The 30–300 Rule: The Countable Range in Plate Counts
The 30–300 rule defines the countable range for standard plate counts: only plates with 30 to 300 colonies give a statistically reliable count. Here is why the limits exist and how to handle plates that fall outside them.
When you perform a viable plate count, you do not count every plate in a dilution series — you count the one that falls in the countable range. By long-standing convention, that range is 30 to 300 colonies per plate. This is the 30–300 rule, and it exists because counts at the extremes are simply not trustworthy.
Why there is a lower limit of 30
Colony counts follow a Poisson distribution, where the random variation of a count is roughly the square root of the count itself. With only 10 colonies, that variation is a large fraction of the result, so two identical samples can give very different numbers by chance. By 30 colonies the relative variation has dropped enough that a single plate is a reasonable estimate of the true concentration. Below 30, the result is reported as too few to count (TFTC).
Why there is an upper limit of 300
At the other end, crowding is the problem. As colonies pack together they merge, overlap and compete for nutrients and space, which suppresses growth and hides colonies behind one another. The result is a systematic undercount. Beyond about 300 colonies on a standard 90 mm plate you can no longer resolve individual colony forming units, so the plate is reported as too numerous to count (TNTC).
How the rule fits into a dilution series
Because you cannot know the concentration in advance, you plate several ten-fold dilutions. Typically one of them lands in the countable range:
- A low dilution may be TNTC (more than 300 colonies).
- The next dilution falls in the 30–300 window — this is the one you count.
- A higher dilution may be TFTC (fewer than 30 colonies).
You then use the count from the countable plate, together with its dilution factor, to calculate the concentration. See how to calculate CFU per mL for the formula.
Edge cases and variations
- Two plates in range: if two consecutive dilutions both fall in 30–300, many methods average the calculated CFU/mL from both.
- All plates below 30: report the count from the lowest dilution as an estimate and note it is below the countable range.
- Different plate sizes or methods: some protocols use a slightly different range (for example 25–250 for certain food-testing standards). Always follow the specific method you are working to.
Counting plates near the limits
Near 300 colonies, accurate counting by eye is hard and error-prone. Photographing the plate and using an online colony counter helps you resolve closely spaced colonies and apply a consistent merged-colony rule, which keeps dense-but-countable plates inside reliable territory. For the full counting method, see how to count bacterial colonies on agar plates.
Frequently asked questions
What is the 30–300 rule?
The 30–300 rule states that, for an accurate standard plate count, you should count agar plates that contain between 30 and 300 colonies. Plates outside this range give statistically unreliable results.
Why 30 as the lower limit?
Below 30 colonies, random sampling variation becomes large relative to the count, so a single plate is a poor estimate of the true concentration. Thirty colonies is the accepted point where the count becomes statistically stable.
Why 300 as the upper limit?
Above roughly 300 colonies the plate becomes crowded: colonies merge, overlap and compete for nutrients, so you undercount. Three hundred is the practical limit for counting individual colonies reliably.
What do I do if every plate is outside the range?
Report the result using the closest convention: 'too few to count' (TFTC) below 30 and 'too numerous to count' (TNTC) above 300, and run additional dilutions so a plate lands in the 30–300 window next time.