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Finance tool

Compound Interest Calculator

Explore how an initial amount and equal recurring contributions may grow under a fixed nominal annual rate—with the timing and calculation conventions shown alongside the estimate.

Free · Private · Runs in your browser

Growth estimator

Enter a rate, term, and contributions

Amounts stay in your browser. Results are estimates, not promises of investment performance or an offer of a financial product.

Starting amount and growth

Enter non-negative dollar amounts. Commas and currency symbols are not accepted in numeric fields.

The nominal rate is divided by the selected compounding frequency; it is not an effective annual yield.

Term and recurring contribution
Contribution timing
Your estimate and schedule will appear here after you submit valid values.

Formula and calculation method

For principal P, nominal annual rate r, compounding frequency m, and term t in years, the principal component is P(1 + r/m)mt. The calculator applies the same growth factor separately to every recurring contribution from its scheduled deposit time to the end of the term, then adds all components.

The effective annual yield shown is (1 + r/m)m − 1. It describes the selected nominal rate and compounding frequency only; recurring contributions do not change that yield.

Assumptions and deterministic conventions

  • Fixed inputs: the nominal rate, contribution amount, frequencies, and timing remain unchanged for the entire term.
  • Contribution schedule: equal periods divide a 365-day model year. Only complete contribution periods are counted; a final partial contribution interval adds no extra contribution.
  • Beginning versus end: both choices use the same number of contributions. Beginning contributions receive one additional contribution interval of growth.
  • Partial terms: months are treated as twelfths of a year. When a term ends between compounding boundaries, the periodic growth factor is extended with a fractional exponent; this is a mathematical convention, not a claim about an institution’s accrual policy.
  • Precision: calculations use JavaScript double-precision numbers without intermediate cent rounding. Currency is rounded to two decimals only for display.
  • Boundaries: zero rates and zero contributions are supported. Negative rates, withdrawals, negative amounts, non-whole years/months, terms over 100 years, and non-finite or excessive values are rejected.

What this estimate does not include

Actual returns may vary and can be negative. This model does not account for taxes, inflation, fees, volatility, changing rates, contribution limits, market losses, institution-specific day-count rules, or the timing of weekends and holidays.

This calculator is educational and is not financial, tax, legal, or investment advice. It does not recommend any account, security, strategy, or expected return.

Common questions

Why is the effective yield higher than the nominal rate?

When interest compounds more than once per year, earlier interest can itself earn interest. At a zero rate, nominal and effective rates are both zero.

What changes when contributions are made at the beginning?

Each contribution is deposited one contribution interval earlier than the matching end-of-period contribution, so it receives one more interval of estimated growth.

Can contribution and compounding frequencies differ?

Yes. Every contribution is placed on its own evenly spaced schedule, then accumulated using the selected compounding growth factor for the time it remains invested.

Version and verification

Method version 1.0.0; deterministic test suite last verified 2026-08-04. Automated tests cover zero-rate, frequency, timing, partial-term, boundary, overflow-guard, schedule, metadata, keyboard, responsive, and accessibility behavior. No manual screen-reader pass is claimed.