How Long to Double Your Money at 7% Interest?

The short answer

At 7% a year with annual compounding, money doubles in 10.24 years. You can reach that two ways:

• The Rule of 72 — divide 72 by the rate: 72 ÷ 7 = 10.29 years. Instant, no calculator needed.
• The exact formula — ln(2) ÷ ln(1.07) = 10.24 years. Precise, but you need a calculator.

The gap between them is 0.04 years, or about two weeks. For a shortcut you can do in your head, that is an excellent trade.

Both methods side by side

The exact method works because doubling means growing by a factor of 2, and logarithms answer “how many periods of 1.07 growth make 2?” The Rule of 72 is a clean approximation of that same relationship — the Rule of 72 explainer covers where the number 72 comes from.

What $10,000 actually does

Doubling is not a single moment — it is a line the balance crosses. Here is $10,000 at 7% annual compounding around that point:

After ten full years you are still about $328 short. The balance crosses $20,000 roughly three months into year 11 — which is exactly what the 10.24-year figure is telling you.

Doubling time at other rates

The same comparison across common return rates shows where the Rule of 72 is sharpest:

All figures are in years. The rule is at its most accurate between about 6% and 10% — it drifts high at low rates and low at high rates. Since 7% sits right in that sweet spot, it is one of the best possible rates to use it on. For the alternatives, see Rule of 72 vs Rule of 70 vs Rule of 69.

What one percentage point is worth

Seven percent is a common assumption, but it is only an assumption — and the interesting question is how much rides on it. Here is the same $10,000 at 6, 7 and 8 percent, which is roughly the width of the disagreement between reasonable long-run equity assumptions:

One percentage point buys you roughly a year and a half off the doubling time: 1.65 years going from 6 to 7 percent, and 1.24 years going from 7 to 8 percent. Note that the second point is worth less than the first — doubling time falls with the rate, but at a decelerating pace, because the relationship is logarithmic rather than linear. Each additional point of return buys progressively less time.

Over a full horizon, though, the effect runs the other way and grows dramatically. Two extra percentage points cut the doubling time by 2.9 years, which sounds modest — but it adds eight-tenths of an extra doubling across 30 years, and the ending balance at 8 percent is 1.75 times the balance at 6 percent. The same $10,000 finishes at $57,435 or $100,627 depending on a two-point assumption you cannot control and cannot verify in advance.

Two conclusions worth drawing from that, in opposite directions. Costs matter more than they look: a 1 percent annual fee is not 1 percent of your outcome, dropping from 7 to 6 percent costs about 28 percent of your 30-year growth, because the fee comes straight off the rate that drives the exponent. And precision in the rate assumption matters less than it looks: if the plausible range of answers spans $57,000 to $101,000, then arguing about whether the doubling time is 10.24 or 10.29 years is not where the uncertainty lives.

How many doublings you get

The interesting part is not one doubling but how many fit into an investing lifetime. At 7%, each takes about 10.24 years:

Each doubling adds far more in dollars than the one before: the first adds $10,000, the third adds nearly $40,000. That is why the last decade of a long investment does so much heavy lifting — the same effect described in how compound interest works. Note these use annual compounding to match the Rule of 72; with monthly compounding, $10,000 over 30 years reaches about $81,165, as shown in how much $10,000 grows in 30 years.

Does compounding frequency matter?

Slightly. More frequent compounding doubles your money a little sooner, because interest starts earning interest earlier:

Monthly compounding gets there about four months sooner. The Rule of 72 is built around annual compounding, so it lines up with the 10.24-year figure — if your account compounds monthly, expect to double marginally faster than the rule suggests.

Assumptions

• A constant 7% return. Real returns swing year to year; a steady rate is an illustration, not a forecast.
• Annual compounding unless stated otherwise, so the figures line up with the Rule of 72. The monthly comparison is labelled above.
• A single lump sum. No additional contributions or withdrawals — adding money would reach $20,000 sooner, but that is not “doubling” in this sense.
• No fees, taxes or inflation. These are nominal figures; after inflation, doubling your purchasing power takes longer.

How these figures were checked

• Exact doubling times use t = ln(2) ÷ ln(1 + r). For monthly compounding the same formula runs on the monthly rate and the result is divided by 12, giving 9.93 years at 7 percent against 10.24 annually.
• Balances use FV = PV × (1 + r)n at full precision, rounded to the nearest dollar only for display — so the $10,000 row crossing exactly $20,000 at 10.24 years is the definition of the doubling time, not a coincidence of rounding.
• Doubling counts are the horizon divided by the exact doubling time (30 ÷ 10.2448 = 2.93), not rounded to whole doublings.
• The 1.75× comparison is 1.08³⁰ ÷ 1.06³⁰, computed directly rather than inferred from the rounded balances.
• No external source is required. Every number here is arithmetic on a stated rate, so it is verified by recomputation rather than cited. The 7 percent rate itself is an illustrative assumption, not a forecast or a promise of returns.

Last verified 8 September 2026.

Frequently asked questions

The bottom line

At 7%, money doubles in about 10.2 years — 10.24 exactly, or 10.29 by the Rule of 72, a difference of roughly two weeks. That makes 7% one of the rates where the mental shortcut is nearly perfect. Over a 30-year horizon you get close to three doublings, turning $10,000 into about $76,123 with annual compounding.

Test any rate with the Rule of 72 calculator, or read what the Rule of 72 is for the shortcut behind it.