Simple Interest Calculator

Calculate interest earned or owed using the simple interest formula. Solve for interest, total amount, principal, rate, or time with step-by-step work.

How to Use

  1. Enter the known values: principal, annual rate, time, and optionally total amount.
  2. Select the time unit and compounding style shown for interpretation. This tool uses simple interest only.
  3. Choose what to solve for, or leave it on Auto.
  4. Review the live results and open Show Work for formulas and steps.
Simple Interest Snapshot
Fast overview of interest, total repayment, and effective growth over the selected term.
Interest
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Total Amount
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Principal
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Annual Rate
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Growth Over Principal —

Formula used: I = P × r × t, where rate is annual and time is converted to years.

Inputs & Settings
Enter any known values. Leave the target field blank or choose a Solve For mode.
Starting amount before interest.
Enter APR-style annual rate as a percent.
Time is converted into years for the formula.
Interest amount only, not total amount.
Principal plus interest.
Display-only formatting choice.
Show Work (step-by-step)
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Work is shown using the simple interest formula with time normalized to years.

Simple Interest Formula

Quick answer: Simple interest is calculated with I = P × r × t.

Where I is interest, P is principal, r is annual rate as a decimal, and t is time in years.

  • Interest: I = P × r × t
  • Total Amount: A = P + I
  • Principal: P = I ÷ (r × t)
  • Rate: r = I ÷ (P × t)
  • Time: t = I ÷ (P × r)
This tool is for simple interest only. It does not compound interest between periods.

FAQ

What is simple interest?

Simple interest is interest calculated only on the original principal. It does not earn additional interest on prior interest.

What is the difference between simple and compound interest?

Simple interest uses the original principal only. Compound interest adds interest onto the balance, so future interest can grow faster.

Can I use months or days?

Yes. The calculator converts months, weeks, or days into years before applying the formula.

Why is my total amount just principal plus interest?

That is exactly how simple interest works: Total = Principal + Interest.

Tool Info

Last updated:

Updates may include UI improvements, formula validation, and edge-case handling for rate and time conversions.