Finance

Compound Interest Calculator

Estimate future balance growth from a starting principal, recurring contributions, annual return, and time horizon.

Last reviewed: April 30, 2026Free toolMethodology

Compound 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

Compounding is easier to understand when you can separate what comes from contributions and what comes from growth.

When to use

  • Testing long-term savings assumptions
  • Comparing contribution levels
  • Showing the cost of waiting to start investing or saving

Inputs & Outputs

Inputs

  • Starting balance is the amount already saved or invested.
  • Monthly contribution is the recurring amount added each month.
  • Annual return is the assumed yearly growth rate before compounding.
  • Years is the total time horizon for the projection.

Outputs

  • Future value is the projected ending balance.
  • Contribution total isolates the money added by the saver.
  • Growth earned shows the portion created by compounding.

Compound growth with contributions

Apply monthly compounding to the starting balance and recurring monthly contributions over the chosen time horizon.

Future value = principal x (1 + r)^n + contribution x (((1 + r)^n - 1) / r)

Worked example

1

Long-term savings plan

A saver starts with 15,000, adds 500 per month, earns an assumed 7% annually, and stays invested for 20 years.

Inputs

  • Starting balance: 15,000
  • Monthly contribution: 500
  • Return: 7%
  • Years: 20

Steps

  • Convert annual return to a monthly rate
  • Apply monthly compounding across 240 months

Result

  • The calculator shows how much of the ending balance came from contributions versus compound growth.

Edge cases & caveats

  • Returns are assumed to be smooth and constant, which does not reflect real markets.
  • This is a planning estimate, not an investment forecast.

Frequently Asked Questions

Why do small rate changes matter so much over long periods?

Because compounding applies growth to previous growth, so small differences widen as the time horizon grows.

Can this work for plain savings accounts?

Yes. Use the expected annual yield and contribution schedule that match the account.

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