CFU/mL Colony Count Calculator

A plate count turns colonies into a concentration using one idea: each visible colony grew from a single viable cell or clump, a colony-forming unit (CFU). Multiply the mean colony count by the dilution factor (the reciprocal of the dilution you plated — 10⁵ for a 10⁻⁵ dilution) and divide by the volume you spread in mL. That gives CFU/mL of the original sample: CFU/mL = (mean colonies × dilution factor) / plated volume. The number is only trustworthy when the mean count lands in the countable range, about 25–250 colonies here (some standards quote 30–300) — too few and sampling error dominates, too many and colonies overlap. Enter your plate counts, dilution factor and plated volume below; the calculator averages replicates, returns CFU/mL and flags any plate outside the countable range.

How many live bacteria are in a sample? You cannot count them under a microscope 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 that appear. Each colony is taken to have grown from one viable, reproducing unit — hence "colony-forming unit", CFU — and the count scales back up to the original concentration. This page is part of the Cell Culture calculator set and does that scale-up, with the reliability check that most tools skip.

The arithmetic is one line but every term matters. The dilution factor is the reciprocal of the dilution you actually plated: a 10⁻⁵ dilution carries a factor of 10⁵. The plated volume is the amount of that diluted sample spread on the agar, 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. Multiply the colony count by the dilution factor and divide by the volume to get CFU/mL. Getting the factor and the volume right is where most errors live; the serial dilution calculator helps you plan the dilution series so one plate lands countable.

A single plate count is noisy, so you plate replicates and often a dilution series. The mean of the replicate counts is what you scale up. Then comes the countable-range rule: a plate with only a handful of colonies carries huge Poisson sampling error, while a lawn of hundreds has overlapping colonies you cannot resolve. The convention is to report only plates whose count falls in a defined window — 25 to 250 colonies here, though you will also see 30 to 300 in other standards. A plate above the ceiling is recorded as TNTC (too numerous to count) and a lower dilution is used instead. This calculator averages your plates and warns when the mean falls outside the range rather than silently printing a number you should not trust.

Two caveats define what a CFU is and is not. A CFU is not necessarily one cell: a clump, a pair or a chain (streptococci, for example) gives one colony from several cells, so CFU/mL typically underestimates the true single-cell count. And a plate count measures only cells that can grow under the exact conditions you used — medium, temperature, oxygen, time. Viable-but-non-culturable cells, and any organism whose needs the plate does not meet, are invisible to it. For total cell numbers regardless of culturability, a direct microscopic count with the cell counting calculator is the complementary method.

Colony counts per plate

Comma-separated colony counts from replicate plates spread at the same dilution. The calculator averages them.

Dilution factor (×)

Reciprocal of the dilution plated — 1e5 for a 10⁻⁵ dilution, 1e6 for 10⁻⁶.

Volume plated (mL)

Volume spread/poured per plate in mL — 0.1 for 100 µL, 0.2 for 200 µL, 1.0 for a full pour plate.

Viable concentration1.48e+8 CFU/mL

Mean colony count148 colonies

Plates averaged1 plates

Dilution factor1e+5 ×

Volume plated0.1 mL

What this calculator gives you that others don’t: Most CFU tools (punnettsquare.org, Westlab, online-colony-counter, ConductScience) take a count, a dilution and a volume and print CFU/mL — some add a confidence interval or a pass/fail countability flag — but they keep the method opaque: no derivation of why the dilution factor is the reciprocal of the dilution, no explanation of the pour/spread/membrane volume conventions, no guidance on which plate of a series to choose, no CFU/mL vs CFU/g distinction, and no statement that a CFU may be a clump so the answer underestimates single cells. This calculator averages replicate plates, shows every intermediate term, explains the reciprocal-dilution and plated-volume steps, applies the countable-range rule (25–250, noting the 30–300 alternative) as an explicit below/above warning with the fix for each case, and links to the serial-dilution, dilution and cell-counting tools so the dilution planning, plating and microscopic counting connect end to end. It carries four worked examples with real numbers, tiered sources including the LibreTexts bacterial-numbers lab, assumptions and a review date.

