Rule of 72: How Long to Double Your Money?

Years to double your money: the rule of 72 vs the exact math.

PrivateFreeSourcedReviewed Sep 29, 2026

Years to double at 7% (rule of 72)

10.3 years

72 ÷ 7 = 10.29. Exact answer: 10.24 years

Exact (ln 2): 10.24 years
$10K in 40 yrs: $149,745
Double in 10 yrs: 7.18%

At 7% a year your money doubles about every 10.2 years, so $10,000 becomes $149,745 in 40 years — 3 full doublings. The rule of 72 says 10.3, essentially spot on.

Try a what-if

$10,000 doubling at 7%Each dot is a doubling (annual compounding, no deposits).

Rule of 72 vs the exact answerYears to double at common rates. The rule is most accurate near 8%.

Rate	72 ÷ rate	Exact	Error
2%	36.0	35.00	+1.00
3%	24.0	23.45	+0.55
4%	18.0	17.67	+0.33
5%	14.4	14.21	+0.19
6%	12.0	11.90	+0.10
7%	10.3	10.24	+0.04
8%	9.0	9.01	−0.01
9%	8.0	8.04	−0.04
10%	7.2	7.27	−0.07
11%	6.5	6.64	−0.10
12%	6.0	6.12	−0.12

Your numbers

Annual return or interest rate

7.00% a year

Starting amount

$10,000

Years to show

40 years

The rule works for anything that compounds — including inflation (at 3%, prices double in about 24 years; see the inflation calculator) and debt. For regular deposits, use the compound interest calculator.

Next steps

At a 7% annual return your money doubles in about 10 years: the rule of 72 says 72 ÷ 7 = 10.3 years, and the exact answer is 10.24. So $10,000 becomes $20,000 in about 10.2 years, $40,000 in 20.5 and $80,000 in 30.7. The rule of 72 is the quickest way to feel what compounding does — divide 72 by any interest rate to estimate how many years it takes to double. This calculator shows the rule's estimate next to the exact answer, ln(2) ÷ ln(1 + r), charts each doubling of your own starting amount, and includes a table of rates from 2% to 12% so you can see where the shortcut is spot on (around 8%) and where it drifts. It works just as well for inflation and credit card debt.

How this calculator works

Enter an annual return or interest rate — an investment return, a savings APY, an inflation rate or a loan APR.
Enter a starting amount to chart, such as $10,000.
Choose how many years to show on the chart.
Read the rule-of-72 estimate at the top and the exact doubling time beside it.
Check the table to see how accurate the rule is at other rates, and use the what-if chips to compare the stock market's long-run return, the effect of fees, or credit card debt.

rule of 72: years ≈ 72 ÷ rate(%)
exact: years = ln(2) ÷ ln(1 + r)
rate to double in t years = 2^(1/t) − 1

Money doubles when (1 + r)^t = 2, so t = ln 2 ÷ ln(1 + r). Because ln 2 ≈ 0.693 and ln(1 + r) is a little less than r, dividing 72 by the percentage rate is a close, easy approximation for annual compounding at typical rates.

r: Annual rate as a decimal (0.07 for 7%)
t: Years to double
ln: Natural logarithm

Frequently asked questions

What is the rule of 72?

The rule of 72 is a shortcut for how long money takes to double: divide 72 by the annual interest rate. At 8%, 72 ÷ 8 = 9 years, and the exact answer is 9.01 years. It works for anything that compounds — investments, savings, inflation and debt — and it's easy to do in your head.

How accurate is the rule of 72?

Very close at typical rates. It's almost exact near 8% (9.0 versus 9.01 years), overestimates at low rates — at 2% it says 36 years versus the exact 35.0 — and underestimates at high ones: at 12% it says 6.0 versus 6.12. The exact formula is ln(2) ÷ ln(1 + r).

How long does it take to double money at 7%?

About 10 years. The rule of 72 gives 72 ÷ 7 = 10.3 years; the exact answer with annual compounding is 10.24 years. So $10,000 invested at 7% becomes $20,000 in about 10.2 years, $40,000 in 20.5 years and $80,000 in 30.7 years — and roughly $150,000 after 40 years.

Can I use the rule of 72 for debt and inflation?

Yes. Unpaid credit card debt at the 21.52% average APR doubles in about 3.35 years by the rule (3.56 exact). Inflation at 3% halves your money's buying power in about 24 years (23.45 exact). And cash earning the 0.38% national savings average would take about 183 years to double.

What's the difference between the rule of 72, 70 and 69?

They're the same shortcut tuned for different cases. 69.3 is exact for continuous compounding, 70 is handy for low rates like inflation, and 72 is the most popular because it divides evenly by 2, 3, 4, 6, 8, 9 and 12 and is most accurate around 6–10% with annual compounding.

Sources used in this calculator

Reviewed by the Money Scale editorial team. How we source our data

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