Rule of 72: How Long for Prices to Double at 4% Inflation?
At 4% annual inflation, prices double in exactly 18 years according to the Rule of 72.
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.
The Rule of 72 applies to inflation too — at 4%, the price of everything doubles in 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.