How to use the cfu calculator

Plate a dilution series so that at least one dilution lands in the countable range after incubation — plan it with the serial dilution calculator.

After incubation, count the colonies on the plates from one dilution. Choose a plate with roughly 25–250 colonies; if several qualify, count the replicates.

Enter the counts comma-separated in Colony counts per plate (e.g. 140, 150, 160). The calculator averages them.

Enter the Dilution factor as the reciprocal of the dilution plated — 1e5 for 10⁻⁵, 1e6 for 10⁻⁶.

Enter the Volume plated in mL — 0.1 for 100 µL, 0.2 for 200 µL, 1.0 for a pour plate.

Press Calculate. The primary card is CFU/mL; supporting cards show the mean colony count, plates averaged, dilution factor and plated volume. Heed any below/above countable-range warning.

CFU/mL Colony Count formula

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

dilution factor = 1 / dilution plated (10⁵ for a 10⁻⁵ dilution)

countable range ≈ 25–250 colonies per plate (some standards 30–300)

CFU/g = (mean colonies × dilution factor) / plated volume, with mass-based dilutions

The dilution factor is the reciprocal of the dilution, not the exponent — a 10⁻⁵ dilution gives 10⁵. Divide by the plated volume in mL because you spread only a fraction of a millilitre; plating 0.1 mL means the count represents one tenth of a mL of that dilution. One colony is assumed to arise from one viable unit, so the answer is in CFU, not cells. Report only plates inside the countable range.

Variables and units

Symbol	Meaning	Unit
mean colonies	Average colony count across the replicate plates	colonies
dilution factor	Reciprocal of the dilution plated (10⁵ for 10⁻⁵)	×
plated volume	Volume of diluted sample spread per plate	mL
CFU/mL	Viable concentration in the original sample	CFU/mL
countable range	Colony counts that give a reliable plate	colonies

How cfu is calculated

Average the colony counts across the replicate plates to get the mean colony count.
Multiply the mean by the dilution factor (the reciprocal of the dilution plated).
Divide by the plated volume in mL to get CFU/mL of the original sample.
Check the mean against the countable range: warn if below ~25 (plate a lower dilution or larger volume) or above ~250 (plate a higher dilution; record as TNTC).

Worked laboratory examples

Example 1 — 100 µL of a 10⁻⁵ dilution

Given: mean colonies 148 · dilution factor 10⁵ · plated volume 0.1 mL

CFU/mL = (148 × 10⁵) / 0.1 = 1.48 × 10⁷ / 0.1 = 1.48 × 10⁸.
148 is inside the 25–250 countable range, so the plate is reliable.

Result: 1.48 × 10⁸ CFU/mL

Example 2 — 200 µL spread, replicate plates

Given: colony counts 140, 150, 160 · dilution factor 10⁵ · plated volume 0.2 mL

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

Result: 7.5 × 10⁷ CFU/mL from three countable plates

Example 3 — Too few colonies to trust

Given: mean colonies 12 · dilution factor 10³ · plated volume 0.1 mL

CFU/mL = (12 × 10³) / 0.1 = 1.2 × 10⁵.
But 12 is below the ~25 floor: a plate this sparse is dominated by sampling error.
Plate a lower dilution (more cells) or a larger volume to bring the count into range.

Result: 1.2 × 10⁵ CFU/mL, flagged: too few colonies — re-plate a lower dilution

Example 4 — Too numerous to count

Given: mean colonies 400 · dilution factor 10⁵ · plated volume 0.1 mL

CFU/mL = (400 × 10⁵) / 0.1 = 4 × 10⁸.
400 exceeds the ~250 ceiling: colonies overlap and the count is unreliable.
Record as TNTC and plate the next higher dilution (10⁻⁶) instead.

Result: flagged TNTC — use a higher dilution; the raw 4 × 10⁸ is not trustworthy

Want the reasoning behind the numbers? Read How to Calculate CFU/mL from Plate Counts.

