The Rule of 72: how fast money doubles

The situation

Someone tells you their savings earns 4% and your eyes glaze over — 4% of what, over how long, and does that even matter? You want a gut-level sense of whether a rate is a big deal or a rounding error, without opening a spreadsheet. There’s a one-step trick for exactly that, and it fits on a napkin.

The idea

Here it is: divide 72 by the interest rate, and you get roughly the number of years it takes for money to double. That’s the whole Rule of 72.

It works because of compounding — the snowball you just saw in the last lesson. Money that earns interest on its interest doesn’t grow in a straight line; it grows by doubling, again and again, and 72 happens to be a number that makes the doubling math come out clean enough to do in your head. It’s an approximation, not the exact formula, but it’s close enough to be genuinely useful.

The magic is that it points both ways:

• On savings, doubling is the goal. A higher rate means a shorter doubling time — your money reaches “twice as much” sooner.
• On debt, doubling is the trap. The same shortcut tells you how fast an unpaid balance can blow up. High-rate debt doubles fast, and that’s the warning the rule gives you for free.

The rule of thumb: 72 ÷ rate ≈ years to double. Memorize that one line and you can size up almost any rate on the spot.

By the numbers

Let’s run a few divisions you can check in your head:

Annual rate	72 ÷ rate	Years to double
2%	72 ÷ 2	about 36 years
4%	72 ÷ 4	about 18 years
6%	72 ÷ 6	about 12 years
8%	72 ÷ 8	about 9 years
24% (a card)	72 ÷ 24	about 3 years

Read down that table and the story jumps out. At 2%, money crawls — 36 years to double. Bump the rate to 6% and the doubling time drops to 12 years. And look at the bottom row: a credit card at 24% would roughly double an unpaid balance in just 3 years. That’s not a savings story anymore — that’s the cost of carrying high-rate debt, made vivid by the exact same trick.

The rule isn’t perfectly precise (the true doubling time at 6% is a hair over 11.9 years, not exactly 12), but it’s close enough to make smart calls without a calculator. A small change in the rate makes a big change in the doubling time — that’s the real lesson.

Why it matters

This is the pocket version of everything in this phase. You don’t need to run the compound calculator for every rate you bump into — just divide 72 and you instantly know if a rate is a slow simmer or a fast burn. Keep it handy for Phase 7 (credit) and Phase 8 (debt), where the fast-burn side is the whole point.