How to use it
- Enter your starting amount and how much you will add each month (either can be zero).
- Enter the yearly interest rate and the number of years.
- Choose how often interest is compounded, and optionally an inflation rate to see the result in today's money.
- Read the final balance, the split between deposits and interest, and the year-by-year table, which you can download as CSV.
Worked example
Start with 10,000, add 200 a month, and earn 7% a year compounded monthly for 20 years:
- Total deposited: 10,000 + 200 × 240 months = 58,000
- Final balance: about 144,573
- Interest earned: about 86,573, more than the deposits themselves
- With 2.5% yearly inflation, the balance is worth about 88,229 in today's money
What makes the biggest difference
- Time. Interest earned in the early years keeps earning interest, so starting five years earlier often matters more than a slightly higher rate.
- Rate. A higher rate compounds faster, but higher-rate investments usually carry more risk.
- Regular deposits. Small monthly amounts add up because each one starts compounding as soon as it is added.
- Compounding frequency. Monthly versus yearly compounding makes only a small difference, as the FAQ example shows.
The rule of 72
A quick way to estimate doubling time: divide 72 by the yearly rate. At 6% money doubles in about 12 years, at 9% in about 8. It is an approximation that works best for rates between about 4% and 12%.
For a loan, where compounding works against you, see the mortgage calculator or the loan EMI calculator.
Frequently asked questions
What is the compound interest formula?
A = P × (1 + r/n)^(n×t). P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. Regular deposits are added on top, each growing for the time it stays invested.
Does compounding monthly instead of yearly make a big difference?
A small one. 10,000 at 7% for 10 years grows to about 19,672 with yearly compounding and 20,097 with monthly compounding. The rate and the time invested matter far more.
What is the rule of 72?
Divide 72 by the yearly interest rate to estimate how many years it takes to double your money. At 7% that is about 10.3 years; the exact figure is 10.2.
Why show the result after inflation?
Prices rise over time, so a balance in 20 years will buy less than the same number today. The inflation-adjusted figure divides the future balance by the cumulative inflation to show its value in today's money.
Are investment returns guaranteed?
No. This calculator assumes the same rate every year. Savings account rates change, and investments such as shares can rise and fall, so treat the result as an estimate.
Last updated · Built and checked by Moeed Hussain