Compound Interest Calculator

About this tool

This compound interest calculator shows how money grows when interest is added back to the balance and then earns interest of its own. Enter your initial principal, the annual interest rate, and a term in years (plus optional extra months), choose how often interest compounds, and — if you save regularly — add a contribution with its own frequency and timing. Everything runs locally in your browser; no numbers are uploaded.

It returns the future value, the total you contributed, the total interest earned, the effective annual rate (APY implied by your nominal rate and compounding), the number of years for the principal alone to double, and a year-by-year schedule of balance, contributions-to-date and interest-to-date.

Under the hood every combination of frequencies is reduced to one effective annual rate (EAR = (1 + r/n)^n − 1, or e^r − 1 for continuous compounding), the principal grows by (1 + EAR)^t, and regular deposits are valued as an annuity at the equivalent contribution-period rate — with a start-of-period (annuity-due) option when you deposit at the beginning of each period. A negative contribution models a regular withdrawal.

Example

Enter principal 1000, rate 5, 10 years, compounding monthly, contribution 0:

{
  "future_value": 1647.01,
  "principal": 1000.0,
  "total_contributions": 0.0,
  "total_interest": 647.01,
  "effective_annual_rate": 5.1162,
  "years_to_double": 13.892,
  "years": 10.0,
  "schedule": [ ... ],
  "summary": "1,000.00 at 5% compounded monthly over 10 years grows to 1,647.01 (0.00 contributed, 647.01 interest)"
}

Compounding monthly instead of annually lifts the effective rate from 5% to 5.1162%, so the same nominal 5% ends at 1,647.01 rather than 1,628.89. Add a 500/month contribution and the future value climbs past 79,000 as the annuity stacks on top of the growing principal.

Limits & notes

• Every field is optional and falls back to a sensible default (principal 1000, rate 5%, term 10 years, monthly compounding, no contributions, deposits at end of period).
• The rate is a nominal annual percent; the tool reports the effective annual rate (APY) separately so you can compare accounts quoted either way.
• Term is capped at 1000 years and the schedule is capped at 121 rows (the final term boundary is always included).
• Money is rounded to 2 decimals and rates to 4; figures are estimates for planning and your institution's exact rounding, fees and day-count may differ.
• Enter amounts as plain numbers — no currency symbols or thousands separators.

FAQ