The free online colony counter for agar plates
Upload agar plate image here
Drop in an agar plate photo and get an automatic colony count with CFU/mL.
The tool
What the online colony counter does
This online colony counter is a free, browser-based tool for counting bacterial colonies on agar plates. Instead of marking colonies by hand with a pen and tally counter, you upload a photo of your petri dish and let the automatic colony counter detect and count the colonies for you.
It is built for everyday lab work: plate counts, viable cell counts, water and food testing, and teaching. Because it runs entirely in the browser, the agar plate counter works on any device with no software to install — and it helps you turn a raw colony count into a reliable CFU/mL value.
Workflow
How to use the colony counter
Count an agar plate and calculate CFU/mL in four steps.
- 01
Upload your agar plate photo
Take a clear, evenly lit photo of your petri dish on a plain background and upload it to the counter.
- 02
Let it detect colonies
The tool scans the plate and automatically marks individual colonies, separating them from the agar and dish edge.
- 03
Review and adjust
Check the highlighted colonies, add or remove any that were missed or merged, and confirm the final count.
- 04
Calculate CFU/mL
Enter your dilution factor and plated volume to convert the count into colony forming units per millilitre.
Benefits
Why use an online colony counter
Faster, more consistent, and more accurate than counting by hand.
Save time
Count a crowded plate in seconds instead of tallying hundreds of colonies by hand.
Improve accuracy
Automatic detection reduces miscounts and eye strain on dense plates, so your numbers are more reliable.
Stay consistent
Apply the same counting logic to every plate to reduce operator-to- operator variation in your results.
Use any device
A browser-based tool that works on phones, tablets and lab computers — free, with nothing to install.
FAQ
Frequently asked questions
Is the online colony counter free?
Yes. The colony counter is completely free to use. There is no signup, no subscription, and no limit on how many agar plates you can count.
Does the colony counter work on mobile?
Yes. The tool runs in your web browser and works on phones, tablets, laptops and desktops. You can photograph a plate with your phone and count it on the same device.
What image format should I upload?
Standard photo formats such as JPEG, PNG and WebP work well. For the most accurate count, use a sharp, evenly lit photo taken straight above the plate against a plain, contrasting background.
What is the countable range for a plate count?
Microbiologists generally count plates with 30 to 300 colonies (the 30–300 rule). Plates with fewer than 30 or more than 300 colonies are usually reported as outside the reliable countable range.
Does the counter calculate CFU/mL?
Yes. After counting, you can enter your dilution factor and plated volume, and the tool converts the colony count into CFU per millilitre using the standard CFU formula.
Do I need to install anything?
No. The colony counter is a browser-based tool. There is nothing to download or install — just open the page and start counting.
Learn
Learn colony counting
Guides on counting bacterial colonies, the 30–300 rule, and CFU calculations.
How to Count Bacterial Colonies on Agar Plates
A complete, lab-tested guide to counting bacterial colonies on agar plates: method, the countable range, tips for accuracy, and how to turn counts into CFU/mL.
Read guideThe 30–300 Rule: The Countable Range in Plate Counts
What the 30–300 rule is, why the countable range matters for accurate plate counts, and how to handle plates that fall outside the range.
Read guideWhat Is CFU? Colony Forming Units Explained
A clear definition of CFU (colony forming unit): what it measures, why microbiologists use it instead of raw cell counts, and how it is reported.
Read guideHow to Calculate CFU per mL: Formula and Examples
The CFU per mL formula explained step by step, with worked examples, dilution factors, and common mistakes to avoid.
Read guide