Doubling Time Calculator

Q: How do I turn doubling time into specific growth rate (µ)?

Use this formula: µ = ln 2 ÷ T d = 0.693 ÷ T d So if your doubling time is 30 minutes (0.5 hours), µ = 0.693 ÷ 0.5 = 1.386 per hour. The calculator also shows an Implied Growth Rate as a percent per period, which is the same idea written a different way.

Q: What time unit should I pick for bacteria?

Use minutes or hours for most bacteria. E. coli doubles in about 20 minutes, so minutes work well. Use hours or days for mammalian cell lines, yeast, and slow growers like M. tuberculosis . Your answer comes back in the unit you pick.

Updated Aug 27, 2026 By Jehan Wadia

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Doubling Time

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Step-by-Step Solution

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Doubling Time in Every Time Unit

Exact result converted into each supported time unit (1 month = 30.4375 days, 1 year = 365.25 days).
Time Unit	Exact Doubling Time	Rule of 70 Approximation

Growth Curve

Introduction

Doubling time is how long something takes to grow to twice its size. This Doubling Time Calculator finds that time for you. It works for bacteria in a flask, cells in a dish, money in a bank, or people in a city.

You can use it in three ways:

Two-Value Method — Enter a start value, an end value, and the time between them. Good for counting cells or reading OD values.
Growth Rate Method — Enter a percent growth rate per hour, day, or year.
Reverse (Future Value) — Enter a start value and a known doubling time to see how big things get later.

The tool picks the right formula and shows each step of the math. You also get the growth rate, the time to triple, the Rule of 70 estimate, a table of the answer in seconds through years, and a growth curve you can look at. Pick any time unit you like, and the calculator does the switching for you. If you want to model the whole curve instead of just the doubling point, pair this page with our Exponential Growth Calculator.

How to use our Doubling Time Calculator

Pick one of the three tabs, type in your numbers, and the doubling time calculator shows how long it takes your value to double, plus the growth rate, a Rule of 70 check, a step-by-step solution, a unit table, and a growth curve.

Method tab: Choose Two-Value Method if you know a start and end count, Growth Rate Method if you know the percent growth per period, or Reverse (Future Value) if you know the doubling time and want a future number.

Initial Value (Two-Value Method): Enter the amount you started with, like your first cell count or OD reading. It must be more than 0.

Final Value: Enter the amount at the end of your run. It must be larger than the initial value, or nothing has doubled. To express that jump as a percentage instead, try the Percent Change Calculator.

Duration: Enter the time that passed between the two readings, such as 30 for a 30-minute count. If you only have clock times for your samples, the Time Duration Calculator will work out the elapsed span first.

Time Unit (Two-Value Method): Pick seconds, minutes, hours, days, weeks, months, or years. Your answer comes back in this same unit.

Growth Rate (%): On the Growth Rate tab, enter the steady percent growth for each period, like 7 for 7% growth. Use a number above 0 and up to 10,000.

Time Unit (Growth Rate Method): Pick the unit that the growth rate applies to, such as "% per hour" for bacteria or "% per year" for population.

Initial Value (Reverse): On the Reverse tab, enter the amount you are starting with today.

Doubling Time: Enter how long it takes the amount to double, such as 20 minutes for E. coli in rich medium.

Elapsed Time: Enter how far ahead you want to look, like 35 for 35 hours of growth.

Time Unit (Reverse): Pick one unit for both the doubling time and the elapsed time.

Calculate and Reset: Click Calculate to see your results, or Reset to clear the form and start over.

What Is Doubling Time?

Doubling time is how long something takes to grow to twice its size. If a jar holds 1,000 bacteria today and 2,000 bacteria in 30 minutes, the doubling time is 30 minutes. It works for anything that grows by a steady percent each period: cells in a flask, tumors, money in a savings account, or a country's population.

How Doubling Time Works

Doubling time comes from exponential growth. In exponential growth, each cell (or dollar) makes more copies, and those copies make copies too. So growth speeds up over time even when the growth rate stays the same. As long as the rate is steady, the time to double never changes. Going from 100 to 200 takes the same time as going from 1 million to 2 million.

