose, the US Consumer Financial Protection Bureau’s loan resources are worth reading.

For a clear explanation of how APR differs from a simple interest rate, the CFPB’s explainer on rate vs APR covers it in detail.

For current consumer credit conditions in the United States, the Federal Reserve’s G.19 Consumer Credit release updates monthly.

Frequently Asked Questions About Loan Interest

It’s a tool that shows how much interest you’ll pay on a loan across the full term, plus your payment amount, total repayment, and a year-by-year breakdown of how each payment splits between principal and interest. This Loan Interest Calculator also supports different payment frequencies and both amortized and simple interest loans.
For an amortized loan, use Payment = P × r × (1 + r)^n ÷ ((1 + r)^n − 1), then subtract the principal from the total of all payments to get total interest. For a simple interest loan, total interest is P × annual rate × years. The Loan Interest Calculator runs both for you.
Under the simple-interest model used by this calculator, interest is charged on the original principal for the whole loan term, so total interest doesn’t drop as the balance is paid down. Amortized loans charge interest on the outstanding balance each period — the interest gets smaller as the balance drops. The two methods give very different totals even at the same headline rate, which is why they shouldn’t be compared on rate alone.
Under the calculation model used here, more frequent payments — biweekly or weekly instead of monthly — reduce the balance more often, so less interest builds up over the life of the loan. Real-world lenders can have different conventions for interest accrual and payment timing, so check how your lender actually calculates it.
On an amortized loan, interest is charged on the outstanding balance. Early on, the balance is at its highest, so interest takes up most of each payment. As the balance falls, the interest portion shrinks and the principal portion grows. That’s why paying extra early has such a big effect on total interest.
Almost always, yes. A longer term lowers the periodic payment but keeps the balance outstanding longer, so more interest builds up. The loan interest payment calculator makes this obvious — compare a 15-year term to a 30-year term at the same rate and the difference in total interest is usually dramatic.
Yes. The calculator shows total repayment (principal plus interest) and interest as a percentage of total cost, so you can see the whole cost at a glance. If you want to factor in lender fees, note them separately — this tool focuses on the interest side of the loan.
Rates depend on the loan type, your credit profile, the term, and the market. Secured loans like mortgages usually price lower than unsecured loans like personal loans, because the lender has collateral. Borrowers with strong credit and steady income typically see the lower end of the range for their loan type.
No. The interest rate is the cost of the principal. APR is broader and can include certain fees on top of the interest rate. Two loans with the same interest rate can have different APRs if their fees differ. The Loan Interest Calculator works from the interest rate you enter — it doesn’t compute APR.

⚠️ Disclaimer: The results from this Loan Interest Calculator are hypothetical calculations based on the inputs you provide and are for educational and informational purposes only. They should not be treated as financial, lending, or tax advice. The calculator uses your inputs as entered and does not verify them against any lender’s actual offer, fee schedule, or loan agreement. Real loan terms vary by lender, region, credit profile, loan amount, and promotional period, and may include charges not captured here — such as origination fees, closing costs, prepayment penalties, late payment charges, or mandatory add-on products. The year-by-year breakdown is an estimate; actual lender schedules may differ because of daily interest, payment dates, rounding rules, fees, or rate changes. Total interest assumes every payment is made on time for the full term and that the interest rate remains unchanged; variable-rate loans will produce different results. Please consult a qualified financial advisor and read your loan agreement carefully before committing to any borrowing.