Compound Interest Calculator

Checked 15 October 2026. What was checked. How these pages are maintained.

Compound interest formula

Without extra deposits: A = P(1 + r/n)^nt. P is the start, r is the annual rate as a decimal, n is compounds per year, t is years. Extra monthly deposits are added each month before that month's growth. This is an estimate, not investment advice. Inflation and taxes are ignored.

Worked example

$10,000 at 4% compounded monthly for 10 years, no extra deposits, is about $14,908. The same start with $200 added each month is much higher because those deposits also compound.

This is not a forecast. Fees, tax on interest in a non-registered account, and inflation all reduce what you can spend. A TFSA or RRSP changes the tax, not this formula.

A lump sum, and the same lump with a monthly deposit

$2,000 at 5 percent for 3 years, with nothing added, finishes near $2,322.94 if the growth is monthly. The same money compounded once a year finishes near $2,315.25. Compounded daily it finishes near $2,323.64. On this small example the gap between yearly and daily is under $9. The gap gets wider with a higher rate, a longer term, or a larger start, which is why the compounds box exists. With no extra deposit the page uses A = P(1 + r/n)^nt.

Add $100 at the start of each month and you are no longer looking at a lump sum. Over 36 months you contribute $2,000 + $3,600 = $5,600. At 5 percent compounded monthly the balance is about $6,214. The interest is the difference, about $614, not a percent of the original $2,000 alone. Each deposit is in the account for a different number of months. The last $100 barely earns anything. The first $100 earns for the whole three years.

What this projection leaves out

Fees, a rate that changes, and tax on interest in a non-registered account are not in the loop. A TFSA or an RRSP changes the tax, not this formula. Inflation is a separate question: $2,323 in three years does not buy what $2,323 buys today. The inflation calculator is that other multiplication. If you already know the date and the dollar target, the savings goal calculator solves for the deposit instead of the ending balance. This page is not a forecast of a market return.

Common questions