In plain words
Knowing the Rule of 72 helps you judge how long compounding may take to double money. Missing it can make growth seem faster or slower than it is, leading to poor time planning.
Sofia puts her savings into a pot that grows at a steady yearly rate. On the back of an envelope, she divides 72 by that rate to work out roughly how many years until the pot is twice as big. She writes "about" next to her answer, because the Rule of 72 only gives an estimate.
You may meet the Rule of 72 when comparing how long savings or investments might take to double. It can also come up when learning about compounding, where growth builds on earlier growth.
Two worked examples
The common mistake
The mistake: Dividing the rate by the rule's number instead of the other way round — or using it for a single year's growth.
What’s true: The Rule of 72 is a quick way to estimate how many years money takes to double: divide 72 by the annual rate. It is like sharing 72 into equal yearly pieces. A common mix-up is dividing the rate by 72, or using the rule for growth in just one year; it is meant for doubling time.
Inflation has a Rule of 72 too
Inflation can make prices double over time, while idle cash loses buying power. The Rule of 72 estimates this: 72 ÷ the inflation rate gives the years for prices to double. The common mix-up is forgetting that the same maths works against cash, so its real buying power halves.
Knowing this helps you see how rising prices can shrink what idle cash can buy over time. If you miss it, cash kept aside may buy only half as much when prices double.
Small rate gaps, huge endings
Growth compounds by multiplying, so a small yearly rate gap can become a huge difference over time. It is like a snowball growing, then adding growth to its new, larger size. A common mix-up is adding the yearly rate across the years; that misses how each year’s growth builds on earlier growth.
Nina leaves the interest in her savings pot at the end of the first year instead of taking it out. In year two, the bank pays her interest on her first deposit and on the interest added the year before. Her balance now grows by a little more each year than it did the year before.
Where you’ll meet it
- Compounding bites both waysGrowth stacks on growth — the last years of compounding earn more than all the early ones combined.
- A monthly rate times 12 hides the real costTwo numbers can describe the same loan; the smaller-looking one leaves out interest on interest.
- Time is the strongest dialRate and contribution have ceilings; starting earlier multiplies both and costs nothing.
Related terms
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