How to use it
- Choose what to find: the monthly deposit, or the time your deposit needs.
- Type the savings goal and the money you have saved so far.
- Enter the yearly interest rate and, for a set time, the number of years. For a set deposit, enter the monthly amount.
- Read the result, then open the year-by-year table, or use Download as CSV to keep it in a spreadsheet.
How the monthly deposit is calculated
Interest is added to the balance each month at one twelfth of the yearly rate, and each deposit is made at the end of the month. The starting amount grows with the same rate, so the calculator first finds how much of the goal it reaches on its own. The monthly deposits must cover the rest, together with the interest they earn.
The deposit is rounded up to the cent, so the balance is at least the goal in the last month.
d = (G − S × (1 + i)^N) × i ÷ ((1 + i)^N − 1)
d monthly deposit
G savings goal
S saved so far
i monthly rate = yearly rate ÷ 12
N number of months = years × 12- At a rate of 0% the deposit is the gap to the goal divided by the number of months.
- If the saved amount already grows to the goal, the deposit is 0.
How the time is calculated
When the monthly deposit is known, the number of months solves the same formula the other way round. The result is rounded up to whole months, so the goal is reached within that month.
No time exists when the goal is never reached: there is no deposit, and the saved amount does not grow because the rate is 0% or nothing is saved.
N = ln((G + d ÷ i) ÷ (S + d ÷ i)) ÷ ln(1 + i)
N months to reach the goal
d monthly deposit
i monthly rate = yearly rate ÷ 12What this calculator does not include
The rate is assumed to stay the same for the whole period, and the interest is taken as it is credited. Tax on interest, fees and minimum balances are not deducted, so enter the rate you expect after tax. The goal is a fixed amount: inflation is not included unless you plan a higher goal yourself.
- Deposits that are not made once a month.
- A rate that changes during the period. Run the calculator again from the balance you reached, with the new rate.
Worked example: 100,000 goal, 10,000 saved, 8% for 10 years
The monthly rate is 8% ÷ 12 = 0.667%, and there are 120 months. The formula gives a deposit of 425.28 a month, which is rounded up to 425.29.
Over the 10 years, 10,000 + 120 × 425.29 = 61,034.80 is deposited. The balance reaches 100,001.52, so the interest earned is 38,966.72.
With 500 a month instead, the same goal is reached after 109 months: 9 years and 1 month.
| Figure | |
|---|---|
| Monthly deposit for 10 years | 425.29 |
| Deposited in 10 years | 61,034.80 |
| Interest earned in 10 years | 38,966.72 |
| Time with 500 a month | 9 years 1 month |
Frequently asked questions
Why is the monthly deposit rounded up?
A deposit rounded down by a cent could leave the balance a little short of the goal. Rounding up to the cent makes sure the goal is reached.
What if I already have some savings?
Enter them as Saved so far. They grow at the same rate as the deposits, so they lower the monthly deposit you need.
Should the interest rate be before or after tax?
Enter the rate you expect after tax, because the calculator does not take tax off the interest. Tax on interest differs by product and by country.
Why can the time be longer than I expect?
Interest is earned on the balance, so the early months add little and most of the growth comes in the later years. The time is therefore sensitive to the rate, so test a lower rate as well.
Does the goal include inflation?
No. The goal is a fixed amount of money. To plan for inflation, enter a higher goal, or use the compound interest calculator with an inflation rate.
Is my data stored?
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.