Compound investment calculator

How this compound investment calculator works

The calculator applies the entered nominal annual rate at the selected compounding frequency. It converts that rate to an equivalent monthly growth rate, applies one month of growth, then adds the monthly deposit at month-end. Starting balance, deposits and rate stay fixed throughout the scenario.

In the model, B_m = B_(m-1) * (1 + i_m) + C, where i_m = (1 + r/n)^(n/12) - 1, r is the nominal annual rate, n is the compounding frequency per year and C is the monthly deposit. For frequencies below monthly, the engine converts the selected rate to equivalent monthly growth; this projection convention is not a bank's actual crediting schedule. The model assumes a smooth rate and does not represent the order of market gains and losses.

$500 monthly for 30 years: compare rate assumptions

These examples use no starting balance, $500 deposited at each month-end, monthly compounding, and a 30-year term. They use the same calculation as the widget above. The rates are sensitivity inputs only, not return forecasts.

A higher rate produces a higher modeled balance because the input applies to every period. Actual investment returns vary, and fees, taxes, withdrawals and changing contributions can reduce or alter the result.

Nominal balance and inflation-adjusted value

The main balance is nominal: it reflects the selected return assumption but not a purchasing-power adjustment. The calculator separately divides that result by the selected inflation factor raised to the number of years. This is a constant-inflation scenario, not a prediction of future prices. For background on compound-interest calculations, see Investor.gov's compound-interest calculator; the Bureau of Labor Statistics CPI provides U.S. price-index data for historical comparisons.

Using the projection for a 401(k), IRA or investment account

1. Enter your current invested balance, or use zero if modeling a new account.
2. Enter the monthly deposit you want to test. Employer contributions can be included in that amount, but verify matching formulas, vesting and plan rules separately.
3. Choose a nominal rate and compounding frequency, then compare more than one rate assumption.
4. Choose the number of years. The output assumes the rate and deposit remain unchanged over that full period.

This is generic growth arithmetic, not an account-specific projection. It does not apply IRS contribution limits, employer-match rules, tax treatment or withdrawal restrictions. For index history, use the separate S&P 500 calculator; for employer-plan inputs, see the 401(k) calculator.

Fees, taxes and investment risk

There is no separate fee or tax field. The rate is applied as entered, so the page does not calculate a fund's expense ratio, account charges or tax bill. Investor.gov explains that investment products involve risk and can lose value; a smooth projection cannot show volatility or sequence of returns. The SEC also describes how fees and expenses affect a portfolio. Review fund costs in current disclosures and use the fee analyzer to model fee assumptions separately.

Solve for a target or monthly deposit

When you have a target balance, the inverse tool below can solve for a starting balance, monthly contribution, rate or timeline under its stated fixed-rate model. A required return is a mathematical result for the entered target, not a promise that markets will deliver it.

Embedding a calculator

For current embedding options and instructions, see Snowballr widgets. The widget page describes its embed details and attribution behavior.

Compound investment calculator FAQ

Methodology and editorial standards

Scenario values use the shared compound calculation engine, nominal-rate convention, and equal month-end deposits. The inflation-adjusted figure uses the separate constant-inflation assumption. The calculation and this page were reviewed on 2026-10-04; no projection is a guarantee. Read our editorial standards and sources index.