Bacterial CFU Estimation Calculator
The colony forming unit (CFU) count is the workhorse measurement of viable microbial concentration in a sample. After plating a diluted sample on agar and incubating, you count colonies and scale the result back to the original concentration. Enter the number of colonies counted, the dilution factor (as a fraction such as 0.001 for 1:1000), and the volume plated in milliliters. The calculator returns the concentration in CFU per mL, along with the effective dilution and total viable cells in your stated sample volume.
CFU per mL formula
CFU/mL = colonies / (volume plated * dilution factor)
Effective scale factor = 1 / (volume plated * dilution factor)
Total CFU = CFU/mL * sample volume
The dilution factor is the fraction of the original concentration that was plated (0.001 means 1:1000). Volume plated and sample volume are in mL. All inputs must be positive for a valid result.
Microbiology counting context
- The standard plate count assumes one colony grows from one viable cell or cell cluster.
- Plates with 25 to 250 colonies are considered statistically reliable in standard methods.
- Serial dilutions multiply: each 1:10 step reduces concentration by another factor of ten.
- CFU is a measure of viable, culturable cells and may underestimate total cells that do not grow on the medium used.
- Report CFU/mL with the dilution and plating volume so others can reproduce the estimate.
Bacterial CFU: frequently asked questions
What is a CFU?
CFU stands for colony forming unit, an estimate of the number of viable microbial cells in a sample. Each colony on an agar plate is assumed to arise from a single viable cell or cluster, so counting colonies and correcting for dilution gives cell concentration.
What is the CFU per mL formula?
CFU per mL equals the number of colonies counted divided by (the volume plated in mL times the dilution factor): CFU/mL = colonies / (plated volume * dilution factor). The dilution factor is entered as a fraction (for example 0.001 for a 1:1000 dilution), so dividing by it scales the count back up to the undiluted concentration.
How do I express the dilution factor?
Enter the dilution factor as the fraction of original concentration plated. A 10-fold (1:10) dilution is 0.1, a 1:100 dilution is 0.01, and a 1:1000 dilution is 0.001. Serial dilutions multiply: three 1:10 steps give a combined dilution factor of 0.001.
What colony count range is reliable?
Standard plate count guidance treats plates with roughly 25 to 250 colonies as statistically reliable. Below 25 the count is imprecise; above 250 colonies merge and are undercounted. Choose the dilution that lands in this range.
Why multiply by the inverse of the dilution?
Plating a diluted sample counts only a fraction of the original organisms. Dividing the colony count by the dilution factor (which is less than 1) scales the result back up to the concentration in the undiluted sample, then dividing by plated volume converts to per mL.
Official sources
- U.S. Food and Drug Administration: BAM Chapter 3, Aerobic Plate Count.
- U.S. Environmental Protection Agency: Clean Water Act Analytical Methods.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.