CFU/mL calculator (plate count)
Count the colonies on a plate, enter the dilution that plate came from and the volume you spread or poured. The calculator gives the concentration in CFU per millilitre of the original sample, its base-10 logarithm, and, if you enter the sample volume, the total number of viable cells. It also tells you whether the plate is in the usual 30–300 countable range.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Count the colonies on one plate, ideally one with 30–300 colonies.
- Enter the dilution factor of the tube that plate came from.
- Enter the volume plated in mL (0.1 mL is typical for spread plates, 1 mL for pour plates).
- Optionally enter the total sample volume to get the total CFU in it.
CFU/mL = colonies × dilution factor ÷ volume plated (mL)
- log₁₀ CFU/mL = log₁₀ of that value
- Total CFU = CFU/mL × sample volume (mL)
Worked example
150 colonies on the plate made from the 10⁻⁶ dilution, 0.1 mL spread:
150 × 1,000,000 ÷ 0.1 = 1.5 × 10⁹ CFU/mL (1,500,000,000), or 9.176 log CFU/mL.
42 colonies from 1 mL of the 10⁻³ dilution: 42 × 1,000 ÷ 1 = 42,000 CFU/mL; in a 10 mL sample that is 420,000 CFU.
Tips
- Average duplicate plates. Count two or three plates of the same dilution and use the mean colony count.
- Serial dilutions multiply. Three 1:10 steps make 1:1,000; a 1:100 step followed by a 1:10 step also makes 1:1,000.
- Grams instead of millilitres. For a food or soil sample homogenised 1:10 (10 g in 90 mL), the result is per gram of the original sample when the homogenate counts as the first 10⁻¹ dilution.
Frequently asked questions
How do you calculate CFU per mL?
CFU/mL = number of colonies × dilution factor ÷ volume plated in mL. With 150 colonies on the plate from the 10⁻⁶ dilution and 0.1 mL spread: 150 × 1,000,000 ÷ 0.1 = 1.5 × 10⁹ CFU/mL.
What is the dilution factor?
The reciprocal of the dilution. A sample diluted 1:10 six times in a row (10⁻⁶) has a dilution factor of 1,000,000. Enter the factor, not the fraction: 1000000 for 10⁻⁶, 1000 for 10⁻³, 1 for an undiluted sample.
Why only plates with 30 to 300 colonies?
Below about 30 colonies chance variation is large; above about 300 colonies merge and compete, so counts are too low. 30–300 is the widely used range for standard plates, and some methods use 25–250. The calculator still computes other counts but flags them.
Why colony forming units and not cells?
A colony may grow from a single cell or from a clump or chain of cells, so the count is of units that formed colonies. Dead cells, and live cells that cannot grow on that medium, are not counted.
What is log CFU used for?
Microbial counts span many orders of magnitude, so they are compared and averaged as base-10 logarithms. 1.5 × 10⁹ CFU/mL is 9.176 log CFU/mL. A difference of 1 log is a tenfold difference.