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Compound Interest Calculator
Enter a principal, annual rate, time and compounding frequency to see the future value and exactly how much of it is earned interest. The maths runs entirely in your browser.
The compound formula
A = P(1 + r/n)nt: each period the interest itself starts earning interest. That is why compounding frequency matters — monthly compounding beats yearly at the same headline rate.
The rule of 72
Divide 72 by your annual rate for a quick estimate of doubling time: at 6%, money doubles in about 12 years; at 9%, about 8 years.
Before tax and inflation
The result is nominal. After tax and inflation, real purchasing power grows slower — a useful mental haircut is 2–3% a year.
Frequently Asked Questions
What does compounding frequency change?
How often interest is added to the balance. Daily compounding earns slightly more than monthly, which earns more than yearly — at the same annual rate.
Simple vs compound interest?
Simple interest pays only on the original principal. Compound interest pays on principal plus all interest earned so far — the gap explodes over long periods.
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