The Main Formulas

From two values: T_d = (t × ln 2) ÷ ln(V₂ ÷ V₁)
From a growth rate: T_d = ln 2 ÷ ln(1 + r)
Future value: V_f = V₀ × 2^{(t ÷ T_d)}

Here, ln means natural log, and ln 2 is about 0.693. The letter r is the growth rate written as a decimal, so 7% becomes 0.07. If you want to check a natural log by hand, the Log Calculator and the Exponent Calculator handle those pieces on their own.

The Rule of 70

The Rule of 70 is a quick shortcut. Divide 70 by the growth rate percent, and you get a close answer. At 7% growth, 70 ÷ 7 = 10 years to double. The exact answer is 10.24 years. The shortcut is good for small rates but drifts off for big ones, so use the exact formula when the numbers matter. Finance folks often use the closely related Rule of 72 Calculator, which fits typical interest rates a bit better.

Doubling Time in Biology

In a lab, doubling time is also called generation time. Scientists measure it to check if cells are healthy and growing normally. A slow doubling time can mean the cells are stressed, starved, contaminated, or too crowded. Cells only grow at this steady rate during the log phase (also called the exponential phase). Before that is the lag phase, when cells adjust and barely divide. After it comes the stationary phase, when food runs out and growth stops. Doubling time math only works during the log phase.

Bench work around a growth curve usually pulls in a few other tools: the Dilution Calculator for setting up your starting inoculum, the Serial Dilution Calculator for plate counts, the Molarity Calculator for media components, and the Tm Calculator when you follow up with PCR.

Typical Doubling Times

E. coli bacteria: about 20 minutes in rich broth
Yeast: about 90 minutes
HeLa human cancer cells: about 24 hours
Human stem cells: 24 to 48 hours
Mycobacterium tuberculosis: about 24 hours (very slow for a bacterium)

Where Else It Is Used

Doubling time shows up in money, disease tracking, and tech. Investors use it to see how fast savings grow, which is the same idea behind the Compound Interest Calculator, the CAGR Calculator, and the Future Value Calculator. Public health teams use it to see how fast an outbreak is spreading, since a short doubling time means fast spread. In computing, Moore's Law says the number of transistors on a chip doubles about every two years. The mirror image of doubling is decay, which you can explore with the Half Life Calculator or the Radioactive Decay Calculator.

Things to Watch For

Doubling time assumes growth stays steady. Real growth almost never does. Bacteria run out of food, populations run out of space, and markets change. Use doubling time for short-term estimates and for comparing growth speeds, not for far-off predictions. If your data wobbles, fitting a line to log-transformed counts with the Linear Regression Calculator gives a steadier rate than any single pair of readings. Also make sure your time units match. If you measure hours, your answer is in hours.

Formulas used

Doubling time from two values

T_d = \frac{t \cdot \ln 2}{\ln\!\left(\frac{V_2}{V_1}\right)}

Doubling time from a growth rate

T_d = \frac{\ln 2}{\ln(1 + r)}, \quad r = \frac{R\%}{100}

Implied growth rate from doubling time

r\% = \left(2^{1/T_d} - 1\right) \times 100

Rule of 70 approximation

T_{70} = \frac{70}{r\%}

Future value after elapsed time

V_f = V_0 \times 2^{\,t / T_d}

Number of doublings

n = \frac{t}{T_d}

Time to triple and time to grow tenfold

T_3 = \frac{\ln 3}{\ln(1+r)}, \qquad T_{10} = \frac{\ln 10}{\ln(1+r)}

Time unit conversion

T_{\text{unit}} = \frac{T_d \cdot s_{\text{from}}}{s_{\text{to}}}

Frequently asked questions

Can I use OD600 readings instead of cell counts?

Yes. OD600 tracks how cloudy your culture is, and it rises with cell number during log phase. Put your first OD in Initial Value and your later OD in Final Value.