How to interpret the result

CFU/mL is a viable concentration: the number of colony-forming units per millilitre of your original, undiluted sample. It is what you quote for an inoculum, a contamination level or a growth-curve density.

Choose the plate that lands inside the countable range. When a dilution series gives one plate at 15 and another at 400, neither is ideal — re-plate an intermediate dilution so the count falls between the limits.

The dilution factor and plated volume scale in opposite directions: a higher dilution (bigger factor) or a smaller plated volume both raise the computed concentration for the same colony count. Double-check the exponent you enter.

Because a CFU may be a clump or chain, treat CFU/mL as a lower bound on single-cell numbers, and as a count of only those organisms that grow under your specific conditions.

Assumptions

Each colony arises from a single viable colony-forming unit that was evenly distributed in the plated volume.
The dilution factor entered is the reciprocal of the actual dilution plated, including all prior dilution steps.
The plated volume is accurate and the sample was spread or poured uniformly.
Incubation conditions allowed every viable unit present to form a visible colony.
The count is only quantitatively meaningful when the mean falls inside the countable range.

Limitations

Measures culturable cells only; viable-but-non-culturable organisms and those needing other conditions are missed.
A CFU can be a clump, pair or chain, so CFU/mL typically underestimates the true single-cell concentration.
Depends on even spreading and an accurate dilution; a poorly spread plate or pipetting error biases the count.
Counts outside the countable range are statistically weak (too few) or unresolvable (too many, TNTC).

Common mistakes

Entering the dilution exponent (5) instead of the dilution factor (10⁵), or the dilution (10⁻⁵) instead of its reciprocal.
Forgetting to divide by the plated volume, or entering it in µL instead of mL.
Reporting a plate that is outside the countable range instead of re-plating a better dilution.
Counting a lawn as "many" rather than recording it as TNTC and using a higher dilution.
Averaging plates from different dilutions, which are not comparable.

FAQ

How do you calculate CFU/mL?

CFU/mL = (mean colony count × dilution factor) / plated volume in mL. The dilution factor is the reciprocal of the dilution you plated (10⁵ for a 10⁻⁵ dilution), and the plated volume is what you spread on the agar (0.1 mL for 100 µL). Report only plates with a count in the countable range.

What is a colony-forming unit?

A CFU is one viable cell, or one clump or chain of cells, that gives rise to a single visible colony. Because several attached cells can form one colony, CFU counts are usually lower than the true number of individual cells — it is a measure of viable, reproducing units, not of cells.

What is the countable range and why does it matter?

It is the window of colony counts that give a statistically reliable plate — about 25–250 here, though many standards use 30–300. Below it, Poisson sampling error is large; above it, colonies overlap and you cannot count them accurately (TNTC). Plating a dilution series ensures at least one plate lands in the window.

What dilution factor do I enter?

The reciprocal of the dilution plated. If you plated from a 10⁻⁵ dilution tube, the factor is 10⁵ (enter 1e5). If the sample was diluted in several steps, the factor accounts for the total dilution of the tube you spread from, not just the last step.

Is CFU/mL the same as cells/mL?

No. CFU/mL counts only viable units able to form colonies under your conditions, and a unit may be more than one cell. A direct microscopic count (hemocytometer) gives total cells/mL, live and dead. CFU/mL is typically lower than a total count; use each for its purpose.

Scientific Sources

LibreTexts — Microbiology Laboratory Manual (Hartline) 1.15: Determination of Bacterial Numbers · Tier 2 source
Wikipedia — Colony-forming unit (definition and the CFU/mL calculation) · Tier 3 source
Wikipedia — Serial dilution (dilution factor feeding the plate count) · Tier 3 source

External links open in a new tab and carry rel="nofollow". We cite primary references (NIST, NIH/NCBI, peer-reviewed papers) and established manufacturer technical documentation (NEB, Thermo Fisher, Promega, IDT). Calculated results are estimates; your protocol and manufacturer guidance take precedence.

Last reviewed: 2026-09 · Method version: 1.0

CFU/mL Colony Count Calculator