📈 Compound Interest Calculator
Compound interest is why saving pays off so much over the long run: you earn interest not just on your deposit, but on the interest from previous years too. This calculator shows what your savings are worth after a chosen period, including an optional fixed monthly deposit.
Final value
£31,998.32
Total deposited
£22,000.00
Total interest
£9,998.32
Find the monthly rate: 5% ÷ 12 = 0.4167%
Calculate the growth factor over the term: (1 + 0.004167)^120 = 1.647
Starting amount times growth factor, plus the deposits with interest: = £31,998.32
Formula
FV = P × (1 + i)ⁿ + PMT × (((1 + i)ⁿ − 1) ÷ i)
The first part calculates what your starting amount alone becomes. The second part adds up the growth of each monthly deposit separately, because a deposit in year 1 has longer to grow than one in year 9.
What the variables mean
- P
- Starting amount — €
- PMT
- Monthly deposit — €
- i
- Monthly rate (annual rate ÷ 12 ÷ 100)
- n
- Number of months
tips
- Starting early outweighs depositing more later — time is the most powerful factor in compound interest.
warnings
- This is an indicative calculation with a fixed rate. Real savings and investment returns fluctuate year to year.
commonMistakes
- Using the annual rate as the monthly rate. Always divide by 12 first.
- Forgetting inflation erodes the purchasing power of the final amount; use a real (post-inflation) rate if relevant.
useCases
- Estimating what a savings account yields over 10 or 20 years.
- Visualising the effect of starting to save earlier.