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Simple Interest Calculator

Calculate simple interest using I = P × r × t — useful for short-term and flat-rate loans.

Principal Amount

Annual Interest Rate

Time Period

years

About the Simple Interest Calculator

The simple interest calculator computes interest that accrues linearly on a principal amount over time. Unlike compound interest, where prior interest earns interest, simple interest is calculated only on the original principal for the entire duration. The result is a straight-line growth curve: each year adds the same dollar amount of interest, and the total interest is just the annual figure multiplied by the number of years.

Simple interest is the standard structure for many short-term financial products. Most auto loans in the United States use simple interest, where interest accrues daily on the outstanding principal. U.S. Treasury bills (T-bills) discount instruments and short-term commercial paper use simple interest conventions. Many car loans, motorcycle loans, and short-term personal loans also use simple interest, often with daily accrual — meaning paying early in the month saves interest versus paying late. The Federal Deposit Insurance Corporation (FDIC) notes that the Truth in Lending Act disclosure distinguishes simple-interest loans from precomputed-interest loans, where the full term's interest is computed up front and built into the loan balance.

The simplest expression of simple interest is I = P x r x t: the interest equals the principal times the rate times the time. Because time is linear, the result is easy to reason about and to verify by hand. For a $5,000 loan at 6% over 3 years, the interest is $5,000 x 0.06 x 3 = $900, and the total repayment is $5,900. No reinvestment of prior interest occurs, so there is no compounding effect.

This calculator supports partial years (e.g. 2.5 years) so you can model short-term instruments accurately. For periods shorter than one year, you can also enter fractional values such as 0.25 for three months or 0.0833 for one month. The result is in nominal dollars and does not adjust for inflation or taxes.

How It Works

The simple interest formula is:

I = P x r x t

where:
  P = principal
  r = annual interest rate (as decimal)
  t = time in years

Total amount to repay is the principal plus the interest:

A = P + I = P x (1 + r x t)

Unlike the compound interest formula A = P x (1 + r)^t, which applies the rate to a growing balance, simple interest applies the rate only to the original principal. The growth curve is linear rather than exponential: at the same rate and time, simple interest always produces less total interest than compound interest, with the gap widening as time grows.

For daily accrual on a simple-interest auto loan, the calculation uses a daily rate:

Daily interest = (principal x annual_rate) / days_in_year

For a $20,000 loan at 6% APR with 365-day year:
Daily interest = (20000 x 0.06) / 365 = $3.29 per day

Most U.S. auto loans use a 365-day year for daily accrual. Some lenders use 360 days, which slightly increases the effective rate. The difference is small but real: 6.0% on a 360-day basis equals an effective annual rate of 6.0% x (365/360) = 6.0833%. Always check the day-count convention in the loan agreement.

This calculator uses annual time periods by default. For monthly periods, divide the rate by 12 and the time by 12, or simply enter the time in years (e.g. 6 months = 0.5 years). The tool validates that all three inputs are non-negative and that time is greater than zero.

Worked Examples

Using the default inputs of $5,000 at 6% for 3 years: I = 5000 x 0.06 x 3 = $900. Total to repay = $5,900. Interest per year = 5000 x 0.06 = $300, which is consistent: $300 x 3 years = $900. Because simple interest is linear, the per-year figure is exactly the same in year 1, year 2, and year 3.

For a $20,000 auto loan at 5.5% APR for 5 years using simple interest: I = 20000 x 0.055 x 5 = $5,500. Total to repay = $25,500. Compare to a compound-interest loan at the same rate and term: 20000 x (1.055)^5 - 20000 = $6,164 — compound interest would cost $664 more over the same period.

For a short-term Treasury bill equivalent, consider a $10,000 face-value instrument at a 4.5% discount rate for 6 months (0.5 years): I = 10000 x 0.045 x 0.5 = $225. The investor pays $9,775 today and receives $10,000 at maturity, earning $225 in interest. Actual T-bill pricing uses a bank discount convention that differs slightly, but the magnitude is similar.

For an auto loan with daily accrual, consider a $25,000 balance at 7% APR with 365-day year. Daily interest = (25000 x 0.07) / 365 = $4.79 per day. If you make a $400 payment 15 days into the billing cycle, 15 days x $4.79 = $71.92 of your payment covers interest and $328.08 reduces principal. Paying early in the cycle means fewer days of accrual before the payment, so more of each payment reduces principal — a real saving on simple-interest loans that does not exist on precomputed-interest loans.

When to Use This Tool

Use the simple interest calculator when you need to:

Estimate interest on a short-term personal loan or auto loan that uses simple interest.
Compute the discount on a short-term note or Treasury bill.
Calculate late-payment interest on an unpaid invoice at a contractual rate.
Determine the interest portion of a single-period loan or bond coupon.
Compare simple vs. compound interest on the same principal to see the compounding gap.
Verify the daily accrual figure shown on your auto loan statement.
Teach the foundational difference between linear and exponential growth in finance.

Limitations & Disclaimer

This calculator uses standard simple-interest math and does not model daily accrual, partial payments, precomputed-interest structures, day-count conventions other than 365-day annual time, or variable rates. For actual lending decisions, review the loan agreement's interest method (simple vs. precomputed) and day-count convention, and consult a licensed financial advisor. See our disclaimer for full details.

Frequently Asked Questions

What is the difference between simple interest and compound interest?

Simple interest accrues only on the original principal. Compound interest accrues on the principal plus any previously accrued interest. At 6% for 5 years on $10,000, simple interest yields $3,000 while compound interest (compounded annually) yields $3,382. The gap widens with time: over 20 years, the same inputs give $12,000 simple versus $22,090 compound.

Do most auto loans use simple or compound interest?

Most U.S. auto loans use simple interest with daily accrual, meaning interest is charged daily on the outstanding principal. Paying early in the billing cycle reduces the days of interest charged before the payment. Precomputed-interest loans, where the full term's interest is computed up front, are legal in some states but uncommon. Check the loan agreement or ask the lender directly.

Does simple interest ever exceed compound interest?

No, not for the same principal, rate, and time period with positive rates. Compound interest always equals or exceeds simple interest, with equality only when the rate is zero or the time is one period. The gap grows with both rate and time. This is why Albert Einstein's (possibly apocryphal) comment about compound interest being 'the most powerful force in the universe' resonates in finance.

Why would a borrower ever prefer simple interest?

Because simple interest costs less than compound interest over the same period. For short-term loans (under one year) the difference is small, but for multi-year loans the savings can be material. The borrower benefits; the lender accepts the lower return. Most jurisdictions require lenders to disclose whether the loan is simple-interest or precomputed so borrowers can compare accurately.

How is simple interest used for late payment of invoices?

Commercial contracts often specify a simple interest rate (e.g. 1.5% per month, equivalent to 18% per year) on overdue invoices. To calculate the late fee on a $5,000 invoice 45 days overdue at 18% per year: I = 5000 x 0.18 x (45/365) = $110.96. Always check the contract for the exact rate, day-count convention, and any minimum fee.

What is the day-count convention and why does it matter?

Day-count convention specifies how interest accrues within a period. Common conventions include 365/365 (actual days over 365), 360/360 (30-day months over 360-day year), and actual/360. At 6.0% nominal, the 360-day convention produces an effective rate of 6.0% x (365/360) = 6.0833%. Always verify the convention in your loan agreement — auto loans commonly use actual/365 or actual/360.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team