Rule of 70 Calculator
The Rule of 70 is a simple shortcut for estimating how long it takes money, earnings or even prices to double. It is similar to the Rule of 72, but many people prefer it for lower growth assumptions or when thinking about inflation and economic growth.
How it works
At 5%, the Rule of 70 gives about 14 years. At 2% inflation, it gives about 35 years for prices to double.
When it is useful
- Lower growth or inflation-style estimates
- Quick mental math in personal finance
- Comparing different growth assumptions side by side
Rule of 70 vs Rule of 72
| Shortcut | Best use | Main idea |
|---|---|---|
| Rule of 70 | Lower growth rates, inflation, simple intuition | Slightly cleaner for slower growth assumptions. |
| Rule of 72 | Common investing returns | Widely used for rates like 6%, 7% and 8%. |
Best related pages
Where the Rule of 70 beats the Rule of 72
Both rules estimate doubling time by dividing a constant by the percentage growth rate. They differ only in the constant, and each is accurate over a different range. The Rule of 70 owns the low end.
| Annual return | Exact doubling time | Rule of 72 says | Rule of 70 says | Rule of 72 error | Rule of 70 error |
|---|---|---|---|---|---|
| 2% | 35.00 years | 36.00 | 35.00 | +2.8% | -0.0% |
| 3% | 23.45 years | 24.00 | 23.33 | +2.3% | -0.5% |
| 4% | 17.67 years | 18.00 | 17.50 | +1.9% | -1.0% |
| 5% | 14.21 years | 14.40 | 14.00 | +1.4% | -1.5% |
| 6% | 11.90 years | 12.00 | 11.67 | +0.9% | -1.9% |
| 7% | 10.24 years | 10.29 | 10.00 | +0.4% | -2.4% |
| 8% | 9.01 years | 9.00 | 8.75 | -0.1% | -2.8% |
| 10% | 7.27 years | 7.20 | 7.00 | -1.0% | -3.7% |
| 12% | 6.12 years | 6.00 | 5.83 | -1.9% | -4.6% |
| 15% | 4.96 years | 4.80 | 4.67 | -3.2% | -5.9% |
| 20% | 3.80 years | 3.60 | 3.50 | -5.3% | -7.9% |
Exact figures use annual compounding: t = ln(2) / ln(1 + r).
At 2% the Rule of 70 is accurate to 0.01% — effectively exact. At 3% it is within 0.5%. By 10% it is understating doubling time by 3.7%, and the Rule of 72 has become the better shortcut. That crossover is why economists reach for 70 and investors reach for 72: they work with different rates.
The exact answer is ln(2) / ln(1 + r), and ln(2) is about 0.693 — so 69.3 is the theoretically correct numerator when the rate is very small and ln(1 + r) is close to r. 70 is that constant, rounded to something divisible. 72 adds a deliberate upward fudge that happens to cancel the curvature of the logarithm at around 8%, and has more convenient divisors.
What the Rule of 70 is normally used for
- Inflation. At 3% inflation, prices double in about 23 years — the fastest way to see why a fixed nominal income shrinks. See compound interest vs inflation.
- Population and economic growth. The rates involved are usually low single digits, which is precisely the range the rule is accurate over.
- Low-rate savings. A 2% account doubles in about 35 years, which is a more honest way to describe it than quoting the rate.
- Any doubling question, in reverse. Divide 70 by the number of years you want to double in, and you get the rate you need.
For investment returns in the 6–10% range, the Rule of 72 is the better shortcut, and the doubling time calculator gives the exact figure when the approximation is not good enough.
Questions about the Rule of 70
What is the Rule of 70?
Divide 70 by the annual percentage growth rate to estimate how many years something takes to double. At 2% that is 35 years; the exact answer is 35.00 years.
Is the Rule of 70 more accurate than the Rule of 72?
At low rates, yes. Below roughly 5% the Rule of 70 is closer; above roughly 8% the Rule of 72 is. They cross in between, which is why both survive.
Why 70 and not 69.3?
69.3 is the mathematically correct constant for very small rates, since ln(2) ≈ 0.693. 70 rounds it to a number with far more convenient divisors, at a cost in accuracy too small to matter for mental arithmetic.
Can I use the Rule of 70 for inflation?
That is its most common use. At 3% inflation, prices double in roughly 23 years, meaning something costing $100 today costs about $200 then. The purchasing power of a fixed sum halves over the same period.