Colony Forming Units (CFU) Calculator
Calculate CFU/mL from plate counts. Account for dilution factor and volume plated to find original bacterial concentration. Convert between CFU/mL and total CFU.
🦠 Cell Biology📐 CFU/mL = colonies / (dilution · volume plated)🧬 Biology
Colonies counted on plate
Dilution factor (e.g. 10^-3 = 0.001)
Volume plated (mL)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Colony Forming Units (CFU) Calculator | — | CFU/mL = colonies / (dilution · volume plated) | CFU/mL |
Step-by-Step Examples
Example 1
Standard plate count
150 colonies on plate after 1:1000 dilution (d=0.001), plated 0.1 mL.
- CFU/mL = 150/(0.001×0.1) = 150/0.0001 = 1,500,000
- = 1.5×10^6 CFU/mL
✓ 1.5×10^6 CFU/mL
Example 2
Dilution series
Three plates: 10^-5: 48 colonies; 10^-6: 6 colonies; 10^-4: too many (TNTC).
- Use 10^-5 plate (30-300 colonies is countable range)
- CFU/mL = 48/(10^-5 × 0.1) = 4.8×10^7
✓ 4.8×10^7 CFU/mL (from 10^-5 plate)
Example 3
Total CFU in sample
50 mL broth culture at 1.5×10^6 CFU/mL.
- Total CFU = 1.5×10^6 × 50 = 7.5×10^7 CFU total
✓ 7.5×10^7 total CFU in 50 mL sample
Real-World Applications
Antibiotic Testing
CFU counts measure bactericidal vs bacteriostatic effects. Killing curves use CFU/mL over time.
Food Safety
CFU/g standard for food safety. Acceptable limits: <10 CFU/g for pathogens in ready-to-eat foods.
Microbiology Research
Quantitative viability assay. Standard method since Koch's era. Simple and direct.
Pharmaceutical Manufacturing
Bioburden testing for sterile products. FDA requires < 10 CFU/mL in injectable drugs.
Common Mistakes to Avoid
⚠️
Count plates with 30-300 colonies
TFTC (too few to count, <30): imprecise. TNTC (too many to count, >300): colonies merge. The 30-300 range gives reliable statistical counts.
⚠️
Dilution factor = decimal, not ratio
10^-3 dilution = 0.001 as decimal in formula. Not 1000.
⚠️
Duplicates or triplicates recommended
Average multiple plates from same dilution. Report mean ± SD. CFU has ~10% CV with good technique.
Frequently Asked Questions
What is a CFU? ▾
Colony forming unit: a single bacterium (or cluster) that grows into a visible colony on agar. 1 CFU represents ≥1 viable cell. Clusters give the same CFU as single cells.
Why 30-300 colonies? ▾
Statistical: 30 colonies = 18% CV; 300 colonies = 6% CV. Below 30: too variable. Above 300: colonies overlap and are hard to count accurately.
What is the difference between CFU/mL and cells/mL? ▾
CFU/mL counts only culturable (viable and not in clumps) cells. Cells/mL from microscopy counts all. Total counts are always ≥ CFU/mL.
How do I count TNTC plates? ▾
Plate next dilution, or use serial dilution results to interpolate. Never estimate from TNTC plates — results are unreliable.
What is the Most Probable Number (MPN) method? ▾
Alternative to CFU for dilute samples. Uses multiple tubes at multiple dilutions; reads as turbid/clear. Used for drinking water coliforms when CFU is impractical.