Compound interest calculator

How to calculate compound interest

1. Enter the principal (starting amount).
2. Enter the annual interest rate as a percentage.
3. Choose how often interest compounds: annually, quarterly, monthly, or daily.
4. Set the time period with the slider. Your final amount and year-by-year table appear instantly.

Common uses

• Projecting how a savings account or ISA balance grows over time with a fixed interest rate, working out the investment window by counting the days between two dates
• Comparing growth across different compounding frequencies to understand the effect of daily versus annual compounding
• Checking an investment's real result against steady compounding by calculating its annualized return

Technical specification

• Algorithm or formula: Standard compound-interest formula A = P(1 + r/n)^(nt) where P is principal, r the annual rate as a decimal, n the compounding periods per year, and t the term in years. This is the future-value formula for a nominal annual rate compounded n times a year, as set out in interest-theory texts such as Kellison, The Theory of Interest (3rd ed., McGraw-Hill/Irwin, 2009).
• Browser API or library: Pure JavaScript arithmetic and Math.pow. No external library.
• Input limits: Any positive principal and any rate from 0% up. Term 1–50 years, with a row per year in the year-by-year table. If the result is too large to calculate, the tool shows a note instead of a number.
• Output: Final balance, total interest earned, and a per-year table of balance growth.
• Known limitations: No support for irregular deposits, withdrawals, inflation adjustment, or tax. Continuous compounding (Pe^(rt)) is not exposed.

Frequently asked questions

What is the difference between compound and simple interest?

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus any previously earned interest, so your interest earns interest over time. Over long periods or at high rates, the difference between the two becomes substantial.

What is the compound interest formula?

A = P(1 + r/n)^(nt). P is the principal (starting amount), r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the number of years. A is the final balance including all interest.

How does compounding frequency affect growth?

More frequent compounding produces slightly higher returns. Daily compounding produces more than monthly, which produces more than quarterly, which produces more than annual. The difference is small at low rates but grows at higher rates and over longer periods.

Is my data sent to a server?

All calculations run locally in your browser using JavaScript. Our servers are not involved at any point.