Simple Interest Calculator

What this calculator does

Simple interest grows in a straight line because interest is calculated only on the original principal. Enter principal, annual rate, and time. ToolGrym calculates total interest, final amount, annual interest, and equivalent monthly and daily accrual.

The result also shows what the same principal and rate would become with annual compounding. This comparison isolates the value created when earned interest begins earning interest itself.

Simple interest formula

The formula is:

I = P x r x t

where:

• I is total interest;
• P is the original principal;
• r is the annual rate as a decimal; and
• t is time in years.

The final amount is:

A = P + I

For a constant principal, the approximate daily interest is P x r / 365. This daily figure is informational; an amortizing loan has a declining principal, so its actual daily interest falls as principal payments are applied.

Worked example

For $10,000 at 5% simple interest over 3 years:

• Interest: $10,000 x 0.05 x 3 = $1,500
• Final amount: $11,500
• Interest per year: $500
• Monthly equivalent: about $41.67
• Daily equivalent: about $1.37

If the same amount compounded annually at 5%, it would become $11,576.25. Compounding adds $76.25 because the first and second years’ interest also earn interest.

Simple interest versus compound interest

Simple interest is appropriate when the agreement explicitly applies a rate only to principal or when you need a short constant-balance estimate. Use the compound interest calculator when interest is credited back to the balance or when you make recurring contributions.

For deposit products, compare the disclosed annual yield with the APY calculator. APY reflects compounding and therefore makes accounts with different crediting frequencies easier to compare.

Using simple interest for loans

Many loans described as simple-interest loans calculate interest on the current outstanding principal, not forever on the original principal. Payments reduce principal, so a full payoff schedule requires month-by-month or day-by-day modeling. Use the student loan calculator or loan payoff calculator for changing balances.

Federal Student Aid describes Direct Loans as simple daily interest loans. It also explains that daily interest is based on the outstanding principal. Capitalization can add unpaid interest to principal in specific circumstances, after which future interest is calculated on the larger principal.

Common mistakes

1. Entering 5 instead of 0.05 inside a manual formula. The calculator accepts 5%, but the written formula uses 0.05.
2. Mixing months and years. Convert months to years by dividing by 12 before using I = P x r x t.
3. Using the original balance for an amortizing loan. The outstanding principal declines after payments, so constant-principal interest overstates later interest.
4. Calling APY simple interest. APY is an effective annual yield that reflects compounding.
5. Ignoring capitalization. Unpaid loan interest may be added to principal under the agreement or program rules, changing future interest.

Methodology and review

This tool assumes a constant principal, fixed annual rate, exact time expressed in years, and a 365-day daily equivalent. It does not model payments, changing balances, leap years, fees, capitalization, or product-specific day-count conventions. Reviewed July 20, 2026.