What is the simple interest formula?

Interest = Principal × Rate × Time, where rate is the annual interest rate as a decimal (or divided by 100 if entered as a percentage) and time is in years. Total repayment = Principal + Interest.

What's the difference between simple and compound interest?

Simple interest is calculated only on the original principal for the entire period — it never grows on itself. Compound interest is recalculated periodically on the growing balance (principal plus previously earned interest), so it produces more interest over time at the same rate. Our Compound Interest Calculator handles that calculation separately.

Where is simple interest actually used?

Simple interest is common on short-term loans, certain auto loans, some personal loans, and certificates of deposit or bonds with a fixed term — situations where the interest doesn't need to compound because the term is short or the structure is straightforward. Most long-term savings, credit cards, and mortgages use compound interest instead.

Can I solve for the interest rate or time instead of the interest amount?

Yes — use the Find rate or Find time tabs. Given any three of the four values (principal, rate, time, interest), the calculator solves for the missing fourth value using the same underlying formula rearranged algebraically.

Does time have to be a whole number of years?

No — enter a decimal, like 0.5 for six months or 2.5 for two and a half years. The formula works identically for fractional time periods.

Is my data stored anywhere?

No. All calculations run locally in your browser. Nothing you enter is transmitted to a server or stored.

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Simple Interest Calculator

Calculate simple interest and total repayment from principal, rate, and time — or solve for any one of the four values.

Last updated August 30, 2026

Principal ($)

Annual rate (%)

Time (years)

Interest earned

$750.00

Total interest

$750.00

Total repayment

$5,750.00

A moderate interest rate over a medium timeframe creates a steady, predictable linear growth pattern that does not compound over time. This straightforward calculation is commonly used for short-term personal loans, auto loans, and certain types of fixed-income bonds. Comparing this simple interest total with a compound interest calculation over the same duration highlights how reinvested earnings can accelerate growth over longer periods.

AI-generated insight

How to use the simple interest calculator

1
Choose Find interest, Find principal, Find rate, or Find time, depending on which value you don't already know.
2
Enter the three remaining values.
3
Your result, total interest, and total repayment update instantly.

The simple interest formula

Principal, rate, and time multiply together to produce interest, which is added back to the principal for the total.

Interest = Principal × Rate × Time

Worked example: a $5,000 principal at a 5% annual rate over 3 years: interest = $5,000 × 0.05 × 3 = $750, for a total repayment of $5,750. Unlike compound interest, that $750 is calculated once on the original $5,000 — it doesn't itself earn additional interest in later years.

The same formula rearranges to solve for any other value: principal = interest ÷ (rate × time), rate = interest ÷ (principal × time), and time = interest ÷ (principal × rate) — each available as its own tab above.

Simple vs. compound interest

The same $5,000 at 5% for 3 years produces exactly $750 in simple interest regardless of how often it's calculated. Under annual compounding instead, that same $5,000 grows to about $5,788 — roughly $38 more — because each year's interest is added to the balance before the next year's interest is calculated. The gap between the two widens significantly at higher rates or over longer periods.

Use our Compound Interest Calculator to see the compounding version of the same numbers side by side.

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