Doubling-time calculator
Rule of 72 Calculator
The Rule of 72 is a shortcut that estimates how many years it takes money to double at a given annual return. It is not perfect, but it is fast, memorable and surprisingly useful when you want rough intuition before running a full projection.
Rule of 72 estimate in years to double
—
Exact compounding result
—
The exact result uses the compound growth formula rather than the shortcut.
How the Rule of 72 works
Years to double ≈ 72 ÷ annual return (%)
At 7%, the shortcut gives about 10.3 years. The exact compound result is about 10.24 years. That small gap is why the rule is useful for intuition but not a replacement for a full calculator.
Why people use it
- It turns abstract percentages into a time estimate you can feel.
- It helps compare return assumptions quickly.
- It is good for rough planning conversations before detailed modelling.
Where it can mislead you
| Situation | Why the shortcut is weaker |
|---|---|
| Very low or very high rates | The shortcut becomes less precise outside the middle range. |
| Monthly contributions | The rule does not account for new deposits being added along the way. |
| Fees, taxes or inflation | Your real doubling time may be longer than the headline estimate. |
Best related pages
When to switch to the main calculator
Use the Rule of 72 for mental math and fast comparisons. Switch to the main compound interest calculator when you want to model a real scenario with starting balance, monthly contributions, contribution timing and a year-by-year breakdown.