Decision tools built for clarity
Compound Interest Calculator
Explore how contributions and compounding can affect long-term growth.
Enter the starting principal, recurring contribution, annual rate, duration, compounding frequency, contribution frequency, timing, and currency.
This educational growth projection is not financial or investment advice and does not guarantee any return.
Methodology
Formula or transformation
Accumulates principal and recurring contributions across the selected compounding and contribution periods. Interest is credited once per compounding period: the balance carried into the period earns the full periodic rate, and a deposit made inside the period earns simple interest for the months it is held until the period boundary, then compounds with the balance. Timeline example: 100 at the end of every month at 12% compounded yearly earns 100 × 12% × (11 + 10 + … + 0) / 12 = 66 in the first year, an ending balance of 1,266. The reference calculator credits such deposits only from the next period, so it shows 1,200 for the same case.
Variables
- Starting principal
- Recurring contribution
- Annual interest rate as a percentage
- Duration in years
- Compounding periods per year
- Contribution periods per year
- Contribution timing within each period
Units
- Currency
- Percent
- Years
- Periods per year
Rounding policy
Displayed money values are rounded to the declared currency scale using half away from zero.
Interest is credited at the selected compounding interval, while contributions are applied at the selected frequency and beginning-or-end timing.
Explore how contributions and compounding can affect long-term growth.
Reviewed sources
- Investor.gov / U.S. SEC — Compound interest calculator methodology Reviewed on 2026-09-20
Formula review history
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Initial formula and source review.
Reviewed 2026-09-04 for formula version periodic-compounding-contributions-v1
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Intra-period accrual defined: a deposit made inside a compounding period earns simple interest for the months it is held, credited at the period boundary; every compounding and contribution combination re-tested (deep audit 2026-09-20).
Reviewed 2026-09-20 for formula version periodic-compounding-contributions-v2
Assumptions
- The entered annual rate and contribution amount remain constant.
- Compounding and contribution timing follow the selected fixed schedules.
Limitations
- Taxes, fees, inflation, withdrawals, and market volatility are excluded.
- Actual investment returns vary and are not guaranteed by a constant-rate projection.
Worked example
A principal of 1,000 compounded annually at 5% for 2 years with no contributions grows to 1,102.50.
Frequently asked questions
Does contribution timing matter?
Yes. A beginning-of-period contribution generally compounds for longer than an end-of-period contribution.
Is the projected balance guaranteed?
No. It is a constant-rate mathematical projection, not a forecast or investment promise.