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Doubling time calculator

How long a value takes to double at a steady growth rate: the exact figure ln2 / ln(1+r), next to the famous mental shortcuts, the Rule of 72 and the Rule of 70.

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The Doubling time calculator turns Growth rate per period (%) into Exact doubling time, Rule of 72, Rule of 70, instantly and for free. For instance, with Growth rate per period (%) = 7 it returns Exact doubling time = 10.245, Rule of 72 = 10.286 and Rule of 70 = 10.

How to use it

  1. Enter your values: Growth rate per period (%).
  2. Read the result instantly: Exact doubling time, Rule of 72, Rule of 70.

Frequently asked questions

What does the Doubling time calculator actually compute?

It takes Growth rate per period (%) and derives Exact doubling time, Rule of 72 and Rule of 70 from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

A single value: Growth rate per period (%). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Growth rate per period (%) = 7, the calculator returns Exact doubling time = 10.245, Rule of 72 = 10.286 and Rule of 70 = 10. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Growth rate per period (%) = 7.7 instead, Exact doubling time goes from 10.245 to 9.344 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Growth rate per period (%) = 6.3, Exact doubling time comes out at 11.345. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Comparing two investments that pay at different times, deciding whether a project clears its cost of capital, and sanity-checking a valuation someone else produced.

What is the most common mistake?

Trusting a valuation without asking what share of it comes from the terminal value. Past 70%, the answer is an assumption about the distant future dressed up as a calculation.

What is the difference between the Doubling time calculator and the Cost of equity calculator (CAPM & DDM)?

This one returns Exact doubling time and Rule of 72; the Cost of equity calculator (CAPM & DDM) returns Cost of equity — CAPM and Cost of equity — DDM. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

DCF calculator (discounted cash flow) is the closest one after this: Value a company from its projected free cash flows: discount each year at the WACC, add a Gordon terminal value, then work down to equity value per share.

What else is worth having open alongside it?

Funding rate calculator and Perpetuity value calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
ComparisonReal vs Nominal Return: Why Subtracting Inflation Is the Wrong AnswerAt 7 percent nominal and 3 percent inflation the real return is 3.883 percent, not 4. The Fisher equation divides, it does not subtract — and over 30 years the shortcut overstates a $10,000 pot by $1,072.ExplainerVolatility Is Not Risk, and the Square Root of Time Is a ChoiceAnnualised volatility = period standard deviation × √(periods per year), and that √t scaling assumes independent increments. It is a model, not arithmetic: at a daily autocorrelation of 0.1 a 60 percent annualised figure should read 66.3. The payload is volatility drag — the arithmetic mean exceeds the geometric by about σ²/2, so at 8 percent average return and 40 percent volatility the compound outcome is zero.ExplainerThe Sharpe Ratio, and What It Quietly AssumesSharpe = (return − risk-free) ÷ standard deviation, so it prices return per unit of volatility — and volatility is symmetric. Two funds can share a Sharpe of 0.4939 while their Sortino ratios are 8.59 and 0.74. Annualising by √12 assumes independent returns: at an autocorrelation of 0.2 the published figure is 20 percent too high.GuideBuying Back Retirement Quarters or Points: From What Age It Stops PayingThe usual advice is that a buy-back gets worse with age, because the price rises. The French scale is written to be actuarially neutral, so that is not quite what is happening — and once you see what actually moves the answer, the decision changes. Computed on the current parameters.ExplainerPresent Value vs Future Value: Why Money in Thirty Years Is Worth About an Eighth of Its FacePV = FV ÷ (1+r)^n. At 7 percent over 30 years the discount factor is 0.131, so a promise of $100,000 in thirty years is worth $13,137 today — and $41,199 if you assume 3 percent instead.ComparisonNPV vs IRR: What to Do When the Two Rules Rank the Same Projects DifferentlyIRR picks the $10,000 project returning 50 percent; NPV picks the $100,000 project returning 30 percent, worth $20,370 against $3,889. And a mine with a cleanup cost has two IRRs, 10 and 20 percent, so the rate answers nothing.