Colony Counter
Click to tally colonies without losing your place, then convert the count to CFU/mL with your dilution factor and plated volume.
Set this to whatever your method specifies - the published ranges differ by method and by standard, so the tool follows your SOP rather than picking one for you.
Crowded plate? Count a sector
Click anywhere on the plate to mark a colony. Undo removes the last mark. Marks are visual only - the count is what matters.
What this tool does
CFU/mL is calculated as the colony count multiplied by the dilution factor, divided by the volume plated in millilitres. A plate is only statistically countable within a specific window - which exact numbers depend on which standard you follow: 30-300 (USDA/AOAC), 25-250 (APHA) or 15-300 (ISO, and the range the FDA's own current BAM Chapter 3 protocol adopts). Below the window the count is unreliable; above it, colonies merge and get undercounted.
Lets you click each colony to build a tally with a visible mark, so you do not lose your place partway through a crowded plate, with an undo for misclicks.
Converts that count into CFU/mL using your dilution factor and plated volume, and tells you whether the plate falls inside your chosen countable range.
Offers three named method presets - USDA/AOAC (30-300), APHA (25-250) and ISO/current FDA BAM (15-300) - so the range matches a real, attributable standard instead of one number presented as universal.
How to use it
- Click each colony on the plate to tally it, or type a count you already have straight into the box.
- Select the dilution factor of the plate you counted.
- Enter the volume plated in millilitres - commonly 0.1 mL for a spread plate or 1.0 mL for a pour plate.
- Read the CFU/mL figure, and check the countable-range note before recording it.
- Tap a method chip to set the dilution, the volume plated and the countable window together.
- Watch the window bar to see how far inside or outside the countable range the plate sits.
Why use this counter
Counting colonies on a plate by eye is easy to lose track of, especially when crowded, and the CFU/mL calculation is another place to slip. A tally with visible marks and the maths built in reduces both errors.
Why we built it
We built the colony counter to include a dilution factor that converts a raw count to CFU/mL, which is the number actually reported. Calculate more with the percentage calculator and the scientific notation converter.
Related tools worth a look: the Clicker Counter and the QR Code Scanner.
Who this tool is for
- Microbiology students and lab technicians counting plates
- Teachers preparing practicals
- Food-safety and water-testing staff
When to use it
- Microbiology teaching labs.
- Counting plates with a dilution series.
- Checking a manual count and its CFU/mL.
- Practising colony counting technique.
The calculation
CFU per mL = colonies counted x dilution factor / volume plated in mL.
Worked example: you count 148 colonies on a plate from a 10^-4 dilution, having spread 0.1 mL. That gives 148 x 10,000 / 0.1 = 1.48 x 10^7 CFU/mL in the original sample.
The dilution factor is the reciprocal of the dilution. A 10^-4 dilution has a factor of 10,000, because you are multiplying back up to undo the dilution you performed. Getting this the wrong way round is the most common arithmetic error in the whole method, and it produces an answer wrong by eight orders of magnitude.
The unit is colony forming units, not cells. One CFU is one colony, which may have grown from a single cell or from a clump of several. CFU/mL is deliberately a count of viable, culturable units rather than a cell count.
Why the countable range depends on which standard you follow
There is no single universal countable range - three real, named standards specify three different windows for the same basic method, and this tool's three method presets are named after each: USDA and AOAC's official methods specify 30-300 colonies per plate; the American Public Health Association's Compendium and Standard Methods specify 25-250; and ISO specifies 15-300. The FDA's own current Bacteriological Analytical Manual, Chapter 3, lists all three explicitly and then adopts 15-300 - the ISO figure - as "the suitable colony counting range" for its own protocol, reasoning that ISO is internationally accepted in regulatory affairs and that published results often report lower colony counts than the older ranges allow for.
The reasoning behind having a window at all is the same regardless of which one you use. Below the lower bound, ordinary sampling variation is large relative to the count - a plate showing 8 colonies could as easily have shown 4 or 14 from the same sample, so multiplying that figure by a large dilution factor amplifies noise rather than signal. Above the upper bound, colonies begin to crowd and merge: two colonies that touch are counted as one, and competition for nutrients suppresses some growth entirely, so the plate systematically undercounts.
