Rule of 72 Calculator
How long money takes to double at a rate, or the rate to double in a set time.
About this tool
Choose whether you want the years to double or the rate needed. Enter the rate or the number of years and the calculator gives the rule-of-72 estimate alongside the rule of 70, the rule of 69.3 and the exact answers for annual and continuous compounding. It also shows how long money takes to triple and quadruple, and a quick-reference strip for common rates.
How the rule of 72 works
Divide 72 by the annual percentage rate to estimate how many years something takes to double. At 8% a year, 72 รท 8 = 9 years, and the exact answer with annual compounding is ln 2 รท ln 1.08 โ 9.01 years. It works backwards too: to double money in 10 years you need about 72 รท 10 = 7.2% a year, and the exact figure is 7.18%. The true constant for continuous compounding is about 69.3, so the rule of 70 or 69.3 is a little more accurate at low rates, while 72 is closest around 6โ10% and divides neatly by many numbers. The same shortcut works for anything that grows or shrinks by a steady percentage. Inflation of 3% a year halves the buying power of cash in about 24 years, and a debt at 18% doubles in about four years if nothing is paid. Honest limits: real investment returns vary year to year, so treat the rule as a rough guide, not a forecast. Not financial advice. The compound interest calculator gives exact growth with contributions, the CAGR calculator measures past growth and the inflation calculator shows the effect on prices. Compound interest, explained with real numbers explains the maths.
Frequently asked questions
What is the rule of 72?
A shortcut for doubling time: 72 รท annual rate % โ years to double. At 6% money doubles in about 12 years.
How accurate is the rule of 72?
Within a few percent for rates of about 4โ12%. At 8% it gives 9 years against an exact 9.01.
What rate doubles money in 10 years?
About 7.2% a year by the rule, or 7.18% exactly with annual compounding.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.