About this calculator
Enter a starting amount, an annual interest rate, the number of years and how often interest is added, and see the final balance. Add a regular payment to see the effect of saving steadily.
It shows how much you paid in and how much is interest, with the working. It does arithmetic only and is not financial advice.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
£1,000 at 5% for 10 years, monthly
- Final balance
- £1,647.01
- Total paid in
- £1,000.00
- Interest earned
- £647.01
£1,000.00 at 5% a year, compounded monthly for 10 years, grows to £1,647.01, of which £647.01 is interest.
Show the working
- Interest rate per month: i = 5% ÷ 12 = 0.416667%.
- Number of months: n = 12 × 10 = 120.
- Growth factor: (1 + i)ⁿ = 1.647009.
- Your starting amount grows to £1,647.01.
- Final balance = £1,647.01.
Same, plus £100 a month
- Final balance
- £17,175.24
- Total paid in
- £13,000.00
- Interest earned
- £4,175.24
£1,000.00 plus £100.00 every month at 5% a year, compounded monthly for 10 years, grows to £17,175.24, of which £4,175.24 is interest.
Show the working
- Interest rate per month: i = 5% ÷ 12 = 0.416667%.
- Number of months: n = 12 × 10 = 120.
- Growth factor: (1 + i)ⁿ = 1.647009.
- Your starting amount grows to £1,647.01. Each month's payment (paid at the end of the month) adds £15,528.23 in total with interest.
- Final balance = £17,175.24.
£5,000 at 3% for 5 years, yearly
- Final balance
- £5,796.37
- Total paid in
- £5,000.00
- Interest earned
- £796.37
£5,000.00 at 3% a year, compounded yearly for 5 years, grows to £5,796.37, of which £796.37 is interest.
Show the working
- Interest rate per year: i = 3% ÷ 1 = 3%.
- Number of years: n = 1 × 5 = 5.
- Growth factor: (1 + i)ⁿ = 1.159274.
- Your starting amount grows to £5,796.37.
- Final balance = £5,796.37.
The compound interest formula
With a starting amount P, an annual rate r, interest added m times a year for t years, the balance is P × (1 + r ÷ m)^(m × t). With a regular payment C at the end of each period, add C × ((1 + i)ⁿ − 1) ÷ i, where i = r ÷ m and n = m × t.
Why compounding matters
Compound interest means you earn interest on your interest. £1,000 at 5% grows to about £1,647 in 10 years with monthly compounding, against £1,500 if only the original amount earned interest (simple interest). The longer the time, the bigger the gap.
What this does not include
The calculator uses a fixed rate and ignores tax, fees and inflation, and assumes payments are made at the end of each period. Real rates change, so treat the result as an illustration, not a forecast.
Frequently asked questions
What is compound interest?
Interest calculated on both your original amount and the interest already added.
How often should interest be compounded?
More often gives slightly more: monthly beats yearly, and daily beats monthly by a small amount.
Does it include regular payments?
Yes: enter an amount in the last box. It is paid in at the end of each compounding period.
Is this financial advice?
No. It is a calculator that does the arithmetic with the numbers you enter.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.