Time to Double Calculator

The Rule of 72 is the back-of-envelope shortcut every investor learns first. Divide 72 by the annual interest rate (as a percentage) and you get the approximate number of years for an investment to double. At 6% it's 12 years, at 9% it's 8 years, at 12% it's 6 years. The tool shows both the Rule of 72 estimate and the precise mathematical answer (using ln(2) / ln(1 + r/100), which is the exact compound-doubling formula).

The rule is accurate within roughly 0.5 years between rates of 4% and 15%, which covers most realistic investment scenarios. It drifts slightly outside that range: at 1% the precise answer is 69.7 years and Rule of 72 says 72, a noticeable error of 3%. At 25%, the precise answer is 3.1 years and Rule of 72 says 2.9. For mental maths during a conversation, the rule is more than good enough; when you need an exact answer for a presentation or model, use the precise calculation the tool provides.

Compound growth follows the formula future value = present value × (1 + r/n)^(n×t), where r is the rate, n is the compounding frequency, and t is years. Setting future value to 2 × present value and solving for t gives t = ln(2) / ln(1 + r). At small interest rates, ln(1 + r) is approximately equal to r, so t ≈ ln(2) / r ≈ 0.693 / r. Multiplying both sides by 100 to convert from decimal to percentage gives t ≈ 69.3 / r%. The number 72 was chosen instead of 69.3 because it has more divisors (2, 3, 4, 6, 8, 9, 12), which makes mental arithmetic easier.

This is also why some sources use the Rule of 70 or the Rule of 69 for slightly different accuracy zones; 72 is the most common because most investment maths happens between 4% and 12% interest, where 72 is a closer approximation than 69 or 70. The tool defaults to the standard Rule of 72 and shows both the rough and precise answers side by side.

UK easy-access cash savings at 4% double in roughly 18 years. Long-run global stock market returns (around 7% real, after inflation) double in roughly 10 years. Property in the UK has averaged 2-4% real growth, doubling in 18-36 years. Speculative assets (crypto, single-stock bets, junior mining stocks) can double in months but can also halve just as fast - the headline 'doubled in 6 months' figures often come with 50% drawdowns weeks later. Rule of 72 maths assumes constant compound growth, which describes diversified index portfolios well and individual gambles poorly.

For business growth, the same maths applies to revenue. If your monthly recurring revenue is growing 6% a month, you double in 12 months (Rule of 72 applied to monthly figures: 72 / 6 = 12 months). Doubling time becomes a useful KPI for early-stage SaaS where investors want to see how long until you hit specific milestones. The [compound interest calculator](/compound-interest-calculator) shows the cumulative path of a single growth rate; this tool just answers the doubling question.