Rule of 70 Calculator

The Rule of 70 Calculator gives you a fast estimate of how long it takes something to double in size when it grows at a constant annual percentage rate. Enter a growth rate — whether that’s an investment return, an inflation rate, or a population growth rate — and the calculator tells you roughly how many years it will take for that quantity to double.

It’s a tool used by investors sizing up potential growth, students learning about compound growth, and anyone who wants a quick mental model for how a steady percentage rate compounds over time — from a savings account to the price of groceries.

What Is the Rule of 70?

The Rule of 70 is a mental-math shortcut for estimating how long it takes a value to double when it grows at a constant annual percentage rate. Rather than working through the full compound-growth math, you divide 70 by the growth rate to get a close approximation in years:

Simple example: at a 7% annual growth rate, doubling time ≈ 70 ÷ 7 = 10 years. That means a value growing steadily at 7% per year would take roughly a decade to double in size.

The number 70 works as a shortcut because of how compound growth behaves mathematically — it’s derived from the natural logarithm of 2, rounded to a number that’s easy to divide in your head. The result is an approximation, not an exact figure, which is covered in more detail further down this page.

How to Use the Rule of 70 Calculator

The calculator above takes two inputs and returns several related results:

Example using the calculator’s own default numbers: Growth Rate 3.5%, Starting Value $1,000 → Doubling Time (Rule of 70) = 20.0 years, Exact Doubling Time ≈ 20.15 years, Approximation Error ≈ 0.74%, Doubled Value = $2,000.

Rule of 70 Formula

In practical terms: at 2% growth — a rate often associated with long-run inflation targets — a value takes 35 years to double, reflecting how slowly small percentage rates compound. At 5%, roughly the historical range of many conservative investment returns, doubling takes 14 years. At 10%, a rate sometimes cited for long-term stock market averages, doubling happens in just 7 years — illustrating how sensitive doubling time is to the growth rate itself.

Rule of 70 Examples

The Rule of 70 can be applied to any quantity that grows or shrinks at a steady percentage rate — the same formula works whether the number represents growth or inflation, since both describe compounding change over time.

1. Investment Growth

2. Inflation

3. Population Growth

4. Economic Growth

Rule of 70 for Investments

Investors sometimes use the Rule of 70 as a quick way to sanity-check how a given annual return might compound over time. Applying it to a range of typical return assumptions:

Rule of 70 and Inflation

The same formula can estimate roughly how long it could take for prices to double at a given inflation rate — which is really asking the reverse question of investment growth: how long until your money’s purchasing power is cut in half.

When prices double, the purchasing power of a fixed amount of money is effectively cut in half — the same dollar buys roughly half as much as it did before. Inflation is not necessarily constant from year to year, so this is a useful way to think about long-run trends rather than a precise forecast for any specific future period. For a deeper look at price-level changes, see the Inflation Calculator and Inflation Rate Calculator.

Rule of 70 vs. Rule of 72

The Rule of 70 and Rule of 72 are both quick-estimate shortcuts for doubling time, and they work the same way — divide the constant by the growth rate. They differ only in which constant they use.

72 is often more convenient for mental math because it divides evenly by 2, 3, 4, 6, 8, 9, and 12 — common growth-rate figures. 70, meanwhile, tends to produce a result slightly closer to the exact mathematical answer for lower growth rates, which is part of why it’s frequently used in economics and inflation contexts. In practice, both are approximations meant for quick estimates, not for precise financial planning — neither should be treated as an exact calculation. If you’d like a side-by-side comparison of your own numbers, the site also has a dedicated Rule of 72 Calculator.

Rule of 70 vs. Exact Doubling Time

The Rule of 70 is an approximation. The precise mathematical formula for doubling time under compound growth uses natural logarithms:

At a 5% growth rate, for example, the Rule of 70 gives 14.0 years, while the exact calculation gives about 14.21 years — a difference of roughly 1.45%. The gap grows slightly wider at higher growth rates: at 10%, the Rule of 70 estimates 7.0 years versus an exact 7.27 years, a difference of about 3.75%.

Despite this gap, the Rule of 70 remains useful precisely because it’s fast and easy to calculate without a calculator or spreadsheet — for most everyday growth rates (roughly under 10%), the approximation stays close enough to the exact figure to be a reliable quick estimate, which is exactly the situation it was designed for.

How Accurate Is the Rule of 70?

The Rule of 70 tends to be most accurate at lower, single-digit growth rates — the kind typically seen with long-run inflation or conservative investment returns. As the growth rate increases, the approximation error grows larger, since the underlying math (logarithmic compounding) departs further from the simple linear “70 ÷ rate” shortcut at higher rates.

Just as important: the result depends entirely on the assumption of a relatively constant growth rate over the full doubling period. Real-world growth rates — whether for investments, inflation, or population — rarely stay perfectly constant for a decade or more, so any Rule of 70 estimate should be read as a reasonable ballpark figure rather than a guaranteed timeline.

Common Uses of the Rule of 70

• Investment planning — quickly estimating how long a portfolio might take to double at an assumed return.
• Inflation analysis — gauging how fast purchasing power could erode at a given inflation rate.
• Population growth — estimating how long it takes a population to double at a steady growth rate.
• Economic growth — projecting how long it takes GDP or economic output to double.
• Business growth — estimating how long it could take revenue or a customer base to double at a steady growth rate.
• Understanding compound growth — building intuition for how percentage rates compound over time.
• Comparing different growth rates — quickly seeing how sensitive doubling time is to small changes in rate.

Frequently Asked Questions

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