Interest Calculator

Calculate simple or compound interest, optional recurring contributions, ending balance, and total interest earned.

How to Use

  1. Choose Simple Interest or Compound Interest.
  2. Enter the starting principal, annual interest rate, and time period.
  3. Optionally add recurring contributions and choose their frequency.
  4. Review ending balance, interest earned, contribution totals, and Show Work steps.
Growth Snapshot
Live readout for principal, growth, and yield.
Ending Balance
$0.00
Interest Earned
$0.00
Total Contributions
$0.00
Effective Annual Yield
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Status: Waiting for input
Contributions: Off
Growth View Projected balance over time
High 0 Start End Projected Balance
Balance Growth area
Inputs & Settings
All calculations run instantly in your browser. Use share only when you want to generate a URL.
Starting amount before any added contributions.
Enter a yearly rate as a percent, not a decimal.
Supports fractional years such as 1.5 or 7.25.
Used only for compound interest mode.
Optional additional amount added each period.
Contribution timing is handled in JS and shown in Show Work.
Beginning-of-period contributions can grow slightly more.
Changes output formatting only, not the underlying math.
Show Work (step-by-step)
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Work is shown using the exact inputs selected above. Simple interest and compound interest formulas are explained automatically.
Growth Schedule
Period Starting Balance Contributions Interest Ending Balance
Enter values to generate a schedule.

Interest Formulas

Simple interest: I = P × r × t

Simple ending balance: A = P + I

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

Compound with recurring contributions: calculated period-by-period based on your contribution frequency and timing.

  • P = principal
  • r = annual rate as a decimal
  • t = time in years
  • n = compounding periods per year
  • A = ending balance
  • I = interest earned
Effective annual yield is derived from the selected compounding frequency when compound mode is active.

FAQ

What is the difference between simple and compound interest?

Simple interest grows only from the original principal. Compound interest grows from both the principal and previously earned interest.

Why does daily compounding usually grow faster than annual compounding?

More frequent compounding means interest is added to the balance more often, so future interest is calculated on a larger amount sooner.

Do recurring contributions change the result much?

Yes. Regular deposits can significantly increase the final balance, especially over longer time periods and with higher rates.

Can this calculator be used for savings, CDs, or investment projections?

Yes, for general planning and estimation. Actual financial products may use different day-count conventions, fees, taxes, penalties, or compounding rules.

Tool Info

Last updated:

Updates may include formula handling, contribution modeling, schedule rendering, and edge-case validation.