Two tips: subtract your blank (plain media) from both readings first, and stay below about OD 0.8. Above that, the reading flattens out and your doubling time will look too slow.

How do I turn doubling time into specific growth rate (µ)?

Use this formula:

µ = ln 2 ÷ T_d = 0.693 ÷ T_d

So if your doubling time is 30 minutes (0.5 hours), µ = 0.693 ÷ 0.5 = 1.386 per hour.

The calculator also shows an Implied Growth Rate as a percent per period, which is the same idea written a different way.

Why does the calculator show an error when my final value is smaller?

Because nothing doubled. Doubling time only works when the value goes up.

If your count dropped, you have decay or cell death instead. Use a Half Life Calculator to find the halving time.

How many cell counts do I need?

Two counts from the log phase is the minimum. But two points can be noisy if one count is off.

For better numbers, take 4 or more counts, take the natural log of each, and fit a line with the Linear Regression Calculator. The slope of that line is µ, and doubling time is 0.693 ÷ slope.

What time unit should I pick for bacteria?

Use minutes or hours for most bacteria. E. coli doubles in about 20 minutes, so minutes work well.

Use hours or days for mammalian cell lines, yeast, and slow growers like M. tuberculosis. Your answer comes back in the unit you pick.

What does the Number of Doublings result mean?

It is how many times your value doubled during the time you entered. It can be a decimal.

For example, 2.5 doublings means the value grew 2^2.5 = about 5.7 times bigger. A fraction is normal, because cells in a flask do not all divide at the same moment.

How does the calculator handle months and years?

It uses average lengths so the math stays even:

1 month = 30.4375 days
1 year = 365.25 days

The year length includes leap years. That is why the unit table may show 365.25 days for a one-year doubling time.

What is the Time to Triple result for?

It shows how long the same growth takes to reach 3 times the start instead of 2 times. It is handy when your assay or protocol needs a set fold-increase, not just a doubling.

The Time to Grow 10× card does the same thing for a ten-fold rise, which is common when you scale up a culture.

Why is my doubling time longer than the textbook value?

Usually one of these:

You included lag phase or stationary phase data
The culture is short on food or oxygen
Temperature or pH is off
The cells are stressed, crowded, or contaminated

Only use readings from the straight part of a log-scale growth curve.

Can I use this for tumor doubling time?

Yes. Enter two tumor volumes or two measured sizes plus the days between the scans on the Two-Value Method tab.

Keep in mind that tumors rarely grow at a steady rate, so treat the answer as a rough speed check, not a forecast.

Do my two values need the same units?

Yes, and this matters a lot. Both values must be the same kind of measure: cells/mL and cells/mL, or OD and OD.

You do not need to convert to a special unit. The math uses the ratio of the two numbers, so 2 million and 8 million give the same answer as 2 and 8.

My samples have clock times, not a duration. What do I enter?

Find the gap between the two times first, then enter that gap in Duration.

The Time Duration Calculator works out the span for you if the times cross the hour or the day.

Should I use the Rule of 70 or the exact answer?

Use the exact answer for lab work, papers, and anything you report. Use the Rule of 70 only for quick head math.

The calculator shows both, plus the gap between them, so you can see how far off the shortcut is for your rate.

How do I plan a culture so it is ready in the morning?

Use the Reverse (Future Value) tab. Enter your starting cell number, the doubling time for your strain, and the hours you plan to leave it.

The tool shows the number you should end up with. If that number is too high, start with fewer cells or a shorter run.

What does the Growth Multiple or Growth Factor mean?

It is how many times bigger the final value is than the start. A growth factor of 2.5× means the value ended up 2.5 times its starting size.

To see that same jump as a percent, use the Percent Change Calculator.

Does doubling time change as cells grow?

In the math, no. During log phase, going from 1,000 to 2,000 takes the same time as going from 1 million to 2 million.

In real flasks, yes. Once food runs low or the dish gets crowded, growth slows and the true doubling time gets longer.