CFU Calculator

Figran

Turn plate counts from several dilutions into CFU/mL or CFU/g, log10 and log reduction.

Weighted mean across dilutions
TNTC, TFTC and estimated counts
Log reduction and time-kill

Samples

Sample

Sample type

Plating

Starting dilution exponent (10ⁿ)Factor

Dilution	Plate 1	Plate 2	Remove

Counting rule

Lower limitUpper limit

Result

2.0 × 10⁹ CFU/mL

2 plates used · 30–300

log₁₀ 9.30

Maximum likelihood, 95% CI: 2.0 × 10⁹ [1.8 × 10⁹, 2.2 × 10⁹]

CFU calculator examples

Explore plate-count scenarios, then load one into the calculator.

E. coli overnight culture

Estimate a liquid culture from duplicate spread plates.

Spread 0.1 mL · 3 dilutions · duplicates

Aerobic plate count of a food sample

Calculate a food sample's aerobic plate count using the FDA BAM method.

Source: FDA BAM Chapter 3, Example 2.1a

Total plate count with dilutions that disagree

Review counts when successive dilutions disagree.

Food sample · 3 dilutions · triplicates

Lowest dilution too numerous to count

Include TNTC plates in the maximum-likelihood estimate instead of ignoring them.

Pour 1 mL · 2 TNTC plates

Disinfectant log reduction

Compare treatment plates with a control and report a detection limit.

3 samples · control vs 2 treatments

Time-kill curve of an antibiotic

Plot viable counts across time for control and drug groups.

2 groups · 5 time points

How to calculate CFU from plate counts

Go from your plates to a result in four steps.

1 Set up the sample
Choose a sample type, plating method and volume. Use Compare samples for log reduction.
2 Enter plate counts
Enter each row's dilution and each plate's count. Type TNTC for crowded plates, or count colonies on a photo.
3 Choose a rule
Choose a countable range (such as 30–300), ISO 7218 or FDA BAM; plates used in the result are highlighted.
4 Read and export
Check the CFU concentration and log₁₀, then download PNG or SVG or copy the result and methods text.

Dilution	Plate 1
10⁻⁵	TNTC
10⁻⁶	212

CFU formula

Choose the formula that matches the plates used in your calculation.

Single plate

CFU/mL = \frac{C}{V \times d}

C = colonies, V = plated volume (mL), d = dilution relative to the original sample.

Multiple plates: weighted result

CFU/mL = \frac{\sum C_{i}}{\sum (V_{i} \times d_{i})}

Cᵢ, Vᵢ and dᵢ are the colony count, plated volume (mL) and dilution for each plate.

ISO: two consecutive tenfold dilutions

CFU/mL = \frac{\sum C}{V (n_{1} + 0.1 n_{2}) d}

ΣC = total colonies, V = equal plated volume (mL), n₁ and n₂ = plate counts at each dilution, d = the less-diluted fraction.

Default example

Parameter	Value
Colonies	212 + 189 = 401
Plated volume × dilution	2.0 × 10⁻⁷ mL
Weighted calculation	401 / 2.0 × 10⁻⁷
Report	2.0 × 10⁹ CFU/mL

CFU/g for food and other solid samples

A 1 + 9 suspension is a 10⁻¹ dilution; enter the total dilution relative to the original sample. For example, 156 and 171 colonies at 10⁻⁷ with 0.1 mL plated give 1.6 × 10¹⁰ CFU/g.

Countable plates, TNTC and TFTC

TNTC means too numerous to count; TFTC means too few to count. Select a range appropriate for your method. Plates outside it may require an estimated result or a detection limit.

Counting rule	Countable range	Plates used	All plates low	No colonies	TNTC
Countable range	30–300 / 25–250 / 15–300	Plates in range	Estimated	Below detection limit	All plates are TNTC. Use a more dilute sample.
ISO 7218	≤ 300	Up to two consecutive eligible dilutions	Estimated	Below detection limit	Up to two consecutive eligible dilutions
FDA BAM	15–300	All exact and TNTC plates	Estimated	Below detection limit	All exact and TNTC plates

ISO 7218 and FDA BAM counting rules

The ISO rule uses up to two consecutive eligible dilutions with at least one plate containing 10 or more colonies. FDA BAM uses a maximum-likelihood estimate and a likelihood-ratio confidence interval. Check your laboratory protocol before reporting.

ISO low-count reporting

Colonies counted	Reporting
0	Below detection limit
1–3	Present below a limit
4–9	Estimated
≥ 10	Calculated result

ISO worked example: 168/215 and 14/25 at 10⁻² and 10⁻³, plated at 1 mL, give 19,181.8 and report as 19,000.

BAM rounds to two significant figures using the third digit; for example 14,500 becomes 14,000.

FDA BAM Chapter 3 · ISO 7218

Log reduction from plate counts

Log reduction is the base-10 decrease in viable counts between control and treated samples.

Log reduction

Log reduction = {log}_{10} (N_{0}) - {log}_{10} (N)

N₀ = control count and N = treated count, in the same units.

If no colonies are detected, use the detection limit and report a minimum reduction (≥).

Percent reduction

% reduction = (1 - 10^{- LR}) \times 100

LR = log reduction.

Log reduction	Percent reduction	Survivors per 10⁶
1	90%	100,000
2	99%	10,000
3	99.9%	1,000
4	99.99%	100
5	99.999%	10
6	99.9999%	1

Time-kill curve

Compare viable counts over time; open points indicate results below the detection limit.

Control
Drug 4×MIC
Control detection limit
Drug 4×MIC detection limit

≥3 log₁₀ reduction from the initial inoculum = bactericidal activity at the specified time. Source: CLSI M26-A

CFU calculator FAQ

What is a colony forming unit (CFU)?

A CFU is a viable cell or group of cells capable of forming a visible colony.

How do I calculate CFU/mL from more than one dilution?

Divide the total colonies on selected plates by their total original-sample volume, ΣC/Σ(V × d). To count transformants per µg of plasmid DNA, use the transformation efficiency calculator.

Why does the calculator say my dilutions disagree?

Successive dilutions should give similar concentration estimates. Check labels, mixing and pipetting, and repeat if needed.

What do I report when plates have no colonies or too few?

Report a detection limit for all-zero plates. Low positive counts may be estimated or reported as present below a limit depending on the selected rule.

How do I convert CFU/mL to log CFU?

Take the base-10 logarithm of the concentration; 2.0 × 10⁹ CFU/mL is log₁₀ 9.30.

How do I calculate CFU/g for food samples?

Include the initial suspension in each total dilution relative to the original sample.

Can I use it for drop plates?

Yes. Select Drop 10 µL and enter each drop in a separate plate column.

Can I count colonies from a plate photo?

Yes. Open the photo counter beside a cell and click each colony. The photo stays in your browser.