What it does
The compound interest calculator works out what a sum grows to when interest is added to it at regular intervals:
- Final amount: the balance at the end of the period, for example 21,589.25 for 10,000 at 8% a year for 10 years, compounded yearly.
- Principal: the sum you start with, 10,000.00 in that example.
- Total interest: the final amount minus the principal, 11,589.25 in that example.
- Effective yearly rate: the growth in one year at your compounding, 8% yearly or 8.2432% quarterly.
- Growth by year: a table with the interest earned in each year and the balance at its end.
The result updates as you type.
How to use it
- Type the principal.
- Type the interest rate as a yearly percentage.
- Type the number of years.
- Pick how often interest is compounded: Yearly, Half-yearly, Quarterly or Monthly.
- Select Copy result to put the figures on your clipboard.
The calculator remembers your numbers and your compounding on this device, so they’re there the next time you open the page.
How it’s calculated
With the principal P, the yearly rate r as a decimal, m compounding periods a year and t years:
A = P × (1 + r ÷ m)^(m × t)
For 10,000 at 8% a year for 10 years, compounded yearly, m = 1, so A = 10,000 × (1 + 0.08 ÷ 1)^(1 × 10) = 21,589.25. Compounded monthly, m = 12 and A is 22,196.40.
The effective yearly rate is (1 + r ÷ m)^m − 1, shown as a percentage to 4 decimals. Each year’s balance in the table uses the same formula with that year’s t, and each year’s interest is that balance minus the one before, so the last balance is the final amount.
The calculator works each amount out exactly and rounds it half up to whole cents once, so no rounding builds up along the way. It assumes the rate stays the same, no deposits or withdrawals are made, and no tax is taken from the interest.
Good to know
- The principal can be from 1 to 1,000,000,000,000, and is rounded to whole cents.
- The interest rate can be from 0% to 50% a year, in steps of 0.01. At 0%, the final amount is the principal.
- The number of years is a whole number from 1 to 50, so the table has at most 50 rows.
- Real rates can change over time, and tax on interest can lower what you keep, so the figure can differ from an account’s.
- No currency is assumed: the amounts are in whatever currency you type.
Privacy
Everything is calculated in your browser, on your device. Nothing you type is uploaded. Only your numbers and your compounding are kept, in this browser’s storage, so the page can show them again.