CFU per mL Calculator
Solve CFU/mL = colonies × dilution factor / volume plated to back-calculate the original bacterial concentration from a countable agar plate. Two 3D diagrams compare an undercountable, overgrown plate to a properly countable one, and charts show how plate colony count and dilution factor relate to final CFU/mL.
Reviewed by the ToolNestr Editorial Team — July 2026
Overgrown vs. countable plate
1. Overgrown (too many colonies)
Colonies overlap and merge — impossible to count accurately, outside the 30-300 rule.
2. Countable plate
Well-separated colonies within the 30-300 reliable counting range.
CFU/mL graphs
How it works
The core idea in one line: a countable agar plate is really just a small, diluted snapshot of the original sample, so multiplying its colony count back up by exactly how much the sample was diluted (and how little of it was actually plated) reconstructs the original bacterial concentration.
CFU/mL = colonies × dilution factor / volume plated (mL)
back-calculates original concentration from a countable plate
Plating an undiluted, heavily-populated bacterial sample would produce so many overlapping colonies that counting becomes impossible — so samples are diluted first to bring the colony count down into a reliably countable range. To reverse this and recover the original concentration, the observed colony count is scaled back up by the dilution factor (how much the sample was diluted) and back down by the fraction of a millilitre actually spread on the plate. The resulting CFU/mL figure represents an estimate of viable, colony-forming cells in the original, undiluted sample — reliable only when the plate's colony count itself falls within the standard 30-300 countable window.
Worked example 1 — a well-diluted, countable plate
Given: 150 colonies counted on a plate made from a 10⁻⁵ dilution (dilution factor = 100,000), with 0.1 mL plated.
A count of 150 falls comfortably within the reliable 30-300 countable range, making this a statistically trustworthy result.
Worked example 2 — a lighter dilution
Given: 45 colonies counted on a plate made from a 1,000-fold dilution, with 0.5 mL plated.
A count of 45 is at the lower edge of the reliable range — still usable, but a slightly less dilute plate would generally give a more statistically confident count.
Interpreting colony counts against the 30-300 rule
The same plate count can be reliable or unusable depending on where it falls.
| Colony count | Interpretation |
|---|---|
| <30 | Too few — statistically unreliable, use a less dilute plate |
| 30-300 ★ | Countable range — use this count |
| >300 | Too many (likely overlapping) — use a more dilute plate |
| TNTC (too numerous to count) | Confluent growth — count is not usable at all |
★ Reference range. Because dilution level is chosen before the true concentration is known, labs typically plate several dilutions in parallel to ensure at least one lands in this window.
Where CFU/mL actually matters
🍔 Food safety testing
Food industry labs quantify bacterial contamination in products using CFU/mL (or CFU/g for solids), comparing results against regulatory safety thresholds.
💧 Water quality monitoring
Drinking water and recreational water testing reports bacterial concentration (like E. coli or coliform CFU/mL) to assess contamination risk against public health standards.
💊 Pharmaceutical and vaccine production
Bioprocessing facilities monitor CFU/mL throughout fermentation to track culture growth and ensure production batches meet target cell densities.
🏥 Clinical microbiology diagnostics
Urine and other clinical sample cultures are quantified in CFU/mL, with a threshold count often used to distinguish a genuine infection from incidental contamination.
Common misconceptions
"CFU/mL counts every bacterial cell in the sample, dead or alive."
CFU specifically counts viable, colony-forming cells — dead cells or those that can't grow under the plating conditions used simply won't form a colony and are not counted, which is exactly why CFU counts are typically lower than total microscopic cell counts.
"Any colony count from a plate can be used directly in the formula."
Counts below 30 or above 300 fall outside the statistically reliable range and shouldn't be trusted at face value — labs are expected to select a dilution that produces a plate within the 30-300 countable window before reporting a result.
"One colony always comes from exactly one original bacterial cell."
A colony usually originates from a single cell, but it can occasionally arise from a small clump of cells that were plated together — this is one reason CFU is described as an estimate of viable cell count, not a perfectly exact one.
"Higher dilution factor always means a more accurate result."
The dilution factor itself doesn't determine accuracy — what matters is whether the resulting colony count falls within the reliable 30-300 range. Too high a dilution can just as easily produce an unreliable (too-few) count as too low a dilution can produce an uncountable (too-many) one.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Microbiology — Chapter 9, "Microbial Growth" (free, peer-reviewed, standard plate count methodology). openstax.org
- • American Public Health Association, Standard Methods for the Examination of Water and Wastewater — standard plate count procedure.
- • Tortora, Funke & Case, Microbiology: An Introduction — Standard plate count chapter.
CFU/mL = colonies × dilution factor / volume plated (mL). The 30-300 colony rule is the standard reference range for reliable plate counting.
How to use this calculator
Enter colony count
The number of colonies counted on the plate.
Enter dilution factor and volume plated
The total dilution applied and the volume of diluted sample spread on the plate.
Read CFU/mL
Original concentration solves instantly, with a countable-range check.
Related tools
Frequently asked questions
What does CFU stand for?
CFU stands for Colony Forming Unit — a measure of viable, culturable bacterial (or fungal) cells, based on the assumption that each visible colony grew from one original living cell (or small cluster).
What is the standard formula for CFU/mL?
CFU/mL = (number of colonies counted) × (dilution factor) / (volume plated in mL) — this reverses the dilution and plating steps to estimate the concentration in the original, undiluted sample.
What is the 30-300 rule?
Plates with fewer than 30 colonies give statistically unreliable counts (too few for confidence), while plates with more than 300 colonies often have overlapping colonies that are difficult or impossible to count accurately — the 30-300 range is the standard target window for a countable plate.
Why do labs plate several different dilutions of the same sample?
Since you don't know the sample's concentration in advance, plating several dilutions in parallel increases the chance that at least one plate lands within the reliable 30-300 colony countable range.
Does CFU/mL count dead bacteria?
No — CFU/mL specifically measures viable, culturable cells, since only living cells capable of dividing will form a visible colony. This is a key difference from total cell counts (like a direct microscope count), which count every cell whether alive or dead.