📊 Compound Interest Calculator
Future value = P × (1 + r/n)^(n×t), money grows on its own interest, which is why 8% for 30 years turns 1 into 10.06. Enter your values below, results update instantly, entirely on your device.
100,000 × (1 + 8%/1)^(1×10)
future value ÷ principal
How the compound interest calculator works
FV = P × (1 + r/n)^(n×t), where r is the yearly rate as a decimal, n the compounding periods per year and t the years. More frequent compounding raises the effective yield slightly. Raising (1 + r/n) to the power n×t compounds every period, so the interest itself starts earning interest, and the gap over simple interest widens the longer the money is left untouched.
Example: 100,000 at 8% for 10 years, yearly compounding → 215,892 (interest 115,892).
The rule of 72 gives the doubling time in your head: 72 ÷ rate ≈ years to double. At 8%, money doubles roughly every 9 years, so 10× in 30 years isn’t magic, it’s three doublings plus change.
Frequently asked questions
What is the compound interest formula?
FV = P(1 + r/n)^(nt). For 5,000 at 6% monthly-compounded for 3 years: 5,000 × (1 + 0.06/12)^36 = 5,983.
What is the rule of 72?
Divide 72 by the annual rate to estimate doubling time: at 6%, ~12 years; at 12%, ~6 years. Accurate within a few percent for rates between 4% and 15%.
How much difference does compounding frequency make?
Less than most expect: 8% for 10 years grows 100,000 to 215,892 yearly-compounded vs 222,196 monthly-compounded, about 3% more. Rate and time dominate; frequency fine-tunes.
What is the difference between compound and simple interest?
Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus previously-earned interest, so it accelerates over time, the "interest on interest" effect.
What is the rule of 72?
A quick way to estimate doubling time: divide 72 by the annual percentage rate. At 8% money roughly doubles in 72 ÷ 8 = 9 years. It's an approximation that works best for rates between about 6% and 10%.
What is the effective annual rate?
It is the true yearly growth once compounding is included: EAR = (1 + r/n)^n − 1. A nominal 12% compounded monthly works out to (1 + 0.01)^12 − 1 = 12.68% effective.
How long to double my money at 6%?
By the rule of 72, about 72 ÷ 6 = 12 years. The exact figure from the formula is ln(2) ÷ ln(1.06) ≈ 11.9 years, so the shortcut lands very close.
Is this compound interest calculator accurate and private?
Yes. It uses the standard published formula, shows its working under every result, and computes locally in your browser, your inputs are never sent to a server, and the page works offline.