Tinker 
How to use this calculator
Type your starting balance, how much you plan to add each month, the annual interest rate and how many years you will leave the money invested. Choose how often interest is compounded: your account terms usually state this. The final balance, total deposited and interest earned update instantly.
The formula
The starting amount grows as P × (1 + r/n)^(n × t), where r is the annual rate as a decimal, n is the number of compounding periods per year and t is the number of years.
Monthly deposits are treated as an annuity paid at the end of each month. The calculator converts the rate into an equivalent monthly rate, m = (1 + r/n)^(n/12) − 1, and adds C × ((1 + m)^months − 1) / m. Interest earned is the final balance minus everything you deposited.
Worked example
Say you deposit nothing up front and add $100 at the end of every month for one year at 12% compounded monthly. The monthly rate is 1%, so the balance is 100 × (1.01^12 − 1) / 0.01 = $1,268.25. You put in $1,200, so $68.25 is interest.
With $1,000 up front and no deposits at the same rate, you would have 1,000 × 1.01^12 = $1,126.83 after a year.
Practical tips
Starting earlier matters more than a slightly higher rate, because compounding accelerates over long periods. Results are estimates: real returns vary, and fees, taxes and inflation are not included. For a realistic picture of buying power, try entering a rate reduced by expected inflation.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on interest already earned, so the balance grows faster over time.
Does daily compounding make a big difference?
Only a little. At 6%, monthly compounding gives an effective 6.17% a year and daily gives about 6.18%. The rate and time period matter far more.
Are deposits added at the start or end of each month?
This calculator assumes the end of each month, which is the standard convention. Deposits made at the start would earn slightly more.
Can I use it for any currency?
Yes. The maths is the same for any currency; just enter all amounts in the same one.