Rule of 72: How to Know When Your Money Doubles
Short answer: To find how many years your money takes to double, divide 72 by the yearly interest rate. At 8% a year, 72 ÷ 8 = 9 years. At 12%, it is 6 years. The Rule of 72 is a shortcut, not an exact formula, but for rates between 6% and 15% it is accurate to within about 3%.
Why 72 works
Money that grows at a fixed yearly rate doubles when (1 + rate) multiplied by itself enough times reaches 2. The exact answer uses logarithms, which are hard to do in your head. Mathematicians noticed that for ordinary rates, 72 divided by the rate in percent lands very close to the exact answer. 72 is also easy to divide: it splits evenly by 2, 3, 4, 6, 8, 9 and 12.
The shortcut against the exact answer
| Yearly rate | Rule of 72 | Exact | Difference |
|---|---|---|---|
| 6% | 12.00 years | 11.90 years | 0.9% |
| 7% | 10.29 years | 10.24 years | 0.4% |
| 8% | 9.00 years | 9.01 years | -0.1% |
| 10% | 7.20 years | 7.27 years | -1.0% |
| 12% | 6.00 years | 6.12 years | -1.9% |
| 15% | 4.80 years | 4.96 years | -3.2% |
Notice how close the two columns are. The rule slightly overestimates at lower rates and slightly underestimates at higher rates, and it is best between about 6% and 10%.
Four ways to use it
- Savings. If a fixed deposit pays 7% and you let it grow without withdrawing, it doubles in about 10.3 years. A lump sum of ₹5 lakh becomes about ₹10 lakh.
- Inflation. Prices double in 72 ÷ inflation years. At 6% inflation, something that costs ₹100 today costs about ₹200 in 12 years. That is why savings that earn less than inflation lose value.
- Comparing returns. A return of 12% doubles money in 6 years, while 8% takes 9 years. Four extra percentage points cut the wait by a third.
- Debt. It works against you too. A card balance at 36% a year doubles in about 2 years if you pay nothing.
Tripling and quadrupling
Similar shortcuts exist: use 114 to find when money triples and 144 to find when it quadruples. At 8% a year, 114 ÷ 8 = 14.3 years to triple (exact: 14.3) and 144 ÷ 8 = 18 years to quadruple (exact: 18.0).
Where the rule does not work
- Regular deposits. The rule is for a single lump sum. A SIP or a PPF with yearly deposits grows differently. Use the PPF calculator or SIP calculator instead.
- Variable returns. Equity returns are not a steady 12% every year. The rule only gives an idea of what a steady average would mean, not a promise.
- Very high or low rates. Below 2% or above 20%, the answer drifts from the exact figure.
To check any case exactly, use the interest calculator, or the CAGR calculator to find the yearly rate your money has actually earned.
Keep reading
- SIP vs FD vs PPF: Where Should ₹10,000 a Month Go? — Investing basics, 7 min read
- How Much SIP Do You Need for ₹1 Crore in 10 Years? — Investing basics, 6 min read
- Home Loan Prepayment vs SIP: Which Is Better? (The Math) — Loans, 7 min read
- How Much Emergency Fund Do You Need? A Simple Formula — Money basics, 6 min read
Frequently asked questions
What is the Rule of 72?
It is a shortcut: divide 72 by the yearly interest rate to estimate the years needed to double your money. At 9% it is 72 ÷ 9 = 8 years.
Is the Rule of 72 accurate?
For rates between 6% and 15% it is within about 3% of the exact answer. At 12%, the rule says 6 years and the exact figure is 6.12 years.
Does the Rule of 72 include inflation?
No. It works with the rate you give it. To see what your money is worth in real terms, use the rate after inflation, for example 8% return minus 6% inflation leaves about 2%, which doubles your buying power in about 36 years.
Can I use it for SIP or PPF?
Not directly. The rule is for one lump sum left to grow. For monthly SIPs or yearly PPF deposits, use a SIP or PPF calculator, which handles each deposit separately.
Sources: National Savings Institute — Public Provident Fund