How to Calculate CFU/mL from Plate Counts

You cannot count bacteria one by one at realistic concentrations, so microbiology uses the plate count: dilute the sample, spread a known volume on agar, incubate, and count the colonies. Each colony is taken to have grown from one viable, reproducing unit, and the count scales back up to the original concentration in CFU/mL. This guide derives the one-line formula, explains why the dilution factor is the reciprocal of the dilution, works the countable-range rule that decides which plate to trust, and states plainly what a CFU is not. The CFU calculator averages your replicate plates, returns CFU/mL and flags any plate outside the countable range.

What a colony-forming unit is

A colony-forming unit (CFU) is one viable cell — or one clump, pair or chain of cells — that gives rise to a single visible colony. The word “unit” matters: because several attached cells can grow into one colony, a CFU count is usually lower than the true number of individual cells. It is a measure of viable, reproducing units, not of cells.

That is also what makes it a viable count. Only organisms able to grow under your exact conditions — medium, temperature, oxygen, incubation time — form colonies. Viable-but-non-culturable cells, and anything whose needs the plate does not meet, are invisible to it. For total cells regardless of culturability, a direct microscopic count with the cell counting calculator is the complementary method.

The formula

CFU/mL of the original sample is:

• CFU/mL = (colony count × dilution factor) / plated volume (mL)

Three terms, each a common source of error:

• Colony count — the number of colonies on the plate (or the mean of replicate plates).
• Dilution factor — the reciprocal of the dilution you plated. A 10⁻⁵ dilution has a factor of 10⁵; a 10⁻⁶ dilution has 10⁶. Not the exponent (5), and not the dilution itself (10⁻⁵).
• Plated volume — the amount of that diluted sample you spread, in mL: 0.1 mL for the common 100 µL spread, 0.2 mL for 200 µL, 1.0 mL for a full pour plate.

Why the reciprocal, and why divide by volume

The dilution factor undoes the dilution. If you plated from a 10⁻⁵ tube, that tube is 100,000-fold more dilute than the original, so every colony on it stands for 10⁵ cells in the undiluted sample — multiply by 10⁵ to scale back up.

Dividing by the plated volume undoes the fact that you spread only a fraction of a millilitre. Plating 0.1 mL means the colonies you counted came from one tenth of a mL of that dilution, so there are ten times as many per mL — divide by 0.1 (equivalently multiply by 10).

A worked example

Spread 100 µL of a 10⁻⁵ dilution; after incubation you count 148 colonies.

• CFU/mL = (148 × 10⁵) / 0.1
• = 1.48 × 10⁷ / 0.1
• = 1.48 × 10⁸ CFU/mL

With replicate plates, average first. Counts of 140, 150 and 160 from 200 µL of a 10⁻⁵ dilution:

• mean = (140 + 150 + 160) / 3 = 150
• CFU/mL = (150 × 10⁵) / 0.2 = 1.5 × 10⁷ / 0.2 = 7.5 × 10⁷ CFU/mL

The countable range and choosing a plate

A plate count is only reliable within a window of colony numbers. Too few colonies and Poisson sampling error dominates — a plate with 8 colonies could easily have had 5 or 12, so the scaled-up concentration is wildly uncertain. Too many and colonies overlap into a lawn you cannot resolve.

The convention is to report only plates in the countable range, about 25–250 colonies here (many standards quote 30–300; state which you use). Outside it:

• Below the floor — plate a lower dilution (more cells) or a larger volume to raise the count. A plate with 12 colonies at 10⁻³ gives 1.2 × 10⁵ CFU/mL but is flagged as too sparse to trust.
• Above the ceiling — record as TNTC (too numerous to count) and plate the next higher dilution. A plate with 400 colonies is not reported; re-plate at 10⁻⁶.

This is why you plate a dilution series: you do not know in advance which dilution will land countable, so you spread several and use the one that falls in the window. Plan the series with the serial dilution calculator. When two dilutions both look usable, prefer the one nearer the middle of the range, and never average plates from different dilutions — they are not comparable.

CFU/mL vs CFU/g

For a liquid sample the answer is CFU/mL. For a solid — soil, food, tissue — you homogenise a known mass into a known volume and report CFU/g, where the dilution factor accounts for the mass-based dilution. The arithmetic is the same; only the denominator’s unit changes, from the plated volume of a liquid to the mass the sample was drawn from.

Pour, spread and membrane filtration

The plated volume differs by method, which is exactly the term that scales your answer:

• Spread plate — a small volume (typically 0.1 mL) spread over the surface. Fast, but limited to a fraction of a mL.
• Pour plate — the sample mixed into molten agar, often 1.0 mL. Handles a larger volume but exposes cells to warm agar.
• Membrane filtration — a large volume filtered and the membrane placed on agar. Used for dilute samples (water testing), where the “plated volume” is the whole filtered volume.

Enter the volume that matches your method; using 0.1 when you actually poured 1.0 is a ten-fold error.

Common mistakes

• Entering the dilution exponent (5) or the dilution (10⁻⁵) instead of the dilution factor (10⁵).
• Forgetting to divide by the plated volume, or entering it in µL instead of mL.
• Reporting a plate outside the countable range instead of re-plating a better dilution.
• Counting a lawn as “many” rather than recording TNTC and using a higher dilution.
• Averaging plates from different dilutions, which are not comparable.
• Assuming CFU = cells, ignoring that a clump or chain forms one colony and that non-culturable cells are missed.

Where this fits in the wider workflow

The plate count is the microbial counterpart of the hemocytometer: the cell counting calculator counts total cells directly under a microscope, while CFU counts only those that grow. Plan the dilutions with the serial dilution calculator or the dilution calculator, then carry two time-point CFU/mL values into the population doubling time calculator to get the generation time of the culture. All of these live together in the cell culture calculators hub.