What this is for: Back-calculates culture density (CFU/mL) from plate counts, flagging plates outside the 30–300 countable range and averaging the countable ones.
The formula: CFU/mL = colonies ÷ (dilution plated × volume plated in mL)
How to use:
- Enter one dilution exponent per plate, in the order the plates were counted.
- Enter the volumes plated in mL and the colonies counted, one per plate in the same order.
- Check the per-plate table for CFU/mL and the 30–300 countable flag.
- Read the mean CFU/mL over countable plates only, the SD over all plates, and log₁₀ CFU/mL.
Bacterial CFU/mL Calculator (spread / pour plate)
Enter the dilution as an EXPONENT (e.g. -5 means a 10^-5 dilution), the volume plated in mL, and colonies per plate. CFU/mL = colonies / (dilution × volume). Plates with 30–300 colonies are the countable range; others are flagged.
Mean CFU/mL, countable plates only
SD of CFU/mL (all plates)
log10 CFU/mL
| Dilution exponent (10^x) | Dilution factor | Volume plated (mL) | Colonies counted | CFU/mL | Countable? (30–300) |
|---|
Prefer to work offline or keep a copy? The download link is right below.
Download the Bacterial CFU/mL Calculator workbook (.xlsx)
Download the Bacterial CFU/mL Calculator workbook (.xlsx)
Notes & limits (from the workbook): Enter the dilution as an EXPONENT (e.g. -5 means a 10^-5 dilution), the volume plated in mL, and colonies per plate. CFU/mL = colonies / (dilution × volume). Plates with 30–300 colonies are the countable range; others are flagged and excluded from the countable mean.