Rule of 72 at 4%: How Long to Double $10,000?
At a 4% annual return, money doubles approximately every 18 years (72 ÷ 4), so $10,000 becomes $20,000 in roughly 18 years.
How to use this tool
- Enter annual return rate, starting amount and projection years in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your years to double (rule of 72) and the full breakdown beneath it.
Conservative investors earning 4% can use the Rule of 72 to estimate how long it takes to double their money — about 18 years.
Frequently asked questions
- How accurate is the Rule of 72?
- It's a good approximation for rates between 6% and 10%. For exact doubling time, use ln(2) / ln(1 + r) ≈ 0.693 / r.
- Does it work for debt too?
- Yes — at 18% APR your credit card debt doubles in roughly 4 years if you make no payments. The rule is a powerful reminder of compounding in reverse.