If no plate in your dilution series lands in range, the standard practice is to report the result as an estimate and note which way it is likely to be biased, rather than presenting a precise figure from an uncountable plate. A plate with confluent, indistinguishable growth across its whole surface isn't a low-confidence count at the top of the range - it's not a count at all, and is reported as TNTC (too numerous to count) regardless of which standard's range you're using.
| Standard | Countable range | Common method |
|---|---|---|
| USDA / AOAC | 30-300 | Spread plate |
| APHA | 25-250 | Pour plate |
| ISO / current FDA BAM | 15-300 | Drop count |
If your lab follows a specific standard or a written SOP, use that one's range rather than guessing - the point of naming these three is to match a real method, not to suggest any one of them is more correct in general.
Serial dilution in practice
A serial dilution transfers a fixed volume through a series of tubes, each step diluting by the same factor. Ten-fold steps are conventional: 1 mL into 9 mL of diluent gives 10^-1, and repeating that gives 10^-2, 10^-3 and so on.
Because you cannot know the concentration in advance, you plate several dilutions and count whichever one lands in range. This is why a plate count is normally reported from one plate of a series rather than from an average across all of them.
Mix each tube thoroughly before drawing the next transfer. An unmixed tube is the second most common source of error after the dilution factor itself, and it produces results that scatter unpredictably across the series.
What you give it, and what you get back
| Item | Details |
|---|---|
| Input | Click each colony on the plate image or counter. |
| Settings | Dilution factor and plated volume. |
| Output | The count and CFU/mL, with undo for misclicks. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Count is off | A colony was missed or double clicked. | Use undo and recount a section. |
| CFU/mL looks wrong | Dilution or volume was entered incorrectly. | Check both against your notes. |
| Result for regulated work | This is an educational aid. | Use validated methods for regulatory use. |
Assumptions and limitations
- This is a counting and calculation aid for education and general laboratory work. It is not validated for clinical, diagnostic or regulatory use, and should not be relied on for any of those.
- Counting is manual. There is no image recognition, so the accuracy of the tally is the accuracy of your clicking.
- The tool does not judge whether your dilution series was performed correctly - it calculates from the figures you enter.
- CFU counts measure viable culturable organisms only. Cells that are alive but non-culturable under your conditions are not represented.
- Marks on the plate are a visual aid to keep your place. The number in the count field is what the calculation uses.
Frequently asked questions
How do you calculate CFU/mL?
Multiply the colony count by the dilution factor, then divide by the volume plated in millilitres. Counting 148 colonies from a 10^-4 dilution with 0.1 mL plated gives 148 x 10,000 / 0.1, which is 1.48 x 10^7 CFU/mL.
What is a countable plate?
A plate with a colony count inside the countable window for whichever standard you're following - 30-300 for USDA/AOAC, 25-250 for APHA, or 15-300 for ISO and the FDA's current BAM protocol. Below the window the count carries too much sampling variation to be reliable; above it, colonies merge and are systematically undercounted. Plates outside that window should not be used for a quantitative result.
How do you count bacterial colonies?
Work systematically across the plate in a fixed pattern so no region is counted twice or skipped, marking each colony as you go. Count colonies touching the plate edge on two adjacent sides only. Clicking to tally, as here, keeps your place without a hand counter.
What is serial dilution?
Transferring a fixed volume through a series of tubes so each step dilutes by the same factor, usually ten-fold. It brings an unknown concentration down into a range where a plate can be counted. Several dilutions are plated because you cannot know in advance which will land in range.
Why do different sources give different countable ranges?
Because there genuinely are three different standards, not one. USDA/AOAC specify 30-300, APHA specifies 25-250, and ISO specifies 15-300 - the FDA's own current Bacteriological Analytical Manual lists all three and adopts the ISO figure, 15-300, for its own protocol. Whichever window you use, the reasoning is the same: below it, random variation dominates and gets amplified by the dilution factor; above it, colonies merge and compete, so the plate undercounts.
Can this be used for clinical or diagnostic work?
No. This is a counting and arithmetic aid intended for teaching and general laboratory use. It has not been validated for clinical, diagnostic or regulatory purposes, and results from it should not inform any clinical decision.
Sources and accuracy
Standard microbiological plate-count / CFU-per-mL method. Countable-range figures verified 2026-09-03 directly against the FDA's current Bacteriological Analytical Manual, Chapter 3 (fda.gov/media/191248, January 2026 edition), which states the range is 30-300 per USDA/AOAC, 25-250 per APHA, and 15-300 per ISO, and adopts 15-300 as its own protocol's range. All three are offered as named method presets rather than one figure presented as universal.
Figures last checked against the source on . Standards and rates can change — if you are relying on this for something that matters, confirm against the source above. Think a figure is wrong or out of date? Send a correction.