Compound Interest Calculator
See how your money grows with compound interest. Enter a starting amount, annual interest rate, time, compounding frequency, and optional monthly contributions to instantly get the final balance, total contributed, and total interest earned.
ToolsSoup's Compound Interest Calculator shows how an investment or savings balance grows over time. Enter a starting amount, an annual interest rate, the number of years, how often the interest compounds, and an optional monthly contribution, and you instantly see the final balance, the total you put in, and the total interest earned. It uses the standard compound interest formula plus a future-value-of-a-series calculation for recurring deposits, so the numbers match what a savings account, certificate of deposit, or investment account would project. Everything runs in your browser: no uploads, no sign-up, and your figures never leave your device.
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already accumulated. Each compounding period, the balance grows by a percentage of its current value, so the interest itself starts earning interest. Over time this snowball effect makes a balance grow much faster than simple interest, which only ever pays a percentage of the original principal. The more often interest compounds — daily instead of yearly, for example — and the longer you leave the money invested, the larger the final balance becomes.
How to calculate compound interest
Working out compound growth takes a few quick inputs, and this tool does the math automatically as you type:
- Enter the starting amount — the principal you begin with.
- Enter the annual interest rate as a percentage and the time in years.
- Choose how often interest compounds and, optionally, a monthly contribution. Read the final balance, total contributed, and total interest below.
What is the compound interest formula?
The core formula is A = P × (1 + r ÷ n)^(n × t), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the time in years. When you add a regular monthly deposit, the tool also applies the future value of a series, FV = PMT × ((1 + i)^m − 1) ÷ i, where PMT is the monthly contribution, i is the monthly rate, and m is the number of months. This calculator runs both parts for you instantly as you change any number.
What's the difference between compound and simple interest?
Simple interest is calculated only on the original principal, so it grows by the same fixed amount every period. Compound interest is calculated on the principal plus all previously earned interest, so the amount added grows each period. Over short periods the two are close, but over many years compound interest pulls far ahead — which is why long-term investing and early saving are so powerful. The total interest figure here reflects compounding, so you can compare it against a simple-interest estimate to see the difference the snowball effect makes.
Why use this compound interest calculator?
- Instantly shows the final balance, total contributed, and total interest earned.
- Supports daily, monthly, quarterly, semiannual, and annual compounding.
- Adds optional monthly contributions using the future-value-of-a-series formula.
- Handles 0% interest and non-round amounts cleanly, rounding money to two decimals.
- Updates live as you type, runs entirely in your browser, and is 100% free with no ads or sign-up.
Frequently asked questions
How do I calculate compound interest?
Use the formula A = P × (1 + r ÷ n)^(n × t), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the years. Enter your numbers above and the calculator does it instantly, including any monthly contributions.
How much does $1,000 grow at 5% over 10 years?
Compounded monthly, $1,000 at 5% grows to about $1,647.01 after 10 years, earning roughly $647.01 in interest. Change any field above to model your own starting amount, rate, time, and contributions.
Does this calculator include monthly contributions?
Yes. Enter a monthly contribution and the tool adds those recurring deposits using the future value of a series, then shows the total you contributed alongside the interest your contributions earned.
How does compounding frequency affect the result?
The more often interest compounds, the more interest you earn, because each period's interest starts earning interest sooner. Daily compounding yields slightly more than monthly, which yields more than annual, for the same rate and time.
What happens if I enter a 0% interest rate?
With a 0% rate there is no interest, so the final balance is just your starting amount plus any contributions, and the total interest earned is zero.