How the calculation works
Compound interest means each period's interest is added to the balance, so the next period earns interest on a larger amount. The calculator steps through time. Between events it accrues interest on the current balance; on every compounding date the accrued interest is added to the balance; deposits and withdrawals are applied on their own dates.
For the common case of monthly deposits at the end of the month, the result matches this closed form:
i = (1 + r / n)n/12 − 1
Balance = A × (1 + i)m + P × ((1 + i)m − 1) / i
A = starting amount · P = monthly deposit · r = nominal annual rate as a decimal · n = compounding periods per year · m = number of months. With a 0% rate the balance is A + P × m.Worked example
Start with 10,000, deposit 500 a month at the end of each month, earn 7% a year compounded monthly, for 20 years (m = 240).
- Monthly rate: i = 0.07 / 12 = 0.005833.
- Growth factor: (1 + i)240 = 4.0387.
- Starting amount grows to 10,000 × 4.0387 = 40,387.
- Deposits grow to 500 × (4.0387 − 1) / 0.005833 = 260,463.
- Balance: 40,387 + 260,463 = 300,851 (rounded). You put in 130,000; the other 170,851 is interest.
The rates shown
- Nominal annual rate: the rate you entered, converted to a yearly figure (for example 0.5% a month is 6% a year).
- Effective annual rate: what the nominal rate becomes after compounding, (1 + r / n)n − 1.
- Return on money put in: interest earned divided by the starting amount plus all deposits.
- Time to double: how long the effective rate alone takes to double a sum, ignoring deposits and withdrawals.
What this calculator does not include
- Taxes and fees. Real returns are lower after they apply.
- A rate that changes over time. Investment returns vary from year to year and are not guaranteed.
- Inflation, unless you enter a rate. It is then applied as a constant yearly rate.
- Currency conversion. The symbol you pick only changes how amounts are shown.
Amounts are rounded to whole units for display only; the calculation itself is not rounded. Last updated: 11 November 2026.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus interest already earned, so growth accelerates over time.
How often should interest compound?
Use the frequency your account states. More frequent compounding gives a slightly higher result, but the difference is small compared with the effect of the rate and the number of years.
Should deposits be at the start or the end of the period?
Choose the one that matches how you pay in. Start-of-period deposits earn interest for one extra period, so the result is a little higher.
How do percentage withdrawals work?
A percentage of the balance takes that share of the whole balance each period. A percentage of earnings takes that share of the interest earned since the previous withdrawal. A fixed amount takes the same sum each period and stops when the balance is gone.
Does the result include tax?
No. The result is before tax and fees. Inflation is only included if you enter a rate.