Enter colony counts and dilutions. Get CFU/mL with countability flags, audit trail, and CSV export. Data never leaves your browser.
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Load example CFU data to see the full workflow
| Dilution | A | B | |
|---|---|---|---|
| 10-6 | |||
| 10-5 | |||
| 10-4 | |||
| 10-3 |
Enter colony counts and hit Calculate
Results with petri dish visualization will appear here
When to use
Do not use for
Only plates within the method-specific countable range yield statistically reliable estimates. Pour plates: 15–300 colonies (FDA BAM Chapter 3). Spread plates: 30–300. Membrane filtration: 20–200 (USP <61>). Plates outside these ranges should be flagged, not averaged in.
The most common calculation error is using the wrong dilution factor. A dilution means the sample was diluted 100,000-fold — the factor is 0.00001, not . Always express the dilution factor as a decimal (e.g., = 0.0001) to avoid sign errors in the denominator.
Colonies growing at the edge of a pour plate or along the periphery of a spread plate can be undercounted or obscured. If more than 25% of colonies are at the plate edge, the result is unreliable. Use the proper agar volume (15–20 mL for standard Petri dishes) and spread evenly.
Under-incubation misses slow-growing organisms; over-incubation lets pinpoint colonies appear and causes swarming species to overgrow the plate. Count at the validated time point (typically 24–48 h at 35–37 °C for mesophilic bacteria) and document the exact duration.
TNTC (Too Numerous To Count) plates must be excluded from the weighted average, not estimated. Colony merger at high density causes severe undercounting. If all dilutions are TNTC, repeat the assay with additional higher dilutions — do not report a lower-bound guess.
Weighted average CFU/mL calculation with FDA BAM countability range flagging. Supports pour plate (15–300), spread plate (30–300), and membrane filtration (20–200) methods. Plates outside the countable range are flagged and excluded from the weighted estimate.
Last validated 2026-03-14. Calculations are designed for planning and documentation support; verify procurement decisions against manufacturer specifications or institutional SOPs.
ConductScience CFU / Dilution Series Calculator (v1.0). ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/cfu-calculator
A colony forming unit (CFU) is a measure of viable bacterial or fungal cells in a sample. Unlike direct cell counts (e.g., hemocytometer or spectrophotometry), CFU counts only living cells capable of dividing and forming a visible colony on agar media.
The term "colony forming unit" rather than "cell" is deliberate — a single colony may originate from one cell or from a clump of cells. CFU counts therefore represent a lower bound on the true number of viable organisms.
Serial dilution is a stepwise reduction of sample concentration. Starting from the neat (undiluted) sample, each transfer dilutes the previous solution by a fixed factor — typically 1:10 (tenfold).
For a 1:10 serial dilution: transfer 1 mL of sample into 9 mL of diluent, mix thoroughly, then transfer 1 mL of that tube into the next 9 mL tube. Each step increases the dilution by one order of magnitude: , , , and so on.
Plating multiple dilutions ensures that at least one dilution yields a countable number of colonies. The optimal range balances statistical precision (enough colonies to reduce sampling error) against colony crowding (too many causes merger and undercounting).
Vortex or shake each dilution tube for at least 5 seconds before transferring. Incomplete mixing is the most common source of inter-replicate variability and can bias counts by 50% or more.
Use fresh pipette tips for each transfer — carryover from higher concentrations can artificially inflate counts at higher dilutions. Pre-wet tips by pipetting up and down once before the transfer.
Record counts within the recommended incubation window (typically 24–48 hours at 35–37°C for mesophilic bacteria). Extended incubation allows pinpoint colonies to emerge and can inflate counts. Document the exact incubation time and temperature.
