Compound Interest Calculator
Enter a starting amount, a monthly deposit, an interest rate and a period. Get the final balance, the interest earned and a year-by-year table you can download.
These figures are estimates only and are not advice. The result assumes a fixed rate, no tax on interest, no fees and no withdrawals. Real accounts change their rates and may pay interest differently.
Worked examples
A lump sum with monthly deposits
Starting amount £1,000, monthly deposit £100, rate 5 per cent, 1 year, monthly compounding, deposits at the end of each month
Balance £2,279.05 after one year, from £1,000 plus £1,200 of deposits.
The £79.05 gained is interest, because each month's interest is added before the next interest is worked out.
Regular saving over ten years
Starting amount £0, monthly deposit £50, rate 4 per cent, 10 years, monthly compounding
Balance £7,362.49 after 10 years, from £6,000 of deposits.
The balance grows from £6,000 of deposits to £7,362.49, and the £1,362.49 difference is the interest earned.
About Compound Interest Calculator
This compound interest calculator shows how a lump sum and regular monthly deposits can grow over time. Enter a starting amount, a monthly deposit, an annual interest rate and a period in years and months. You get the final balance, the total you deposited and the interest earned, followed by a table that shows every year in turn. It is useful for savings accounts, cash ISAs and any investment where you expect a steady rate of growth, and the figures update as you type.
You can choose how often interest is added: monthly, yearly, half-yearly, quarterly, weekly or daily. You can also decide whether deposits go in at the start or the end of each month. The year-by-year table lists the opening balance, deposits, interest and closing balance for every year, and a button downloads it as a CSV file you can open in a spreadsheet. It is a quick way to see why starting early and keeping going matter. Try changing the rate by a point or two to see how much it matters.
How to use Compound Interest Calculator
- Enter your starting amount and monthly deposit.
- Type the annual interest rate and the number of years and extra months.
- Choose how often interest is added and when deposits go in.
- Read the totals and the year-by-year table, then download the CSV if you need it.
Common uses
- Projecting a savings account or cash ISA balance
- Comparing different monthly deposit amounts
- Seeing the effect of monthly against yearly compounding
- Building a table for a spreadsheet or a family budget
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest already added. Because each period's interest becomes part of the balance, growth speeds up over time, which is why the same deposits can produce much more over thirty years than over ten.
How is the balance calculated here?
The annual rate is split by the compounding frequency you choose to give the growth each period. Each month the balance grows by the matching amount and your deposit is added, at the start or end of the month depending on your choice. The year table adds the months up.
Does compounding frequency make a big difference?
Usually a small one. Daily compounding gives slightly more than monthly, and monthly slightly more than yearly, at the same stated rate. The rate, the amount you deposit and the length of time have a much bigger effect on the final balance.
Does the result include tax, fees or changing rates?
No. It assumes a fixed rate, no tax on interest, no charges and no withdrawals. Real accounts can change their rates, and interest above your allowance may be taxable. This is an estimate only and is not advice.
What is in the CSV download?
The file holds the same year-by-year table you see on screen: the period, opening balance, deposits, interest and closing balance, with figures to the penny. It opens in any spreadsheet program, so you can chart it or add your own columns.