CFU Serial Dilution Calculator
The viable plate count (standard plate count) determines how many live bacteria or fungi are in a sample by diluting it until a countable number of colonies grow on an agar plate. The formula is: CFU/mL = colonies / (volume plated in mL x dilution factor). Enter the number of colonies counted, the volume of diluted sample plated (typically 0.1 mL), and the dilution factor at which that plate was made. The calculator returns the estimated concentration in the original undiluted sample in CFU/mL and CFU/g (when counting from a known weight/volume sample).
CFU/mL formula
CFU/mL = colonies / (volume plated (mL) x dilution factor)
Example: 156 colonies, 0.1 mL plated, 10^-5 dilution: CFU/mL = 156 / (0.1 x 0.00001) = 1.56 x 10^8 CFU/mL.
Serial dilution best practices
- Use TSAPC/nutrient agar for total aerobic count; selective agars for specific organisms.
- Count only plates with 30-300 colonies (FDA Bacteriological Analytical Manual guideline).
- If multiple dilutions give countable plates, average the CFU/mL values.
- Express results to two significant figures and note the dilution used.
- Incubate at the appropriate temperature and time for the organism (e.g., 37 C for 24-48 h for E. coli).
Frequently asked questions
What is CFU?
CFU stands for colony-forming unit. It is a measure of viable bacteria or fungi in a sample. Each colony visible on an agar plate is assumed to have grown from a single viable cell (or a cluster), so colony count reflects the number of live microorganisms.
How does serial dilution work?
Serial dilution involves making a series of successive dilutions of a sample. For example, adding 1 mL of sample to 9 mL of diluent makes a 1:10 (10^-1) dilution. A further 1:10 step gives 10^-2, and so on. At the end, a known volume is plated to give a countable number of colonies.
What is the formula for calculating CFU/mL?
CFU/mL = colonies counted / (volume plated in mL x dilution factor). For example, 150 colonies on a plate inoculated with 0.1 mL of a 10^-5 dilution: CFU/mL = 150 / (0.1 x 0.00001) = 1.5 x 10^8 CFU/mL.
What is the ideal colony count per plate?
The countable range is typically 30-300 colonies per plate (AOAC and FDA standard plate count guidelines). Plates with fewer than 30 colonies may underestimate due to chance; plates with more than 300 are too crowded for accurate counting.
What volume is typically plated for viable plate count?
For spread plates, 0.1 mL (100 uL) is the standard volume. For pour plates, 1.0 mL is typical. This calculator accepts any volume; just ensure it matches what was actually plated.
Official sources
- FDA Bacteriological Analytical Manual: BAM Chapter 3: Aerobic Plate Count.
- CDC: Microbiology laboratory methods.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.