Enter a growth rate, and you'll see exactly how long it takes for a value to double. That's the whole idea behind the Rule of 72 — a 500-year-old mental-math shortcut that turns compound growth into something you can estimate in your head. Whether you're sizing up an investment, figuring out how fast inflation is chipping away at your savings, or gut-checking a credit card balance, this calculator gives you a clean answer in seconds.
One input, one result, no spreadsheet required. And by the time you've scrolled through this page, you'll be able to run the same math without the calculator at all.
What Is the Rule of 72?
The Rule of 72 answers one specific question: at a fixed rate of compound growth, how long does something take to double?
The answer is always the same formula — divide 72 by the growth rate. That's it. If your money earns 8% a year, it doubles in about 9 years. If inflation runs at 3%, your purchasing power halves in about 24 years. If a startup grows users 12% a month, the user base doubles roughly every 6 months.
It works because compound growth is counterintuitive. Ask most people how long $10,000 takes to double at 7% a year, and you'll get a shrug or a wild guess. Ask someone who knows the Rule of 72, and they'll tell you "about ten years" before you finish the sentence. That speed is the whole point.
The rule traces back to 1494, when Italian mathematician Luca Pacioli — the same monk who standardized double-entry bookkeeping — casually dropped it into a section of his treatise Summa de Arithmetica. He didn't invent the math, but he was the first to write it down as a handy trick. Five centuries later, it's still how professional investors sketch returns on a napkin.
How to Use This Calculator
- Enter your growth rate in the "Increase" field. Use the percentage itself, not the decimal form — so enter 7 for 7%, not 0.07.
- Read the doubling time. The result appears instantly below the input.
- Mind your units. The calculator doesn't care whether your rate is annual, monthly, or quarterly — but the result comes back in the same unit. Feed it an annual rate, you get years. Feed it a monthly rate, you get months.
No submit button, no sign-up, no fine print. As soon as you change the rate, the doubling time updates.
The Formula Behind the Calculator
Here's what's happening under the hood:
Doubling time ≈ 72 ÷ growth rate (%)
So why 72, and not some other number? It's a close approximation of a more complex formula involving logarithms. The mathematically exact version looks like this:
Exact doubling time = ln(2) ÷ ln(1 + r)
Where r is the growth rate written as a decimal. Since ln(2) is about 0.693, and for small rates ln(1 + r) behaves roughly like r, the exact answer is close to 69.3 ÷ rate. But 69.3 is a miserable number to divide by in your head. 72, meanwhile, is divisible by 2, 3, 4, 6, 8, 9, and 12 — making almost every common interest rate snap cleanly into whole-year answers. Mental-math-friendliness beat mathematical purity, and 72 won the day.
For the rates most people actually care about — roughly 5% to 12% — the error is under half a year. That's close enough for napkin math.
Practical Examples
1. Retirement Investing
You put $25,000 into an index fund averaging an 8% annual return.
72 ÷ 8 = 9 years to double
That $25,000 turns into about $50,000 after 9 years, $100,000 after 18, and $200,000 after 27. This is how financial planners explain compound growth without ever touching a spreadsheet.
2. Inflation Eating Your Cash
Inflation runs at 4% a year. You leave $10,000 in a non-interest account.
72 ÷ 4 = 18 years to halve in value
Eighteen years from now, that $10,000 buys what $5,000 buys today. The Rule of 72 works in both directions — it shows doubling or halving depending on whether you're growing or losing value.
3. Credit Card Debt You Aren't Paying Down
You have a $5,000 balance at 24% APR and only pay the interest (or skip payments entirely).
72 ÷ 24 = 3 years to double
If nothing changes, $5,000 becomes $10,000 in 3 years. This is the Rule of 72 at its scariest — and the reason financial planners shout about high-interest debt.
4. A SaaS Business Scaling
Your company adds 6% to monthly recurring revenue every month.
72 ÷ 6 = 12 months to double MRR
MRR goes from $50,000 to $100,000 in a year. From $100,000 to $200,000 the year after. That's why early-stage investors fixate on monthly growth rates — small percentages compound into enormous trajectories.
5. A City on the Grow
A metro area's population grows 2% per year.
72 ÷ 2 = 36 years to double
This is how urban planners and demographers think about capacity. If a city hits 500,000 today and keeps that rate, it's planning for a million by year 36 — and needs to start thinking about housing, water, and transit now.
Rule of 72 Accuracy Table
Here's how the shortcut compares to the exact doubling time across rates people actually use:
Growth Rate | Rule of 72 | Exact Value | Error |
|---|---|---|---|
1% | 72.0 yrs | 69.7 yrs | +2.3 yrs |
2% | 36.0 yrs | 35.0 yrs | +1.0 yr |
4% | 18.0 yrs | 17.7 yrs | +0.3 yr |
6% | 12.0 yrs | 11.9 yrs | +0.1 yr |
8% | 9.0 yrs | 9.0 yrs | 0.0 yr |
10% | 7.2 yrs | 7.3 yrs | -0.1 yr |
12% | 6.0 yrs | 6.1 yrs | -0.1 yr |
15% | 4.8 yrs | 5.0 yrs | -0.2 yr |
20% | 3.6 yrs | 3.8 yrs | -0.2 yr |
25% | 2.9 yrs | 3.1 yrs | -0.2 yr |
Accuracy is strongest right around 8% — which, not coincidentally, is close to the long-run average stock market return. At very low rates (under 3%) the rule slightly overestimates; at very high rates, it slightly underestimates. For most everyday use, the gap is small enough to ignore.
When to Use Rule of 70 or 69.3 Instead
The Rule of 72 has two lesser-known siblings:
- Rule of 70 — favored in demography and for low-growth contexts (1% to 3%). It's slightly more accurate at those rates.
- Rule of 69.3 — the most mathematically precise version, pulled straight from ln(2). Economists and academics lean on this one when every decimal counts.
For investing, debt, and inflation, 72 is almost always the right choice. It's accurate enough, and it's the only version you'll remember a month from now.
Tips for Smarter Use
- Keep your units consistent. A monthly rate returns a doubling time in months. A quarterly rate returns quarters. Don't mix.
- Compound growth only. The Rule of 72 assumes interest or growth compounds each period. For simple interest (where earnings don't build on earnings), the doubling time is just 100 ÷ rate.
- Fees and taxes matter. A 10% gross return becomes a 7% net return after typical fees and taxes. Run the rule on your actual rate, not the headline one.
- Flip the formula when useful. If you know the doubling time you want and need to find the required rate, divide 72 by the number of years. Want to double your money in 8 years? You need a 9% return.
- Expect variance. Real-world returns aren't steady. The rule gives a fair estimate, not a guarantee — it's most useful as a sanity check, not a forecast.
One Last Thought
The Rule of 72 is worth learning for one reason: it turns compound growth from an abstract concept into something you can feel. A 2% return sounds boring until you realize it means 36 years to double. A 24% credit card rate sounds unpleasant until you realize your balance doubles in 3 years. Numbers that used to be background noise start to tell you something.
Use the calculator when you need a quick answer. But commit the rule to memory, and you'll never look at a rate of return — or an interest charge — the same way again.