Interest Rate Calculator

Estimate the annual interest rate from principal, ending balance, and time. Supports both simple and compound interest.

How to Use

  1. Select Simple Interest or Compound Interest.
  2. Enter the starting principal, ending amount, and time period.
  3. For compound mode, choose the compounding frequency.
  4. The tool calculates the annual interest rate and shows the formula steps below.
Rate Snapshot
Annualized result based on your entered values.
Annual Rate
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Interest Earned
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Growth Multiple
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Years
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0% Annual Rate Gauge 50%+
Enter values to estimate the annual rate.
Inputs & Settings
Provide enough values to solve for the annual interest rate.
Simple uses linear growth. Compound uses periodic growth.
Display-only formatting. Does not change the math.
The original starting balance.
The value after interest has been applied.
Time is converted to years for the annual rate calculation.
Used only in compound mode.
10,000
3 years
Show Work (step-by-step)
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Work is shown using the selected mode and annualized units where applicable.

Formulas

Simple interest: A = P(1 + rt)

Solve for rate: r = (A / P - 1) / t

Compound interest: A = P(1 + r / n)^(nt)

Solve for rate: r = n[(A / P)^(1 / nt) - 1]

Where A = ending amount, P = principal, r = annual rate, t = time in years, and n = compounding periods per year.

FAQ

What is the difference between simple and compound interest?

Simple interest grows linearly from the original principal. Compound interest grows on both principal and accumulated interest.

Can I use months or days?

Yes. The calculator converts months or days into years so it can estimate an annual rate.

Why is the compound result different from the simple result?

Because compound formulas include growth within each compounding period, which changes the annual rate needed to reach the same ending amount.

Is this APR or APY?

This tool solves for the annual nominal rate based on the selected formula. In compound mode, the effective yield depends on compounding frequency.

Tool Info

Last updated:

Updates may include UI improvements, financial formatting, and edge-case handling for annualized rate calculations.