Doubles in
17.7 yrs
Rule of 72: 18 yrs
Doubling your money at 4%
How long does it take to double your money at 4%? Estimate it with the Rule of 72 and compare it with the exact value — with a growth curve and a live calculator. Illustrative only, not investment advice.
At 4%, money doubles in about 18 years (Rule of 72), or 17.67 years exactly.
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Explore doubling time
Pick a starting amount and a target — the rate stays fixed at 4%. The estimate and the exact value update live.
Fixed rate: 4% per year. $10,000 grows to 2× its value.
Estimate (rule)18 yrs
Exact17.67 yrs
Resulting amount$19,998
Growth of $10,000 at 4%
8× after about 53 years
Double, triple, quadruple at 4%
Rule estimate (72, 114, 144 ÷ 4) versus the exact logarithmic value and the difference.
| Target | Rule | Estimate | Exact | Difference |
|---|---|---|---|---|
| Double | 72 | 18 yrs | 17.67 yrs | +0.33 |
| Triple | 114 | 28.5 yrs | 28.01 yrs | +0.49 |
| Quadruple | 144 | 36 yrs | 35.35 yrs | +0.65 |
Doubling time by rate (1–20%)
4% · 18 years
Accuracy: Rule of 72 vs. exact
Rule of 72Exact
4% · rule 18 vs. exact 17.67 yrs
Common amounts doubled at 4%
What $1,000 to $100,000 becomes when doubled at 4% — and after how many years.
| Starting amount | Doubled | Doubles in |
|---|---|---|
| $1,000 | $2,000 | 17.7 yrs |
| $5,000 | $10,000 | 17.7 yrs |
| $10,000 | $20,000 | 17.7 yrs |
| $25,000 | $50,000 | 17.7 yrs |
| $100,000 | $200,000 | 17.7 yrs |
Milestones at 4%
Triples in
28 yrs
Exact, ln(3) ÷ ln(1 + r)
Quadruples in
35.3 yrs
Exact, two doublings
Tenfold in
58.7 yrs
Exact, ln(10) ÷ ln(1 + r)
Why 72 — and where the estimate drifts
The exact doubling time comes from ln(2) ÷ ln(1 + r). For small rates that is roughly 100 · ln(2) ≈ 69.3 — but 72 divides far more easily in your head (by 1, 2, 3, 4, 6, 8, 9, and 12). At 4%, the rule estimates 18 years versus 17.67 exactly. The rule is most accurate near 8%; at very low or very high rates the gap grows.
Every figure on this page is illustrative only and not investment advice — real returns vary and are never guaranteed. For your own rate, use the Rule of 72 calculator. Background: Investopedia — Rule of 72.
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Frequently asked questions
How long does it take to double money at 4%?
The Rule of 72 estimates it fast: 72 ÷ 4 ≈ 18 years. The exact formula ln(2) ÷ ln(1 + 4%) gives 17.67 years. For most rates the estimate and the exact value sit only a fraction of a year apart.
When does money triple or quadruple at 4%?
Use Rule of 114 for tripling (114 ÷ 4) and Rule of 144 for quadrupling (144 ÷ 4). Exactly, money triples at 4% after about 28 years and quadruples after about 35.3 years.
Why the number 72 specifically?
The exact doubling time is ln(2) ÷ ln(1 + r). For small rates this is roughly 100 · ln(2) ≈ 69.3 — but 72 divides cleanly in your head (by 1, 2, 3, 4, 6, 8, 9, 12). That convenience is why 72 became the standard shortcut.
How accurate is the Rule of 72 at 4%?
At 4%, the rule estimates 18 years versus 17.67 exactly. It is most accurate near 8% and drifts a little at very low or very high rates — but as a mental shortcut it stays useful across the whole range.
How long until money grows tenfold at 4%?
Tenfold takes about 58.7 years at 4% exactly (ln(10) ÷ ln(1 + 4%)). A handy shortcut is the "Rule of 231": 231 ÷ 4 gives a similar estimate.
Are these figures investment advice?
No. This page explains the math of doubling time at 4% and is not investment, tax, or legal advice. Real returns vary and are never guaranteed.
Run your own rate
The Rule of 72 calculator opens pre-filled with 4% and shows doubling, tripling, and exact times for any rate.