📊 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.
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%.
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.