CFU Calculator
Count colonies on a plate, enter the dilution factor and the volume you plated, and the calculator returns CFU/mL (or CFU/g for solid samples), with a confidence note for countable plate ranges. Works for any plate format.
Calculator
Enter values and calculate to see the result
Original sample
Bench note
CFU/mL = (colonies × dilution factor) ÷ volume plated (mL)
Each colony arises from one viable cell (or clump) in the volume you plated. Dividing the count by the plated volume in millilitres scales to per-mL, and multiplying by the dilution factor scales back up to the original, undiluted sample. Plates outside the 30–300 countable range are flagged because counts there are statistically unreliable.
About this calculation
What this is
CFU/mL is a way to express the concentration of viable cells in a culture by counting the colonies they form on a plate. The plate count is scaled up by the dilution factor and back-corrected for the volume you plated, giving the concentration in the original sample.
Why it is used
Microbiology needs a titer that connects a bench observation (colonies on a plate) back to a sample concentration. CFU/mL is that bridge — and the same formula handles CFU/g for soil or food samples by entering the homogenate volume.
Why this calculator exists
The CFU/mL formula is short (count × dilution / volume) but the units and the countable range (typically 25–250 colonies per plate) are easy to forget. The calculator flags plates outside that range so you can re-pick a dilution rather than trust a number at the limit of the assay.
Key assumptions
- Each colony originated from a single viable cell (no chains or clumps counted as one).
- The dilution series is accurate; any error in the dilution propagates linearly into CFU/mL.
- The plating volume was measured precisely (small pipetting errors are amplified at low volumes).
Limitations
- It cannot detect cells that do not form colonies on the chosen medium or conditions.
- It does not distinguish species — that requires selective media or molecular confirmation.
- It does not model crowding effects on heavily overgrown plates.
What this calculates
- CFU per mL CFU/mL
- Viable concentration of the original, undiluted sample.
- CFU per mL (scientific) CFU/mL
- The same result in scientific notation, e.g. 8.7 × 10⁷.
Frequently asked questions
What is CFU/mL and why do plate counts use it?
CFU/mL — colony-forming units per millilitre — estimates how many viable microorganisms were present in a sample by counting the colonies that grow on a diluted plate and scaling them back to the original sample. Plate counts are the classical viable measure of microbial load in food, water, and culture work because, unlike optical density, they count only organisms able to grow. Each colony is treated as one viable cell or one unit of growth, so clumps count as a single CFU.
How do I calculate CFU per mL of a bacterial sample?
Divide the colonies counted by the volume plated and the dilution it came from: CFU/mL = colonies ÷ (volume plated in mL × dilution), which is the same as colonies × dilution factor ÷ volume plated. For example, 87 colonies from plating 100 µL of a 10⁻⁶ dilution gives 8.7 × 10⁸ CFU/mL in the original sample. This calculator also offers the same result in scientific notation, because plate-count figures span many orders of magnitude.
What do I enter into this CFU calculator?
Three values: the number of colonies you counted on the plate (a whole number), the volume of diluted sample you spread or poured, in µL, and the dilution factor of the tube that volume came from — entered as the reciprocal of the dilution, so a 10⁻⁶ dilution means entering 1000000. All inputs must be positive, and the result is reported as CFU/mL of the original, undiluted sample.
What if my plate has fewer than 30 or more than 300 colonies?
The calculator still returns the result but flags it with a warning, because 30–300 colonies is the statistically countable range. Counts below 30 are less reliable, and counts above 300 risk underestimation from colony overlap on the crowded plate — the warning advises counting a higher dilution instead. A plate with zero colonies is rejected: report that sample as below the detection limit rather than as 0 CFU/mL.
How does the CFU calculation fit into a serial dilution workflow?
CFU/mL is the readout at the end of a dilution plating series: you prepare the dilution series, plate a known volume from one tube, count the colonies, and this calculator scales the count back through that tube’s dilution factor to the original sample. Any error made in the dilutions propagates linearly into the CFU/mL result, so the dilution factor you enter has to match the tube you actually plated. The related-tools panel on this page links to the serial dilution calculator for planning the dilution series itself.
For research and educational use only. Not for clinical or diagnostic decisions. Always verify calculations independently before use in critical applications.