Compound interest calculator

Methodology

This calculator applies the mechanics explained in compound interest to a starting amount and equal contributions made at the end of each selected compounding period. It is an educational illustration rather than a prediction or recommendation.

Formula and units

For a non-zero periodic rate, the projected balance is P × (1 + r)^n + C × ((1 + r)^n − 1) ÷ r. P is the starting amount in US dollars, C is the contribution in US dollars at the end of each period, r is the annual percentage rate divided by the number of compounding periods per year, and n is the total number of periods. At a zero rate, the result is simply the starting amount plus all contributions.

Investor.gov uses the same core inputs in its public compound-interest tool: an initial amount, regular contribution, time, estimated annual rate, and compounding frequency. ^[1]

Supported inputs and boundary behavior

• The starting amount must be between $0 and $1 trillion, and the contribution per selected period must be between $0 and $1 billion.
• The annual return must be between 0% and 100%. Negative returns are outside this calculator's supported range.
• The time period must be a whole number from 0 through 100 years.
• Compounding can be annual, quarterly, or monthly.
• At zero years, the projection equals the starting amount because no contribution period has elapsed. At a zero rate, it equals the starting amount plus all scheduled contributions.

Assumptions and limitations

• The annual return stays constant and compounds annually, quarterly, or monthly.
• Each contribution is made at the end of its selected period.
• Inputs and results use US dollars; the formula itself is currency-neutral.
• The estimate does not model fees, taxes, inflation, withdrawals, changing returns, or investment risk.
• Returns in real markets vary and can be negative. The displayed result is not guaranteed.

Rounding

The calculation keeps full floating-point precision. Displayed US-dollar amounts are rounded to the nearest cent only after the projection is complete.

Worked example

With a $10,000 starting amount, a $100 contribution at the end of every month, a 5% annual return compounded monthly, and 10 whole years, the projected balance is $31,998.32. Total contributions are $22,000.00 and the estimated compound growth is $9,998.32.