BCBetter Calculators

Investment Doubling Time Calculator

Calculate how long it will take for an investment to double using the Rule of 72 and exact compound interest.

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Enter your values and click Calculate

How It Works

Exact doubling time = ln(2) ÷ (n × ln(1 + r/n)), where r is the annual rate as a decimal and n is the number of compounding periods per year. This formula inverts the compound interest equation FV = PV × (1 + r/n)^(n×t) and solves for t when FV = 2 × PV. The tripling time uses the same formula with ln(3) instead of ln(2). The Rule of 72 approximates the same result as 72 ÷ annual rate percentage — it is accurate to within 1–2% for rates between 6% and 10% and less accurate outside that range. Higher compounding frequency (monthly vs annual) reduces doubling time because interest is applied and reinvested more often.

Examples

Stock Market Average
8% annual return, monthly compounding — a typical index fund assumption.
Result: Exact doubling time: 8.69 years. Rule of 72: 9 years.
High Growth
12% annual return, monthly compounding — aggressive portfolio assumption.
Result: Exact doubling time: 5.81 years. Rule of 72: 6 years.
Savings Account
4% annual return, monthly compounding — high-yield savings account.
Result: Exact doubling time: 17.36 years. Rule of 72: 18 years.

Frequently Asked Questions

How accurate is the Rule of 72?
The Rule of 72 is very accurate for rates between 6% and 10%, where the error is less than 1%. For rates above 20% or below 3%, the approximation diverges from the exact answer by several percent. This calculator shows both so you can see the difference.
Does compounding frequency matter much?
Yes, but the effect is small at typical investment return rates. Moving from annual to monthly compounding at 8% reduces doubling time by about 0.3 years. Moving from monthly to daily compounding reduces it by another few weeks.
How do I use this to compare two investments?
Run the calculator twice — once for each return rate. The difference in exact doubling years shows how much faster one investment compounds relative to the other. Even a 2% difference in return rate (say, 6% vs 8%) leads to a gap of about 3 years in doubling time.

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