Calculate& Decide

Money & Finance calculator

Compound Interest Calculator

This calculator projects a future balance by applying compound growth to a starting amount and equal end-of-month contributions.

Updated · Inputs stay in your browser

Educational estimate: This result is not financial, tax, legal, or investment advice. Check real terms and consider qualified advice for important decisions.

Enter your values

Choose the symbol used to display amounts.
Amount available at the beginning.
Added at the end of each month.
A scenario rate, not a guarantee.
The projection period.

Your values stay on this device. Nothing is sent to Calculate & Decide unless you deliberately use your device's share feature.

What this result means

This calculator projects a future balance by applying compound growth to a starting amount and equal end-of-month contributions. The results panel explains the main number and the quantities that produced it, so you can check whether the estimate answers your real question.

Formula used

FV = P(1 + r)ⁿ + PMT × ((1 + r)ⁿ − 1) ÷ r, using a monthly rate r and n months. When r is zero, contribution growth is PMT × n.

Step-by-step calculation

  1. Validate every required value and reject values outside the stated field limits.
  2. Convert rates and time periods to the units required by the formula.
  3. Calculate with full JavaScript number precision; do not round intermediate values.
  4. Round only the displayed result and explain the practical meaning.

Worked example

Starting with $5,000, adding $200 at each month-end, and earning an estimated 5% annually for 10 years produces about $39,291.50. The amount contributed is $29,000; the difference is estimated growth.

Assumptions and limitations

  • Interest compounds monthly at a constant entered rate.
  • Contributions are made at the end of each month.
  • Taxes, fees, inflation, and market volatility are excluded.

How to use this calculator

  1. Choose a currency symbol when the tool uses money.
  2. Enter values in the units shown beside each field.
  3. Select Calculate, then read both the main result and interpretation.
  4. Change one uncertain input at a time to understand sensitivity.
  5. Use Copy result or Share only when you choose to move the result outside this page.

Common mistakes

  • Treating an estimated return as guaranteed.
  • Forgetting that fees and taxes reduce actual growth.
  • Comparing future money with today's purchasing power without considering inflation.

Frequently asked questions

What is compound interest?

It is growth calculated on the starting principal and on growth accumulated in earlier periods.

Can the rate be negative?

Yes, down to -99% annually in this model, but a constant negative rate is only a scenario.

Are contributions made at the start or end of the month?

This calculator assumes end-of-month contributions.

Methodology and sources

See the site-wide calculation methodology and editorial standards. Formula errors can be reported through the error-reporting instructions.

Published and reviewed .