Rule of 72 Calculator

About the Rule of 72 Calculator

Divide 72 by an annual growth rate and you get roughly the years it takes for money to double. This calculator shows that quick estimate next to the exact answer for annual compounding, so you can sanity-check investment returns, savings rates or how fast inflation halves your purchasing power.

How to use it

1. Enter an annual rate in percent.
2. Compare the rule-of-72 estimate with the exact figure.
3. Try different rates to see how much a few points matter.

Tips and good to know

• At 7% a year, money doubles in about 10 years.
• At 3% inflation, prices double in about 24 years.
• The rule is most accurate between about 6% and 10%.
• Fees reduce your effective rate — a 1% fee can add years to doubling.

About money & business tools

Financial calculators are only useful if you understand the formula behind them. Each calculator on this page shows its working: the inputs it uses, the standard formula it applies, and what the result does and does not include. A mortgage figure that ignores property tax, or an ROI that ignores time, can be technically correct and still misleading.

These calculators are educational. They use widely published formulas — the same ones in finance textbooks and bank disclosures — but they cannot know your local taxes, fees or lender terms. Use them to understand the shape of a decision and to compare options, then confirm the final numbers with the institution or a qualified adviser. Nothing you enter is saved or sent anywhere.

Two habits make any calculator more useful. First, change one input at a time and watch how the result moves — that shows you which assumption actually drives the decision, whether it is the interest rate, the term or the price. Second, run a pessimistic and an optimistic scenario rather than a single 'expected' one; real rates, returns and costs rarely stay fixed for years. All figures are shown in whatever currency you are using, because the formulas are the same in dollars, euros or Icelandic krónur. Rounding is applied only for display, so the underlying maths stays precise.

Popular ways people use money & business tools

• Comparing a 15-year and a 30-year mortgage to see the real difference in total interest paid.
• Checking whether a shop's 'extra 10% off sale items' is as generous as it first sounds.
• Converting a contractor's hourly rate into an annual figure to compare it fairly with a salaried job.
• Setting retail prices that reliably hit a target profit margin after product costs.
• Estimating how regular monthly savings could grow over ten or twenty years with compounding.

How we build and test our free tools

Every tool in this collection is built and tested by The Curator team. Before a tool is published we check it against known reference results — textbook loan schedules, standard test vectors for hashes, sample files for conversions — and we re-check it whenever browsers change. Each page explains the method so you can verify the answer yourself rather than take it on trust.

The tools run as JavaScript inside your browser tab. When you choose a file, your browser reads it from your disk into memory; the tool processes it and gives you the result as a download. No copy is sent to us or to anyone else, and nothing is kept after you close the tab. That is why these tools are safe for personal documents, and why they keep working on a slow or interrupted connection once the page has loaded.

The collection is free because it is supported by advertising on these guide pages. We never charge for a tool, never add watermarks and never sell what you type. If something doesn't work as described, or you would like a tool we don't have yet, please contact us — new tools are added in regular batches based on what readers ask for.

Frequently asked questions

Why 72?

It's close to 100 × ln 2 (69.3) adjusted for typical rates, and divides neatly by many numbers.

Is this investment advice?

No — it's arithmetic. Returns are never guaranteed.

Is my input stored or sent anywhere?

No. Everything runs inside your browser tab and disappears when you close it.

Is the Rule of 72 Calculator really free?

Yes. There is no account, no trial and no watermark. The Curator is supported by advertising on its guide pages, never by charging for tools or selling your data.

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