Savings calculator
Work out what your savings grow to when you add a fixed amount every month on top of a starting balance. The calculator separates your own contributions from the interest earned.
After 10 years you will have 48231.69, of which 37000.00 is what you put in and 11231.69 is interest.
Formula
P is the starting amount, M the monthly top-up, r the rate per period and t the number of periods. The calculator compounds monthly.
How we got there
Total contributions are the starting amount plus every top-up. The difference between the result and that total is the interest.
How to work out savings growth
- Convert the rate to a monthly one
5% a year is 0.05 ÷ 12 ≈ 0.004167 a month.
- Grow the starting amount
The starting balance grows as it would under compound interest: 1000 × (1 + 0.004167)¹²⁰.
- Add the growth of the monthly top-ups
Each top-up works for less time than the one before, so they are summed with the annuity formula: M · ((1 + r)ᵗ − 1) ÷ r.
- Separate contributions from interest
Subtract total contributions from the result — what remains is what the interest earned.
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Frequently asked questions
Add the growth of the starting balance to the growth of all the top-ups. The calculator does both in one formula and shows each part.
Yes — the top-ups are monthly, so interest is added monthly too.
Because the most recent top-ups have barely had time to work. Interest accrues for however long each amount was actually invested.
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