Finance

Simple Interest Calculator

Calculate interest earned or owed when interest is based only on the original principal, not on prior interest.

Last reviewed: April 30, 2026Free toolMethodology

Simple Interest Calculator

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These fields start with sample inputs. Keep them or replace them, then run the tool to show a fresh result.

Number fields accept plain values and common formatted input such as 250000, 250,000, or 1,234.56.

Result

Calculating the sample result.

Why it matters

Simple interest still appears in basic lending, short-term agreements, and quick comparison math where full amortization is unnecessary.

When to use

  • Estimating straightforward short-term interest
  • Checking a quoted simple-interest agreement
  • Comparing simple and compound scenarios

Inputs & Outputs

Inputs

  • Principal is the amount borrowed or invested.
  • Annual rate is the yearly simple interest rate.
  • Years is the total term over which simple interest applies.

Outputs

  • Interest amount is the financing cost or earnings over the full term.
  • Ending amount is principal plus simple interest.

Simple interest formula

Multiply principal by the annual rate and the number of years. Since interest does not compound, the rate is applied to the original principal only.

Simple interest = principal x rate x time

Worked example

1

Short-term note estimate

A 12,000 note charges 8% simple interest for 18 months.

Inputs

  • Principal: 12,000
  • Rate: 8%
  • Time: 1.5 years

Steps

  • Interest = 12,000 x 8% x 1.5 = 1,440

Result

  • The simple interest total is 1,440 and the ending amount is 13,440.

Edge cases & caveats

  • Simple interest often understates the cost relative to compounding structures.
  • Always confirm whether the quoted rate is simple, nominal, or effective.

Frequently Asked Questions

How is simple interest different from compound interest?

Simple interest only applies to the original principal. Compound interest also applies to interest that has already accrued.

Can I use months instead of years?

Yes. Convert the term into years by dividing months by 12.

Keep going

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