Calculating your time to double is straightforward once you know the Rule of 70 formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Rule of 70 Calculator.
What is Rule of 70?
The Rule of 70 calculation tells you your time to double from a few simple inputs. The figure you are solving for here is the time to double.
The Rule of 70 formula
The core formula is:
Time to double = 70 ÷ Growth rate per periodHere is what each input means:
- Growth rate per period — a percentage, such as an annual rate. Example: 7%.
How to calculate it step by step
- Write down the growth rate per period (for example, 7%).
- Apply the formula above to get your time to double.
- Double-check the result with the Rule of 70 Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Growth rate per period | 7% |
| Time to double | 10.00 |
With growth rate per period of 7%, the time to double works out to 10.00.
Example 2
With growth rate per period of 14%, the time to double works out to 5.00.
| Result | Value |
|---|---|
| Time to double | 5.00 |
Example 3
With growth rate per period of 3.5%, the time to double works out to 20.00.
| Result | Value |
|---|---|
| Time to double | 20.00 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
- Annual rates must be converted to the period you are calculating for (for example, divide an annual rate by 12 for a monthly figure).
Prefer not to do the maths by hand? — the Rule of 70 Calculator does it instantly, for free, with the formula and a worked example built in.
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