📈 Rule of 72: How Long to Double Your Money?

Compound interest is powerful but hard to picture in your head — that is why investors have used the Rule of 72 for decades. It turns a complicated growth curve into one piece of mental arithmetic: divide 72 by the annual interest rate (as a whole number, not a decimal), and the answer is about how many years it takes for your money to double. If your money grows at 8% a year, 72 ÷ 8 = 9 years to double. That is the whole trick — and it is surprisingly accurate.

The formula, both ways

There are two versions of the rule, and each answers a different question:

Years to double = 72 ÷ interest rate. At 6% per year, your money doubles in about 12 years (72 ÷ 6). At 10%, about 7.2 years (72 ÷ 10). At 12%, about 6 years.

Rate needed = 72 ÷ years. This is the reverse form: if you need your savings to double in 10 years, you need an annual return of about 7.2% (72 ÷ 10). If you only have 6 years, you need about 12% a year (72 ÷ 6).

The rule assumes the rate stays fixed and interest compounds once a year. You can still use it with real-world numbers like a savings account rate or an investment's long-run average — just remember it is an estimate, not a guarantee.

Worked example: $1,000 at 8%

Say you invest $1,000 in a fund earning 8% a year. The rule says 72 ÷ 8 = 9 years to double to $2,000. After another 9 years (year 18), the $2,000 doubles again to $4,000. After a third 9-year stretch (year 27), you reach $8,000. This is the power of compounding: the doublings stack, so your money grows from $1,000 to $8,000 in 27 years without adding a single dollar more. Our free Compound Interest Calculator runs the exact compounding math for any rate and timeline.

Why 72?

The number is not random. The exact doubling formula comes from the math of continuous growth (the natural log of 2, about 0.693, adjusted for percentage rates), and 72 is simply the most convenient nearby whole number. It was chosen because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12 — all the rates people commonly ask about. That is what makes the mental arithmetic effortless. (A "Rule of 69.3" would be slightly more precise, but dividing by 69.3 in your head is nobody's idea of fun.)

The same rule works for inflation and debt

The rule of 72 applies to anything that grows exponentially — including the things that eat your money. If inflation runs at 3% a year, cash sitting in a drawer loses half its purchasing power in about 72 ÷ 3 = 24 years. At 5% inflation, that halves in only about 14.4 years. And debt compounds the wrong way: an unpaid credit-card balance at 15% interest doubles in 72 ÷ 15 = 4.8 years if you never touch it. The same one-second division can estimate both how fast wealth grows and how fast it disappears.

Where the rule breaks down

Three limits worth knowing. First, the rule is most accurate for rates roughly between 4% and 20%; outside that band the approximation drifts and you should use the exact compounding formula. Second, it assumes a fixed rate — real investment returns bounce around year to year, so use a long-run average and treat the answer as a planning estimate. Third, it does not account for taxes, fees or deposits: if you keep adding money (like a monthly SIP), your balance grows faster than the rule alone suggests, which is exactly what a SIP calculator is built for.

The quick cheat sheet

Pin these to your mental wall: at 4% your money doubles in ~18 years; at 6% in ~12 years; at 8% in ~9 years; at 10% in ~7.2 years; at 12% in ~6 years. Flipped around: doubling in 10 years needs ~7.2%; doubling in 15 years needs ~4.8%; doubling in 5 years needs ~14.4%. Whenever someone quotes you a return — a bank, a scheme, a friend — divide 72 by it and you immediately know what that number actually means in years.