How to use
Enter your starting balance, how much you pay in each month, the AER or annual return you expect and how many years you’ll keep going. The final balance updates as you type.
- For a single lump sum, set the monthly payment to 0.
- For a regular saver with no starting balance, set the starting balance to 0.
- Open “Year-by-year breakdown” to see what you’ve paid in, the growth and the balance at the end of each year.
Choose how the rate works:
- AER or annual return: the default. Use it for a savings rate quoted as an AER, and for funds, ETFs and stocks and shares ISAs.
- Savings (gross rate, interest paid monthly): use it when you only know the gross rate of an account that pays interest monthly.
How it’s calculated
Each month, the balance grows by the monthly rate and then your payment is added:
balance at month end = previous balance × (1 + monthly rate) + monthly payment
- AER or annual return: monthly rate = (1 + AER)^(1/12) − 1, so the balance grows by exactly the AER each year.
- Gross rate paid monthly: monthly rate = gross rate ÷ 12. A 4% gross rate paid monthly is an AER of about 4.07%.
Examples
| Scenario | You pay in | Final balance | Growth |
|---|---|---|---|
| £10,000 once, 5% a year, 10 years | £10,000 | about £16,289 | about £6,289 |
| £10,000 plus £200 a month, 5%, 20 years | £58,000 | about £107,694 | about £49,694 |
| £20,000 in a cash ISA, 4% AER, 5 years | £20,000 | about £24,333 | about £4,333 |
| £200 a month, 5%, 30 years | £72,000 | about £163,075 | about £91,075 |
| £200 a month, 5%, 40 years | £96,000 | about £296,505 | about £200,505 |
Time makes the biggest difference. Saving £200 a month for 40 years instead of 30 adds £24,000 of payments but about £133,000 to the final balance.
About investment returns
Over long periods, shares have historically grown faster than cash savings, but past returns are no guarantee. The value of investments can fall as well as rise, and fund charges reduce what you keep. Savings with a UK bank or building society are protected by the Financial Services Compensation Scheme up to £120,000 per person, per banking licence.