Simple Interest Calculator
Compute interest charged on principal only — no compounding. Enter principal, annual rate, and years to get interest and total repayment.
Reviewed by the Calcura editorial team · Last reviewed
- Question
- How do I calculate simple interest?
- Answer
- Simple interest is calculated as I = P × r × t, where P is the principal, r is the annual interest rate (as a decimal), and t is the time in years. For example, $5,000 at 4% for 3 years produces $5,000 × 0.04 × 3 = $600 in interest. Simple interest does not compound — only the original principal earns interest.
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- Formula
I = P × r × t- Caveats
- Most real loans and savings use compound interest. Check the contract before assuming simple.
- If the rate is stated as APR, assume monthly compounding unless the agreement explicitly says simple.
- Time must use the same unit as the rate — an annual rate × time in years, not months.
What This Calculator Does
This calculator computes simple interest — interest charged only on the original principal, with no compounding. Enter a principal, an annual rate, and a time period in years, and it returns the interest amount and the total repayment. Use it for short-term loans, some bonds, and any product whose contract explicitly specifies simple interest.
When To Use This Calculator
Checking a short-term personal or auto loan
A $15,000 car loan at 6% simple interest for 4 years produces $3,600 of interest (15,000 × 0.06 × 4). The calculator gives the same figure directly so you can compare it to the lender's disclosure.
Estimating interest on a Treasury bill or short bond
Some short-duration government and corporate debt is quoted as simple interest over the remaining term. Enter face value as principal, the yield as the rate, and the term in years to estimate the interest payout.
Sanity-checking a promissory note between individuals
A family or friend loan at an agreed simple-interest rate — say $5,000 at 4% for 2 years — produces $400 of interest. The calculator documents the math cleanly for both parties.
Comparing a simple-interest offer to a compound-interest offer
Run the same principal, rate, and horizon here and in the compound-interest calculator. The gap between the two outputs shows how much compounding would add over that period.
How It Works
Simple interest is a flat percentage of the principal, multiplied by the number of years the money is borrowed or lent. No interest is ever earned on previously earned interest. That makes the math linear: doubling the time doubles the interest, and doubling the rate doubles the interest. For sub-year periods, convert days to fractions of a year (days ÷ 365) or months to fractions (months ÷ 12).
Formula
I = P × R × T, where P = principal, R = annual rate (as a decimal), T = time in years. Total repayment = P + I.Real-World Example
Scenario
A borrower takes $8,000 at 7% simple interest for 3 years, with no compounding.
Calculation
I = 8,000 × 0.07 × 3 I = 8,000 × 0.21 I = 1,680
Result
Total interest: $1,680. Total repayment: $9,680. On a straight-line repayment schedule, that's roughly $269/month over 36 months.
Common Mistakes to Avoid
Mistake: Mixing up simple interest and the lender's actual method
Solution: Not every loan advertised as "simple interest" uses this exact formula — auto loans, for example, often accrue interest daily against the outstanding balance. Always read the loan disclosure to confirm the interest method before planning payoff.
Mistake: Entering the rate as a whole number instead of a decimal
Solution: If you manually apply the formula, use 0.05 for 5%, not 5. This calculator accepts a percent in the rate field and converts internally, but hand calculations need the decimal form.
Mistake: Forgetting to pro-rate partial years
Solution: For a 90-day note, T is 90/365, not 0.25. For 18 months, T is 1.5. Entering whole years for sub-year terms overstates interest noticeably.
Mistake: Using simple interest where compounding actually applies
Solution: Credit cards, most mortgages, and most investment accounts compound. Using simple-interest math on those products produces the wrong answer. Use the compound interest calculator for anything that credits interest more than once.
Simple Interest vs. Related Calculators
Compound Interest Calculator
Interest that earns interest. Nearly all long-term investments and savings products work this way — use it whenever the product credits interest more than once.
Loan Payment Calculator
For amortizing loans, the payment and total interest depend on an amortization schedule, not a flat I = P × R × T. Use loan payment for mortgages and anything with monthly principal-and-interest payments.
APR-to-EAR Calculator
Translates a quoted annual percentage rate into an effective annual rate when compounding is involved. Useful when a lender quotes APR but the product actually compounds.
When This Isn't the Right Tool
Amortizing loans (mortgages, most personal loans)
Use instead: Use the loan payment or mortgage calculator. Amortization schedules have principal and interest that shift each month, and simple-interest math will mis-state both total cost and monthly payment.
Credit card balances
Use instead: Credit cards compound daily. Use the credit card payoff calculator for accurate interest and payoff-timeline estimates.
Long-term investments or savings
Use instead: Investment and savings products almost always compound. Use the compound interest or savings growth calculator to see realistic long-term outcomes.
How to Verify This Calculation
To check the result, multiply the three inputs together: principal × (rate ÷ 100) × years. The interest figure should match exactly. Interest plus principal should equal the total. If you're comparing to a lender's quote and the numbers differ, the lender is almost certainly compounding or accruing daily.
Sources
- Consumer Financial Protection Bureau — Understanding loan terms — Plain-language guides to auto loans, personal loans, and loan-cost disclosures.
- U.S. Securities and Exchange Commission — Investor.gov — Background on Treasury bills, bonds, and other simple-interest-style products.
- TreasuryDirect — Treasury bills and notes — Official interest and yield conventions for U.S. government debt.
Reviewed by the Calcura editorial team · Last reviewed