CFU Calculator (Colony-Forming Units per mL)

Background.

After a day or two in the incubator, a plate with 68 separate colonies tells you how many viable particles were in the drop you spread, not how many were in your sample. Getting back to the sample means undoing the dilution: divide the colonies by the volume of original, undiluted material that actually reached the agar. Spread 0.1 mL from a 10⁻⁵ dilution tube and that volume is one millionth of a millilitre, so 68 colonies stand for 6.8 × 10⁷ colony-forming units per millilitre.

Two refinements separate a count that would stand up in a food or water laboratory from a back-of-envelope one. When two consecutive ten-fold dilutions both give countable plates, the FDA's Bacteriological Analytical Manual (BAM) and ISO 7218 combine them in a weighted mean instead of making you choose one. And the result is reported to two significant figures, ties rounded half to even, because a plate count cannot support more. You get both the full-precision titre and the rounded figure to write down.

The countable range is where the standards part company. BAM Chapter 3 counts plates holding 25 to 250 colonies, APHA's Standard Methods uses 30 to 300 for heterotrophic plate counts in water, and ISO sets a range within each individual method. BAM's limits are the default and both can be changed; a plate outside them still gives a number, flagged as an estimated count.

A blank plate is not accepted as zero. It shows only that the concentration was below what your plating scheme can detect, and that detection limit, the concentration a single colony would represent, is reported with every result.

What is cfu calculator?

A colony-forming unit is whatever lands on the agar and grows into one visible colony. The awkward name is deliberate, because a CFU is not a cell: a chain of streptococci or a clump of staphylococci that stays together through dilution and plating produces one colony, so a plate count reads low for organisms that chain or clump. Cells that are alive but cannot grow on the medium, at the temperature or in the atmosphere you provided, the viable-but-non-culturable state, are not counted at all. A CFU/mL figure is therefore tied to its method, and the medium, incubation temperature and time belong next to the number whenever you report it.

How to use this calculator.

1. 01Choose a single plate, or two consecutive dilutions when plates at both a lower and the next ten-fold higher dilution fell in the countable range.
2. 02For a single plate, enter every colony on it, pinpoint colonies included. For two dilutions, enter the colony total across all retained plates at each dilution and how many plates that was, usually 2 for duplicates; set the second dilution's plates and colonies to 0 to use the first alone.
3. 03Enter the volume of diluted sample put on each plate: 1 mL for a pour plate as in BAM, 0.1 mL for a typical spread plate.
4. 04Enter the dilution as a power of ten, −5 for 10⁻⁵ and 0 for neat sample. With two dilutions, give the lower one, the less diluted tube with the larger count.
5. 05Enter the original sample volume if you want the total CFU it contained, or 0 if not, and set the countable range to the standard you work to.
6. 06Copy the two-significant-figure reported value into your record, and read the countability note to see whether it must be labelled as an estimate.

The formula.

For one plate, CFU/mL = C ÷ (V × d), where C is the colonies, V the millilitres of diluted sample plated and d the dilution as a fraction. V × d is the volume of original sample on the plate, so the formula is simply colonies divided by volume. For two dilutions, N = ΣC ÷ [V × (n₁ + 0.1 n₂) × d], where ΣC is every colony on every retained plate, n₁ and n₂ are the numbers of plates at the lower and next dilution, and d is the lower dilution. The 0.1 is not an empirical correction: a plate at the next ten-fold dilution carries one tenth as much original sample, so the bracket adds up the total original volume across all plates. BAM writes the formula without V because its pour plates always receive 1 mL; setting V = 1 gives BAM's version exactly. The detection limit is 1 ÷ [V × (n₁ + 0.1 n₂) × d], the concentration one colony would represent. The reported value keeps two significant figures under BAM's rule: if the third digit is 6 to 9 round up, if 1 to 4 round down, and if it is 5, round up when the second digit is odd and down when it is even, so 15,500 becomes 16,000 and 14,500 becomes 14,000. The logarithm is taken from the unrounded titre. Colony counts must be whole numbers, the dilution exponent cannot be positive, and at least one colony is required.

A worked example.

Frequently asked questions.

References& sources.

1. [1]US Food and Drug Administration, Bacteriological Analytical Manual, Chapter 3 'Aerobic Plate Count', January 2001 Edition (original source BAM 8th ed. Rev. A, 1998; L. Maturin and J. T. Peeler). Introduction: 'The suitable colony counting range (10) is 25-250.' §B: decimal dilutions by transferring 10 mL of the previous dilution into 90 mL of diluent, 1 mL plated in duplicate. §C: EAPC, TNTC and no-colony reporting rules. §D: 'report only the first two significant digits', the round-half-to-even tie rule with four worked examples, and the weighted-mean formula N = Σc/[(n₁+0.1n₂)d] with the 537/0.022 = 24,409 ≈ 24,000 example this calculator reproduces. Read directly from the FDA PDF, 2026-07-29. Free.
2. [2]ISO/TC 34/SC 9 (Food products — Microbiology), 'Excel tool to implement the calculations of the colony-count technique according to ISO 7218 — Verification Report', S. Grosz, 28 August 2020. A free ISO committee document reproducing the ISO 7218:2007/Amd 1:2013 §11.2.6 calculation examples: 168 + 14 at 10⁻²/10⁻³ → 1.7E+04; 168+215 and 14+25 → 1.9E+04; 53 at 10⁻² → 5.3E+03; and 0 colonies at 10⁻¹ → '<1,0E+01'. This calculator reproduces all four. Read directly, 2026-07-29.
3. [3]ISO 7218:2007/Amd 1:2013, Microbiology of food and animal feeding stuffs — General requirements and guidance for microbiological examinations, §11 (enumeration methods, including the weighted-mean colony-count calculation). PAYWALLED — cited bibliographically. Its content is verified here only through the free ISO committee verification report above, which is precisely why the countable range on this page is an editable input rather than a fixed number.
4. [4]ISO 6887-1:2017, Microbiology of the food chain — Preparation of test samples, initial suspension and decimal dilutions for microbiological examination — Part 1: General rules. The governing standard for the decimal dilution series that feeds a plate count. PAYWALLED — cited bibliographically; scope confirmed from the ISO catalogue entry, retrieved 2026-07-29.
5. [5]World Health Organization, WHO Laboratory Manual for the Examination and Processing of Human Semen, §2.8.3: 'Report the average sperm concentration to two significant figures.' Cited as an independent statement of the same reporting rule from a completely different field, confirming that two significant figures is a general convention for counting-based concentration measurements rather than an FDA idiosyncrasy. Public PDF, retrieved 2026-07-29.