What is compound interest?

Compound interest is interest earned on both the original principal and previously earned interest.

Can I include monthly contributions?

Yes. Enter the regular contribution and choose how often it is made. Monthly contributions are the most common planning scenario.

Does this calculator guarantee an investment return?

No. The return is an assumption for planning. Actual savings and investments can be affected by taxes, fees, inflation, market risk, and changing rates.

What is the Rule of 72?

The Rule of 72 is a rough shortcut: 72 divided by the annual return percentage estimates the number of years needed to double at a steady compound rate.

Compound Interest Calculator

Project savings or investment growth with an initial investment, regular contributions, estimated return, time horizon, and compounding frequency.

Initial investment, $

Estimated annual return, %

Time horizon, years

Compound frequency

Regular contribution, $

Contribution frequency

Contribution timing

Target amount, $

What compound interest means

Compound interest means earning interest on the original principal and on interest that has already been added. The effect becomes more visible when time, rate, and regular contributions all work together.

Projection, not a promise

This is a mathematical projection. It does not account for taxes, fees, inflation, changing returns, investment risk, or product-specific rules.

Compound interest formula

A = P (1 + \frac{r}{n})^{n t}

A is future value, P is the initial investment, r is annual return as a decimal, n is compounding periods per year, and t is years.

Regular contributions are added on their contribution schedule. Beginning-of-period contributions have slightly more time to grow than end-of-period contributions.

Inputs that matter most

Initial investment sets the starting balance.
Regular contributions add new principal through time.
Estimated annual return drives the growth assumption.
Compounding frequency controls how often interest is added.
Time horizon usually has the largest effect on long-term compounding.

Rule of 72

As a quick approximation, divide 72 by the annual return percentage to estimate how many years it takes to double at a steady compound rate.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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