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Compound Interest Calculator
See how savings grow with compound interest and regular contributions — with the effective rate, a chart, a year-by-year table and the value in today's money.
A projection that assumes a constant rate and regular contributions. Real returns vary, and taxes and fees aren't included. Not financial advice.
Year by year
The formula and the model
Without contributions
A = P × (1 + r/n)^(n × t)P is the starting balance, r the nominal annual rate, n the compounding periods per year and t the years. $1,000 at 10% compounded once a year for one year: 1,000 × 1.1 = $1,100.
With regular contributions
Each deposit starts earning interest from the day it's made. Interest accrues in proportion to time within each compounding period and is added to the balance at the end of the period. When deposits and compounding happen at the same frequency this gives exactly the standard annuity formulas; when they differ — say monthly deposits into a yearly-compounding account — it stays consistent instead of assuming the rate divides neatly by 12.
Frequently asked questions
What is compound interest?+
Interest earned on both your money and the interest it has already earned. $1,000 at 10% a year becomes $1,100 after one year and $1,210 after two, because the second year's interest is also paid on the first year's $100.
What's the difference between a nominal and an effective rate?+
The nominal rate is the headline annual rate. The effective annual rate includes compounding within the year: 6% compounded monthly is (1 + 0.06/12)^12 − 1 = 6.17% effective. Enter the nominal rate and the calculator shows the effective one.
How are monthly contributions handled if interest compounds yearly?+
Interest builds up day by day in proportion to how long each deposit has been in the account, and is added at each compounding date. A deposit made halfway through the year earns half a year's interest — it isn't treated as if the rate were split into twelve monthly steps.
Should contributions be at the start or end of each period?+
Choose start if you pay in at the beginning of each month (for example on payday), end if you pay in at the end. Deposits at the start earn slightly more because they're invested longer.
What does “in today's money” mean?+
The final balance divided by the price rise from the inflation rate you enter, compounded over the years. It shows roughly what the future amount would buy now — it depends entirely on your inflation assumption.
Is this a guarantee or advice?+
No. It's a projection with a constant rate. Real savings and investment returns change over time, and taxes and fees aren't included.