CFU per mL Calculator
Counting bacteria on a plate only tells you part of the story, because the sample was diluted and only a fraction was spread. This calculator recovers the original concentration in colony-forming units per millilitre from three numbers. Multiply the colonies you counted by the dilution factor, then divide by the volume plated in millilitres. The dilution factor is the reciprocal of the dilution, so a ten-thousand-fold dilution written as ten to the minus four has a factor of 10,000, and multiplying by it reverses the dilution. Dividing by the plated volume, often a tenth of a millilitre, scales the count up to a full millilitre. With 150 colonies, a dilution factor of 10,000 and 0.1 mL plated, the concentration is 150 times 10,000 divided by 0.1, which is 15,000,000 CFU per mL. For a reliable count, choose a plate holding roughly 30 to 300 colonies, since crowded or sparse plates give noisy numbers. This is the standard viable-count method used in microbiology and food safety testing, where a known concentration must be recovered from a spread plate. Use it to report a result or compare samples. Every figure is computed deterministically from your inputs, never estimated, with the method and a worked example shown below for verification.
Concentration is colonies times dilution factor over plated volume: CFU/mL = colonies x dilution factor / volume. With 150 colonies, a factor of 10,000 and 0.1 mL plated, the result is 15,000,000 CFU/mL.
CFU per mL formula
CFU/mL = colonies x dilution factor / volume plated (mL)
colonies = number counted on the plate
dilution factor = reciprocal of the dilution (10^4 = 10,000)
volume plated = millilitres spread on the plate
Multiplying by the dilution factor reverses the dilution, and dividing by the plated volume scales the count to a full millilitre.
Worked example
A plate from a 10 to the minus 4 dilution shows 150 colonies, with 0.1 mL plated.
- Dilution factor for 10^-4 = 10,000
- Colonies x factor = 150 x 10,000 = 1,500,000
- Divide by plated volume = 1,500,000 / 0.1
- CFU/mL = 15,000,000
The concentration is 15,000,000 CFU per mL. These are the calculator's default inputs, so the result above matches the widget exactly.
Dilution to factor
The dilution factor is the reciprocal of the dilution.
| Dilution | Dilution factor |
|---|---|
| 10^-2 | 100 |
| 10^-3 | 1,000 |
| 10^-4 | 10,000 |
| 10^-6 | 1,000,000 |
Microbiology and viable-count methods: US Geological Survey (USGS).
CFU per mL calculator: frequently asked questions
How do you calculate CFU per mL?
Multiply the number of colonies counted by the dilution factor, then divide by the volume plated in millilitres. With 150 colonies, a dilution factor of 10,000 and 0.1 mL plated, the result is 150 times 10,000 divided by 0.1, which is 15,000,000 CFU per mL.
What is the dilution factor?
The dilution factor is the reciprocal of the dilution. A ten-thousand-fold dilution, written as 10 to the minus 4, has a dilution factor of 10,000. Multiplying the colony count by this factor reverses the dilution to recover the original concentration.
Why divide by the plated volume?
You usually plate only a fraction of a millilitre, often 0.1 mL. Dividing by that volume scales the count up to a full millilitre so the result is expressed in CFU per mL, a standard concentration unit.
Which plate should I count?
Choose a plate with a countable number of colonies, typically between 30 and 300, so the count is statistically reliable and not crowded. Plates outside that range are usually recorded as too few or too many to count.
Is the result computed automatically?
Yes. The page multiplies the colony count by the dilution factor and divides by the plated volume deterministically. No value is estimated or hard-coded, so changing any input updates the concentration instantly.
Official sources
- Microbiology and viable-count methods: US Geological Survey (USGS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.