The Rule of 72: How Long Will Your Money Take to Double?

Compound interest is easy to understand and hard to feel. The rule of 72 fixes that. Divide 72 by the annual return in percent and you get the rough number of years it takes for money to double.

The shortcut in practice

• At 6%, money doubles in about 12 years.
• At 8%, about 9 years.
• At 12%, about 6 years.
• At 18%, about 4 years.

It is an approximation, and a surprisingly accurate one for rates between about 5% and 15%. Our compound interest calculator gives the exact figure; the shortcut is for thinking, not for spreadsheets.

Why it reframes the question

Most investors think in percentages. Doubling periods are more intuitive and, more importantly, they show why time matters more than the rate. ₹1,00,000 at 12% becomes:

• ₹2,00,000 in about 6 years
• ₹4,00,000 in about 12 years
• ₹8,00,000 in about 18 years
• ₹16,00,000 in about 24 years

Every doubling takes the same number of years, which is why stopping five years early can cost an entire final doubling. It also explains why a 30-year-old investing modest amounts ends up ahead of a 45-year-old investing larger ones: the first has three doublings before the second has one.

The hidden message about inflation

The rule works on prices too, and that is where it gets uncomfortable. At 6% inflation, prices double in about 12 years. So money sitting in a savings account at 3% doubles in 24 years, while prices double in 12 — meaning it buys half as much in about the time it takes to double in nominal terms.

This is the clearest way to see why idle money is not safe. It is not standing still; it is losing ground, just slowly enough to go unnoticed.

What it says about debt

The rule applies to debt as well, and there it works against you. A credit card balance at 36% doubles in about two years if unpaid — which is why minimum payments keep a balance alive for years. A personal loan at 14% doubles in roughly five years. On loans, the equivalent insight is that the interest saved by prepaying compounds in your favour. Our prepayment calculator shows that saving.

Choosing a realistic rate

The shortcut is only as useful as the rate you feed it, so avoid optimistic assumptions:

• Equity mutual funds: 10–12% over long periods is a defensible planning figure, not a promise.
• Fixed deposits: 6–7% before tax, which drops to roughly 4–5% after tax for many taxpayers.
• PPF: currently around 7%, but tax-free, which makes it worth more than a taxable 7.5%.
• Inflation: 5–6% generally, and 8–10% for education and healthcare.

Our CAGR calculator tells you what rate you have actually been earning, which is the most honest input of all.

Using it for planning

Two useful applications:

Goal check. If you need ₹40,00,000 in 18 years and have ₹5,00,000 today, you need three doublings — an 8x multiple. That requires roughly 12.5% a year, which is at the optimistic end for equity. Knowing that early is far better than discovering it three years before the deadline.

Deciding whether to wait. If an investment has already doubled once at your expected rate, the next doubling is the same distance away as the first. There is nothing special about waiting for a round number; what matters is whether the investment still fits your plan.

The one-line version

Divide 72 by the rate to get the doubling time. Then count how many doublings you have left before your goal. That count — not the percentage — is the number that decides whether your plan works.