The Rule of 72, explained

One division sum tells you how long your money takes to double — and makes the true cost of fees and inflation obvious in a way percentages never do.

The shortcut

Divide 72 by your annual rate of return. The answer is roughly how many years it takes your money to double.

• At 6% → 72 ÷ 6 = 12 years
• At 8% → 72 ÷ 8 = 9 years
• At 10% → 72 ÷ 10 = 7.2 years
• At 3% → 72 ÷ 3 = 24 years

That's the whole thing. No calculator, no spreadsheet — you can do it while someone is still explaining their investment idea to you.

How accurate is it?

Surprisingly good in the range that matters. The exact doubling time at 8% is 9.01 years; the Rule of 72 says 9. At 6% the exact figure is 11.90 years against a predicted 12.

It drifts at the extremes. At 1% the rule says 72 years while the true answer is 69.7. At 20% it says 3.6 years against a true 3.8. For anything between roughly 4% and 12% — which covers most real decisions — it's accurate to within a couple of months.

If you want to know why 72: the mathematically exact constant is about 69.3, but 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, which makes the mental arithmetic trivial. It's a deliberate trade of a little precision for a lot of usability.

Where it's genuinely useful

Sanity-checking a projection

Someone shows you a plan where $10,000 becomes $80,000 in fifteen years. That's three doublings, so each doubling takes five years, so the implied return is 72 ÷ 5 ≈ 14.4% a year, every year, for fifteen years. Now you know what's actually being assumed, and you can decide whether it's plausible.

Understanding what fees really cost

This is the use most people miss. A fund charging 1% a year doesn't reduce your return by 1% — it changes your doubling time. Going from 8% to 7% moves doubling from 9 years to 10.3. Across a 40-year working life that's roughly four doublings instead of four and a half, and the gap compounds into a very large number.

Seeing inflation clearly

It works in reverse too. At 3% inflation, prices double in about 24 years — meaning money under a mattress loses half its purchasing power in that time. At 6% inflation it's 12 years.

Related shortcuts

The same idea scales. Use 114 for tripling and 144 for quadrupling. So at 8%, money triples in about 14 years and quadruples in about 18.

What it doesn't tell you

The Rule of 72 assumes a single constant rate with no additional contributions and no withdrawals. Real investing involves none of those things cleanly — markets fluctuate, you add money over time, and taxes and fees take a cut along the way.

Treat it as a way to build intuition and catch nonsense, not as a planning tool. For an actual projection with regular contributions, run the numbers properly.