THE RULE OF 72

the time it takes steady growth to double: the division you can do in your head, how far it misses and why the divisor is what it is

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Formats: A4 · A3 · A2
What you get: Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject: maths · everyone · memo · study

Everything here is computed for a single model: a quantity grows at a steady rate, compounded once a year. “Exactly” is the time it genuinely takes to become twice as large under that growth. The rule advises nothing and predicts nothing: it only divides, and it works the same for prices, for readers or for bacteria in a dish.

72 WAS CHOSEN FOR ITS DIVISORS, NOT FOR ITS ACCURACY

Seventy-two stands in the rule not because it is the most accurate. No number is an exact constant divisor: the right one grows with the rate, from 69.7 at one per cent to 73.4 at twelve. So the choice is among the whole numbers between 69 and 74, and every one of them is equally defensible.

What settles it is not accuracy but mental arithmetic. 72 divides evenly by 8 of the first twelve numbers — 1, 2, 3, 4, 6, 8, 9, 12 — while its nearest rival 70 manages only 5. A rule you cannot work without paper is not a rule, so divisibility won.

On the sheet

Everything here is computed for a single model: a quantity grows at a steady rate, compounded once a year. “Exactly” is the time it genuinely takes to become twice as large under that growth. The rule advises nothing and predicts nothing: it only divides, and it works the same for prices, for readers or for bacteria in a dish.
Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
Nothing about how it was made: the sheet carries only what the reader needs.

Printing

Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
White background and sparing fills — it prints on a home printer without draining the cartridge.
Margins of at least 10 mm: a home printer cannot print to the edge.
No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 9 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

THE RULE OF 72

72 WAS CHOSEN FOR ITS DIVISORS, NOT FOR ITS ACCURACY

On the sheet

Printing

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