How to use it
- Type the starting amount and the regular deposit, then choose whether you deposit monthly or yearly.
- Enter the yearly interest rate and choose how often the interest is added to the balance.
- Enter the number of years. Add inflation to see the value in today's money, and a tax rate if the interest is taxed.
- Read the balance at the end, then open the year-by-year table, or use Download as CSV to keep it in a spreadsheet.
How the balance is calculated
Compound interest is interest paid on the balance, including the interest that was added earlier. Each period the balance grows by its rate, so the growth itself grows. The formula below gives the balance at the end when you start with an amount and add a deposit each period.
The rate per period is worked out from the yearly rate, the number of times the interest is added and the number of deposits in a year. With no tax, the balance is the sum of the starting amount grown over all periods and the deposits, each grown from the period it was made.
FV = P × (1 + i)^N + C × ((1 + i)^N − 1) ÷ i
FV balance at the end
P starting amount
C deposit each period
i rate earned over one deposit period
N number of deposit periods = years × deposits a year- With a rate of 0% the balance is the starting amount plus all deposits, with no interest.
- Deposits at the start of a period earn one more period of interest. Choose the timing that matches your account.
When adding and depositing differ
A yearly rate cannot simply be divided when the interest is added more often than you deposit, or the other way round. The calculator first finds the rate earned over one deposit period: i = (1 + r ÷ m)^(m ÷ p) − 1, where r is the yearly rate, m the number of times the interest is added in a year and p the number of deposits in a year.
For example, 12% added monthly and deposited once a year earns (1 + 0.12 ÷ 12)^12 − 1 = 12.68% over the year. Entering 12% directly would understate the balance.
Tax and inflation
Tax is taken from each year's interest and paid out of the account, so less interest is added and the next year's interest is smaller. The tax rate you enter is applied to every year; tax rules often have exemptions, limits or a different rate for each product, so enter the rate that applies to you.
Inflation does not change the balance. The value in today's money is the balance divided by (1 + inflation) for each year, which shows how much the money will buy.
What this calculator does not include
- Fees, minimum balances and the cost of an account.
- Changes of rate during the years. Run the calculator again with the new rate and the balance you reached.
- Taxes on gains from shares, funds or property, which are calculated on the sale and not on each year's interest.
Worked example: 10,000 start, 500 a month, 8% for 10 years
The rate is 8% a year added monthly, so each month earns 8% ÷ 12 = 0.667%. Over 10 years there are 120 monthly deposits of 500, which is 60,000 deposited, plus the starting 10,000: 70,000 in total.
The formula gives a balance of 113,669.42 at the end, so the interest earned is 43,669.42. With 2% inflation each year, the same balance is worth 93,248.52 in today's money.
| Figure | |
|---|---|
| Balance at the end | 113,669.42 |
| Total deposited | 70,000.00 |
| Interest earned | 43,669.42 |
| Value in today's money (2% inflation) | 93,248.52 |
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on the starting amount. Compound interest is added to the balance, so each period earns interest on the interest of earlier periods too, and the difference grows with time.
Does it matter how often the interest is added?
Yes, but less than the rate itself. At 8% a year, daily adding gives an effective yearly rate of 8.33% and monthly adding gives 8.30%, so the difference over one year is small.
Why does the balance grow faster than the deposits?
Each deposit earns interest from the day it is added, and the interest earns interest as well. Over a long term the interest can become larger than the total you deposit, as the chart shows.
What does the value in today's money mean?
It is the balance expressed in the money of today. At 2% inflation a year, 100 in ten years buys about what 82 buys today, so the balance in today's money is lower than the figure at the end.
Is the result guaranteed?
No. The calculator assumes one rate for the whole period. Returns on shares, funds and savings accounts change, and banks can change their rates, so the result is an estimate for planning only.
Are my figures sent to a server?
No. The calculation runs in your browser, and nothing you enter is sent to us.
Sources
Figures and text last checked on Tuesday, November 10, 2026.