Why is my result lower than a compound-interest calculation with the same numbers?

Simple interest doesn't compound, so the gap between the two grows the longer the time period — for short terms the difference is small, for long terms it can be substantial.

Does the rate need to be annual?

This calculator assumes the rate and time period use matching units — an annual rate paired with years. Mixing units, like a monthly rate with a time in years, will give an incorrect result.

Why would anyone use simple interest if compound interest grows faster?

It isn't a choice you usually make — some products (certain short-term loans, older-style bonds) are structured to use simple interest by design. It's also useful as an easy hand-check for roughly what a compound calculation should be in the ballpark of over short periods.

How much difference does compounding actually make?

Over short periods, not much — but the gap widens with time, since compound interest earns interest on interest and simple interest doesn't. Compare the same numbers in the Compound Interest Calculator to see the difference for your figures.

What if my time period is in months, not years?

Convert it to a decimal year first — 6 months is 0.5 years, 18 months is 1.5 years — since this calculator's Time field and annual rate are both expected in years.

Why does entering 0% or 0 years still give a valid result?

Simple interest is a straight multiplication of principal, rate, and time — if either the rate or the time is zero, the product is zero, which correctly shows as $0 interest rather than an error.

Can I use a fractional rate, like 6.25%?

Yes — enter the percentage figure directly (6.25, not 0.0625), the same way you'd read it off a loan or bond term sheet.

Does the result change if I repay early?

Not automatically — this figure reflects the full time period you entered. If you actually repay before that, re-run the calculation with the shorter actual time to see the smaller interest amount.

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Finance

Simple Interest Calculator

Interest and total payable on a fixed-rate loan.

🔒 Runs entirely in your browser — nothing here is ever uploaded

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Last reviewed 2026-08-29

About the Simple Interest Calculator

Calculates interest that accrues only on the original principal — unlike compound interest, it never earns interest on previously earned interest, which makes it simpler (and slower-growing) to work out by hand.

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How to use it

Enter the Principal amount.
Enter the annual Rate (% p.a.).
Enter the Time in years.
Read the interest and total payable in the result panel.

Formula

Interest = (Principal × Rate × Time) ÷ 100
Total payable = Principal + Interest

The standard simple-interest formula (I = PRT), the textbook counterpart to the compound-interest formula used in the Compound Interest Calculator.

Worked example

A $10,000 loan at 6% simple interest for 3 years accrues $1,800 in interest, for a total payable of $11,800.

Recommendations

• Confirm whether your loan or investment actually uses simple interest — most modern products use compound interest instead.
• Simple interest is easy to sanity-check by hand: rate × time × principal, no exponents involved.
• For a more realistic projection on savings or loans that compound, use the Compound Interest Calculator instead.

Frequently asked questions

Mostly for short-term loans, certain bonds, and some auto loans. Most mortgages, credit cards, and savings accounts use compound interest instead, which grows faster over time.