How long to double your money
At 6% a year your money doubles in roughly 12 years, and at 8% in about 9. The rule of 72 gets you there in your head: divide 72 by the annual return. It holds well between about 4% and 12% and drifts outside that range, which is why the year-by-year table below is worth reading rather than trusting the shortcut.
Year by year
| 1 | $10,000.00 | $722.90 | $10,722.90 |
| 2 | $10,000.00 | $1,498.06 | $11,498.06 |
| 3 | $10,000.00 | $2,329.26 | $12,329.26 |
| 4 | $10,000.00 | $3,220.54 | $13,220.54 |
| 5 | $10,000.00 | $4,176.25 | $14,176.25 |
| 6 | $10,000.00 | $5,201.06 | $15,201.06 |
| 7 | $10,000.00 | $6,299.94 | $16,299.94 |
| 8 | $10,000.00 | $7,478.26 | $17,478.26 |
| 9 | $10,000.00 | $8,741.77 | $18,741.77 |
| 10 | $10,000.00 | $10,096.61 | $20,096.61 |
An estimate, not financial advice. Figures are illustrative and depend on assumptions listed below. Check anything you plan to act on with a qualified adviser or the provider itself.
The worked example below, drawn. The gap between the lines is interest: narrow for years, then most of the balance.
The rule of 72 is an approximation. Between 4% and 12% it is within a few months; at 20% it is out by more than a year.
Doubling time ignores contributions entirely. If you are still paying in, you will get there sooner than this suggests.
Inflation roughly halves the answer in real terms: money that doubles in 12 years at 6% buys about 40% more, not twice as much.
How it works
The formula
monthlyFactor = (1 + r/n) ^ (n/12)
For each of the 12 x years months:
balance = balance x monthlyFactor + contribution
where
r = annual return, as a decimal
n = compounding periods per year (1, 4, 12 or 365)
Present value, when an inflation rate is given:
real = balance / (1 + i) ^ years
What it assumes
- The return is the same every year. Real returns vary, sometimes a great deal, and a steady average will overstate a bad decade and understate a good one.
- Contributions arrive at the end of each month, so a payment does not earn interest during the month it is made. This is the conventional and slightly conservative treatment.
- Contributions never change. Saving that rises with your income produces a materially larger figure than this shows.
- Tax, platform charges and fund fees are not deducted. A 0.5% annual fee is roughly a 0.5 percentage point cut to the return you enter.
- Compounding frequency is converted to a monthly growth factor so that contributions and compounding can differ without changing the method.
Common questions
Where does the number 72 come from?
From logarithms: doubling time is ln(2)/ln(1+r), and 72 is a number with many divisors that happens to approximate it well between about 4% and 12%. At 8% the rule says 9 years and the exact figure is 9.01. At 20% the rule says 3.6 and the truth is 3.8, so it drifts as rates get large.
Does the second doubling take the same time as the first?
Yes, at a constant rate — that is what compounding means. Money at 6% doubles in about 12 years, quadruples in 24 and is eight times its start in 36. Each doubling adds as much as every previous one combined, which is why the last decade of a long horizon looks so unlike the first.
Sources
Method written and checked by Tessalor on Jul 28, 2026.
The full method, worked example and every assumption behind this figure are on Compound Interest Calculator.