Rule of 72 — Doubling Time Calculator
By Santaji GadepatilinUpdated October 27, 2026
The rule of 72 estimates how long money takes to double: divide 72 by the annual interest rate. At 8%, money doubles in about 9 years. This free doubling time calculator solves for whichever value you leave blank and shows the formula and steps.
Exact years to double (years) vs Annual return (%) — other inputs held at your current values.
How to use the doubling time calculator
Enter an annual return to see the exact years needed to double your money and the Rule of 72 estimate. Or press × on the return and type how many years you want — the required rate is calculated.
Doubling time formulas
Exact: t = ln 2 ÷ ln(1 + r)
Rule of 72: t ≈ 72 ÷ (r × 100)
Comparison table
| Return | Exact years | Rule of 72 |
|---|---|---|
| 4% | 17.7 | 18 |
| 6% | 11.9 | 12 |
| 8% | 9.0 | 9 |
| 10% | 7.3 | 7.2 |
| 12% | 6.1 | 6 |
Time to double, triple and grow 10×
| Return | Double (years) | Triple | 10× |
|---|---|---|---|
| 3% | 23.4 | 37.2 | 77.9 |
| 5% | 14.2 | 22.5 | 47.2 |
| 7% | 10.2 | 16.2 | 34.0 |
| 9% | 8.0 | 12.7 | 26.7 |
| 12% | 6.1 | 9.7 | 20.3 |
| 15% | 5.0 | 7.9 | 16.5 |
For tripling there is a similar shortcut, the rule of 114 (114 ÷ rate). For 10× growth use about 230 ÷ rate.
Rule of 69 and 70
The rule of 72 is easiest to use because 72 divides evenly by many numbers. For other cases:
- Rule of 70: slightly more accurate for low rates (1–5%), often used for population and GDP growth.
- Rule of 69.3: exact for continuous compounding, because ln 2 ≈ 0.693.
Inflation halves your money too
At 6% inflation, prices double in about 12 years, which means your money’s buying power halves in that time. If your savings earn less than inflation, they are losing value even as the balance grows.
Real-life uses
- Investing: how long until a lump sum doubles at a given return.
- Debt: a credit card at 24% APR doubles an unpaid balance in about 3 years.
- Business: how long until sales double at a given growth rate.
- Population: a 2% yearly growth rate doubles a population in about 35 years.
Doubling again and again
At 8%, 10,000 becomes 20,000 in 9 years, 40,000 in 18 years and 80,000 in 27 years. Each extra 9 years doubles it again — the essence of compounding.
Frequently asked questions
What is the Rule of 72?
A shortcut to estimate doubling time — divide 72 by the annual percentage return. At 8%, money doubles in about 72 ÷ 8 = 9 years.
How accurate is it?
Very close for rates between about 6% and 10%. It slightly overestimates at low rates and underestimates at high ones.
Does it work for inflation too?
Yes. At 6% inflation, prices double in about 12 years — so money halves in value in the same time.
How many years does it take to double money at 7%?
Exactly 10.24 years with yearly compounding. The rule of 72 gives 72 ÷ 7 ≈ 10.3 years.
What return do I need to double my money in 5 years?
About 14.87% a year. The rule of 72 gives 72 ÷ 5 = 14.4%.