Compound Interest Calculator

The Compound Interest Calculator projects how a lump sum and a stream of regular contributions grow over time when the interest they earn is reinvested. You enter a starting amount, an annual interest rate, how long the money stays invested, and how often interest is compounded - annually, monthly, daily, or continuously - and the tool returns the final balance, the total interest earned, and a year-by-year schedule showing how the pot builds.

Compounding is the effect of earning interest on your interest. In the first period you earn interest only on the money you put in; in every period after that you earn interest on the original amount plus all the interest already credited. Over years and decades that feedback loop is what turns modest, steady saving into a meaningfully larger sum, and it is why the compounding frequency and the length of the term matter as much as the headline rate.

A worked example shows how it works as a retirement calculator with compound interest. Enter a Starting amount of 25000, an Annual interest rate (%) of 6, 30 Years, Compound frequency Monthly and a Recurring contribution of 500 made Monthly at End of period. Future value reads 652,821.90 at an effective annual rate of 6.17%, from 205,000.00 total deposited and 447,821.90 total interest. The schedule shows 1,709.73 of interest in year 1 and 37,734.93 in year 30. Switch to Start of period and the result rises to 655,333.19.

Every calculation runs client-side in plain JavaScript. Nothing you type - the amounts, the rate, or your contribution plan - is uploaded or stored, and the calculator keeps working with no connection once the page has loaded. It is built for quickly comparing scenarios: nudge the rate, extend the term, or add a monthly deposit and watch the future value and interest update instantly.

A projection is only as good as its assumptions, and three of them are worth stating. The rate is treated as constant for the whole term, which no real investment is, so the output is a scenario rather than a forecast. Compounding frequency changes the result even at the same nominal rate, which is the difference between a nominal and an effective annual rate: 12 per cent compounded monthly is 12.68 per cent effective. And the figures are before tax and before inflation. A projection showing your money doubling over twenty years at 4 per cent is describing the number in the account, not what it will buy. Adjust the rate down by expected inflation if you want the result in today's money.