Compound Interest Calculator

See how savings grow with compound interest

Compound Interest Calculator shows what a starting amount grows to at a fixed annual interest rate, with or without regular deposits. It gives the final balance, how much of that you paid in, how much is interest, the effective annual rate, and a year-by-year table.

You choose how often interest is compounded — yearly, half-yearly, quarterly, monthly or daily — and whether deposits go in monthly or yearly, at the start or the end of each period. Results update as you type and are calculated in your browser. It works with plain numbers, so use any currency.

How to calculate compound interest

1. Enter the initial amount (0 is fine if you’re only making regular deposits).
2. Enter the annual interest rate as a percentage, such as 5 or 4.25.
3. Enter the number of whole years, from 0 to 100, and optionally 0–11 extra months. The total has to be at least 1 month.
4. Choose the compounding frequency. Your bank or account terms usually say which one applies.
5. To add regular deposits, enter the amount, choose monthly or yearly, and choose whether each deposit is paid in at the start or the end of the period.
6. Read the totals and scroll the year-by-year table.

What the results show

• Final balance: the amount at the end of the whole period.
• Total deposits: the initial amount plus every regular deposit.
• Total interest: the final balance minus total deposits.
• Effective annual rate (APY): what the rate is worth over a full year once compounding is included. 5% compounded monthly is about 5.116% a year.
• Year-by-year table: deposits, interest and the end balance for each year, starting from the initial amount. A final part-year is labelled with its number of months.
• Rounding: the calculation keeps full precision and rounds each year-end balance to 2 decimal places once, so the columns and totals add up exactly.

Questions it can help with

• Savings goals: see roughly what a monthly saving could add up to.
• Comparing accounts: compare rates with different compounding using the effective annual rate.
• Rates in context: for simple percentage questions, such as how much a balance changed, use the Percentage Calculator.

Limits to keep in mind

• The rate is fixed for the whole period. Real savings rates change, and investment returns go up and down.
• Tax on interest, account fees and inflation are not included, so the real value of the final balance can be noticeably lower.
• Negative rates aren’t supported, rates are limited to 0–100%, and results too large to show to the cent are refused with a message.
• The figures are estimates for comparing options, not financial advice. Check the account terms or speak to a qualified adviser before deciding.

How the compound interest maths works

The formula without deposits

With no regular deposits, the final balance is P × (1 + r/n)^(n × t): the starting amount P, the annual rate r as a decimal, n compounding periods a year and t years. For 1,000 at 5% compounded yearly for 10 years, that’s 1,000 × 1.05^10 = 1,628.89. Compounded monthly, it’s 1,647.01. The calculator shows this sum for your numbers.

Regular deposits and compounding that don’t match

When deposits are made monthly but interest is compounded, say, quarterly, the calculator uses the standard equivalent-rate method. It converts the annual rate into the rate per deposit period, (1 + r/n)^(n/p) − 1 for p deposits a year, then adds deposits and interest one period at a time. When compounding matches the deposits, this is simply r/12 a month. With no deposits, it gives exactly the same result as the formula above.

Some banks pay simple interest between compounding dates instead, so their figures can differ slightly.

Start or end of the period

A deposit made at the start of each month earns interest for that month; one made at the end starts earning the following month. Over 10 years, 100 a month at 6% compounded monthly grows to 16,387.93 with end-of-month deposits and 16,469.87 with start-of-month deposits.

Compound Interest Calculator: common questions