Compound Interest Calculator (2026)
See roughly how a balance could grow when interest earns its own interest — and how much regular contributions add over time. Enter your numbers below for an instant estimate. Everything runs in your browser; nothing is stored or sent anywhere.
Estimate updates as you type. It assumes contributions are made at the end of each month and a constant rate.
How compound interest works
Compound interest is interest calculated on both your original money and the interest it has already earned. Over short periods the effect is small, but over many years it can become the largest driver of growth — which is why starting early and contributing regularly tends to matter more than chasing a slightly higher rate.
The standard formula for a lump sum is A = P × (1 + r/n)n×t, where P is the principal, r is the annual rate as a decimal, n is how many times it compounds per year, and t is years. When you add regular deposits, each deposit compounds for the time it remains invested, so the calculator above sums the growth of your starting amount and every contribution.
What this estimate does and does not include
The tool assumes a single fixed rate for the whole period and that every contribution is the same size. In reality, savings rates move, investment returns rise and fall (and can be negative), and you may change how much you save. The estimate also does not subtract taxes or fees, and it does not adjust for inflation — so the future dollar figure will buy less than the same number today. Treat the result as a rough illustration, not a prediction.
Common questions
Does more frequent compounding really matter?
Daily or monthly compounding produces a slightly higher balance than annual compounding at the same stated rate, because interest is added back sooner. The difference is usually modest compared with the rate itself and how much you contribute. When comparing savings accounts, compare the APY, which already reflects compounding.
Should I use a savings rate or an investment return here?
That is up to you. For a savings account, use the account's current APY. For long-term investing, people sometimes model a range of assumed returns — but investment returns are not guaranteed and can be negative in any given year, so try lower assumptions too. This calculator does not recommend any specific rate.
Is my data saved?
No. The calculation runs entirely in your browser. Nothing you type is stored, transmitted, or shared.
Method: standard compound interest formula A = P(1 + r/n)^(nt) plus a future-value-of-a-series calculation for regular contributions. General references include published explainers from NerdWallet, Bankrate, and The Calculator Site. Always verify current rates with the provider.