Planning concepts
How long until it doubles? The rule of 72, checked
A rule of 72 calculator: clients enter a return, a starting amount and a time horizon to see years to double, the number of doublings, the future value and the exact doubling time.
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What is the rule of 72?
Divide 72 by the annual return to estimate the years it takes money to double. At 8%, about 9 years; at 6%, about 12. It's a shortcut, most accurate near 8%; the exact doubling time is ln 2 ÷ ln(1 + rate), which the calculator shows alongside.
Any time compounding needs a quick explanation
Clients hear the rule in a podcast or class and want to try it with their own numbers. It's a gateway to understanding compounding.
What the client types into a search box
“how long does it take to double my money”
How a client works through it
Enter a return
Annual rate.
Enter an amount
Starting sum.
Enter a horizon
Years to project.
See doublings
Years to double, doublings, value and exact time.
What lands in your inbox
When a client asks your firm to review the result, the request arrives as a contact record with their inputs and results attached, by email and, on paid plans, by webhook to your CRM.
Received
Return, amount and horizon show a client's expectations. High assumed returns may need tempering in conversation.
Shortcut versus exact
Years to Double uses 72 ÷ rate. Exact Doubling Time uses logarithms with annual compounding. Number of Doublings and Growth Multiple show what happens over the horizon; Future Value applies it to the amount.
The two doubling figures diverge at very high or very low rates.
Results shown: Years to Double · Number of Doublings · Future Value · Growth Multiple · Exact Doubling Time.
At 8%
72 ÷ 8 = 9 years. Exactly: ln 2 ÷ ln 1.08 ≈ 9.01 years. Over 36 years, money doubles about four times, a 16x multiple.
Why a teaching tool belongs on a firm site
It's memorable, shareable and makes compounding intuitive, which helps clients appreciate long-term planning and the cost of fees and inflation.
It also works in reverse for debt and inflation: prices double too.
Follow-ups this request opens
- Set realistic return expectations
- Show the cost of fees with the same logic
- Plan long-term investing
- Explain inflation's doubling effect
What this calculator leaves out
- Approximation.
- Annual compounding for the exact figure.
- Returns not guaranteed.
- No taxes or fees.
Platform disclaimer shown under the calculator: The Rule of 72 is an approximation (most accurate near 8%); the exact doubling time assumes annual compounding. Investment returns are not guaranteed.
Compliance note for firms
Doubling illustrations rely on assumed returns; present them as arithmetic examples, not expected performance.
Not legal advice. See our disclaimer and the guide to compliance for client calculators.
Terms your client sees in the calculator
These definitions appear in the calculator's collapsible Definitions block, so clients don't have to call to ask what a label means.
- Rule of 72
- A shortcut for estimating how long money takes to double: divide 72 by the annual return in percent. At 8%, money doubles in about 9 years. It is an approximation, most accurate near 8%; at very low or very high rates the exact formula gives a noticeably different answer, which this calculator shows alongside it.
- Exact doubling time
- The precise number of years to double at a given rate with annual compounding: ln(2) ÷ ln(1 + rate). At 8%, the exact figure is about 9.01 years, very close to the rule’s 9. At 2%, the rule gives 36 years against an exact 35.0, which shows where the shortcut starts to drift.
- Compound interest
- Growth in which each year’s return is added to the balance and then earns returns itself. It is what makes repeated doubling possible: $10,000 at 8% becomes about $20,000 in 9 years and about $40,000 in 18. Simple interest, paid only on the original amount, would take 12.5 years to double at the same rate.
- Number of doublings
- The time horizon divided by the doubling time: how many times the starting amount doubles over the period. Over 36 years at 8%, money doubles about four times, so it grows roughly sixteenfold. Each doubling adds more dollars than the one before, which is why the last years of a long horizon add the most.
- Growth multiple
- The future value divided by the starting amount. A multiple of 16 means each dollar became about $16. It equals 2 raised to the number of doublings and does not depend on the starting amount, so it is a quick way to compare how different rates perform over the same horizon.
- Future value
- What the starting amount grows to at the given rate after the time horizon, compounded yearly. It is shown in nominal dollars, not adjusted for inflation or taxes, and assumes the rate holds every year. For example, $10,000 at 8% for 36 years grows to about $159,700.
In our glossary: Rule of 72 · Compound interest
Rule of 72 calculator: questions from firms and clients
How accurate is the rule of 72?
Very close around 6% to 10%; less so at extremes.
Can I use it for inflation?
Yes, 72 ÷ inflation estimates years for prices to double.
What about debt?
It shows how fast unpaid debt doubles at its interest rate.
Why 72?
It's divisible by many numbers and approximates ln 2 × 100 adjusted for typical rates.
Is the exact figure shown?
Yes.
Does it include taxes?
No.
What return should I use?
A realistic long-term figure for your investments.
Which plan includes it?
Firm and Network. See pricing.
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Clients who ask this usually ask next
Future Value
how much will my money grow if I invest it
Inflation Impact
how much will things cost in 20 years
Financial Independence (FIRE)
when can I retire early
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