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🔢 Foundations

Rule of 72

A mental model for doubling time: divide 72 by the rate — useful, approximate, and easy to abuse.

The Rule of 72 estimates years to double money at a constant compound rate — a pocket approximation, not a prospectus.

Rule of 72 sits in the foundations family of this atlas: mechanisms first, products second, hype never.

Use the calculator for scenarios, then return to the section pages when you want the vocabulary behind the curve.

Formula72 ÷ rate %
OutputYears to 2×
Best for5–12% bands
SisterRule of 115
TrapFee-blind use
ToneEducational

Profile

Time sensitivity
88
Rate sensitivity
92
Contribution leverage
30
Behaviour risk
50
Tax & fee drag
40
Horizon fit
70

Seven chapters

The idea

The Rule of 72 estimates years to double money at a constant compound rate — a pocket approximation, not a prospectus.

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The math

Formulas, worked examples and rules of thumb for rule of 72, with inflation and fees kept in view.

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Levers

What you can actually pull when thinking about rule of 72: contributions, time, costs, behaviour and wrappers.

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History

A short history of ideas and institutions behind rule of 72.

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Pitfalls

Common traps and myths around rule of 72, with practical fixes.

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Practice

A practical sequence for applying rule of 72 without turning life into a spreadsheet cult.

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Today

Modern context for rule of 72: rates regimes, apps, FIRE debates and longevity.

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Frequently asked questions

What is rule of 72 in one sentence?
The Rule of 72 estimates years to double money at a constant compound rate — a pocket approximation, not a prospectus.
Is this financial advice?
No. Tool-Lifes pages are educational atlases. Decisions need your goals, constraints and, when appropriate, a licensed adviser.
Do I need a high return for compounding to matter?
Higher net rates help, but time and contributions often dominate — especially early. Costs and behaviour can erase a “good” rate.
How should I use the calculator?
Treat outputs as scenario sketches. Vary contribution, rate, fees and inflation to see which lever moves your goal date.
What about inflation?
Always ask whether a figure is nominal or real. A high nominal path can still lose purchasing power.
Are past returns a promise?
No. Illustrations use round teaching numbers. Markets, inflation and taxes change.
Where do fees show up?
In the net rate that compounds. Expense ratios, advice charges and spreads all reduce terminal wealth.
What page should I read next?
From foundations, continue to contributions-vs-rate or fee-drag; for behaviour, open consistency-habit or panic-selling.

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