Calculate CFU/mL from colony counts, dilution, and plated volume, or from serial dilution transfers to estimate microbial concentration.
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CFU/mL Formula
The calculator uses one of two formulas depending on the mode you select.
Plate count mode:
CFU/mL = N / (V * D)
- N = number of colonies counted on the plate
- V = volume plated, in mL
- D = dilution plated, expressed as a decimal (e.g., 10^-6 = 0.000001)
Serial dilution mode: the calculator first builds the final dilution from the per-step transfer and diluent volumes.
D_final = ( V_t / (V_t + V_d) )^n CFU/mL = N / (V * D_final)
- V_t = sample volume transferred at each step
- V_d = diluent volume added at each step
- n = number of serial transfers
- N, V = colonies counted and volume plated, as above
Assumes each colony arose from one viable cell, transfer and diluent volumes are constant across steps, and plates are read within the standard countable range. Results outside that range carry more uncertainty.
Reference Tables
Use these to sanity check your inputs and your result.
| Colony count on plate | Interpretation |
|---|---|
| 0 | Below detection limit for that dilution |
| 1–29 | TFTC (too few to count); high statistical error |
| 30–300 | Standard countable range; report this plate |
| > 300 | TNTC (too numerous to count); use a higher dilution |
| Sample type | Typical CFU/mL range |
|---|---|
| Drinking water (potable) | < 500 |
| Pasteurized milk | 10^3 – 10^4 |
| Raw milk | 10^4 – 10^6 |
| Overnight E. coli broth | 10^8 – 10^9 |
| Saturated yeast culture | 10^7 – 10^8 |
Worked Example
You count 154 colonies on a plate. You plated 0.1 mL of a 10^-6 dilution.
- Reciprocal dilution = 10^6 = 1,000,000
- CFU/mL = 154 × 1,000,000 ÷ 0.1 = 1.54 × 10^9 CFU/mL
FAQ
What is a CFU? A colony forming unit is one viable cell or cluster of cells that grows into a single visible colony on agar. It is a count of culturable cells, not total cells.
Why is 30–300 the countable range? Below 30, random variation dominates the count. Above 300, colonies merge and get miscounted. The 30–300 window keeps statistical error low.
What if I plated more than one volume? Calculate each plate separately and average the CFU/mL values from plates that fall in the countable range.
How do I enter a 10-fold serial dilution? In serial mode, set the transferred volume to 1 mL and the diluent volume to 9 mL. Each step then equals a 1:10 dilution.
