Microbiology · Plate counts
CFU per mL calculator.
Turn a colony count into colony-forming units per mL, with replicate plates averaged and a countable-range check.
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How CFU/mL is calculated
A colony-forming unit (CFU) is a viable cell or clump that grows into one visible colony. The standard plate-count formula is CFU/mL = colonies x dilution factor of the plate / volume plated (mL). The dilution factor is the reciprocal of the dilution: a 10-6 tube has a factor of 1,000,000. Teaching-lab practice is to count only plates with 30 to 300 colonies.
Worked example. 156 colonies from 0.1 mL of the 10-6 tube: 156 x 1,000,000 / 0.1 = 1.56 x 109 CFU/mL. If you plated 1 mL instead, the same count would mean 1.56 x 108 CFU/mL.
With replicate plates the calculator averages the counts first. CFU is a count of viable, culturable units, so it will be lower than a total-cell count (for example from a hemocytometer or an OD600 estimate). To plan the dilutions, use the serial dilution calculator.
Frequently asked questions
What is a countable plate?
A plate with 30 to 300 colonies is the usual range for counting. Fewer than 30 carries high sampling error and more than 300 means colonies overlap and are under-counted.
How do I convert a 10^-6 dilution to a dilution factor?
Take the reciprocal. 10^-6 becomes 1,000,000, which is what you enter in the dilution factor box.
Why is the result different from my OD600 estimate?
CFU counts only cells that grow into colonies under your plating conditions. OD600 also reflects dead cells and debris, so it usually reads higher.
Do I count the volume plated or the volume of the tube?
Use the volume actually spread on the plate, in mL, for example 0.1 mL for 100 uL.
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Research use and scope
This is arithmetic for research-bench planning, not clinical, diagnostic or safety advice. Conversion factors are standard approximations; strain, instrument, reagent and protocol differences matter. Check results against your own protocol, kit manual and lab supervisor, and follow your institution's biosafety rules.