Colony Count Calculators

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Colony counting is the standard method for enumerating viable bacteria and other microorganisms by growing them on agar plates and counting the resulting visible colonies. Each colony is assumed to arise from a single colony-forming unit (CFU) — one bacterial cell or clump of cells. Combined with dilution factor and volume plated, colony counts allow calculation of the original sample concentration in CFU/mL or CFU/g. Accurate colony counting requires careful technique, appropriate dilution, and observance of the 30–300 countable range for statistically reliable results.

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Colony-Forming Unit (CFU)

A CFU is the smallest unit that produces a visible colony on a growth medium — it may be a single bacterium, a pair, a chain, or a clump of cells, depending on the organism's growth habit. Because chains and clusters count as single CFUs, colony counts may underestimate absolute cell numbers for chain-forming bacteria (Streptococcus) or biofilm-dispersed clumps.

Pour Plate vs. Spread Plate

  • Pour plate: Diluted sample is mixed with molten agar (45°C) and poured into a Petri dish. Colonies grow throughout the agar depth. Can use higher sample volumes (1 mL). Heat may kill some sensitive organisms.
  • Spread plate: Diluted sample is spread over the surface of a pre-solidified agar plate using a glass spreader or loop. All colonies grow on the surface — easier to count and isolate. Typically 0.1 mL is plated.

Countable Range: 30–300 Colonies

  • < 30 colonies: Too few for statistical reliability (high sampling error). Report as 'TFTC' (Too Few To Count).
  • 30–300 colonies: Acceptable range for accurate counting.
  • > 300 colonies: Colonies merge — uncountable. Report as 'TNTC' (Too Numerous To Count).

CFU/mL Calculation

CFU/mL = Colony count / (Volume plated in mL × Dilution factor)

Example: 65 colonies on a 0.1 mL spread plate from a 10⁻⁴ dilution:
CFU/mL = 65 / (0.1 × 10⁻⁴) = 65 / 10⁻⁵ = 6.5 × 10⁶ CFU/mL

Counting Tips

  • Count with plate inverted under adequate lighting
  • Use a colony counter with light box and marker pen or digital software
  • Mark colonies with a fine marker as you count to avoid double-counting
  • Average results from duplicate plates (should agree within 10–15%)
  • Report to 2 significant figures

Glossary

Colony-Forming Unit (CFU)
The smallest unit that produces a visible colony on culture medium — may be a single cell, pair, chain, or clump. CFU/mL = colonies / (volume × dilution factor). Used to quantify viable microorganisms.
Spread Plate
A colony count method where diluted sample is spread over pre-solidified agar surface. Colonies grow on surface only — easier to count and isolate than pour plates. Standard volume: 0.1 mL.
TNTC / TFTC
Too Numerous To Count (>300 colonies) and Too Few To Count (<30 colonies) — designations for plates outside the 30–300 countable range. Repeat with different dilutions to achieve countable plates.

Frequently Asked Questions

CFU/mL = Colony count / (Volume plated in mL × Dilution factor). Example: 80 colonies on a plate inoculated with 0.1 mL from a 10⁻⁵ dilution: CFU/mL = 80 / (0.1 × 10⁻⁵) = 80 / 10⁻⁶ = 8.0 × 10⁷ CFU/mL. Always use plates with 30–300 colonies for the most accurate results — fewer than 30 has too much sampling error; more than 300 colonies merge and cannot be accurately counted.

The accepted countable range is 30–300 colonies per plate. Plates with fewer than 30 colonies have high relative sampling error — a difference of a few colonies significantly changes the calculated result. Plates with more than 300 colonies have overlapping colonies that merge and cannot be individually distinguished, leading to undercounting. Report plates below range as TFTC (Too Few To Count) and above range as TNTC (Too Numerous To Count).

Pour plate: diluted sample is added to a tube of molten agar (45°C), mixed, and poured into a plate. Colonies form both on the surface and throughout the agar. Useful for larger volume inoculation (1 mL). Heat from molten agar can kill heat-sensitive organisms. Spread plate: diluted sample is spread over the surface of pre-solidified agar with a sterile spreader. All colonies grow on the surface, making counting and isolation easier. Limited to 0.1–0.2 mL inoculation volume to avoid flooding the plate.

Colony count measures viable cells capable of forming visible colonies — but may underestimate total cells because: (1) bacterial chains, clumps, or aggregates count as one CFU per cluster; (2) stressed, viable-but-non-culturable (VBNC) cells do not form colonies on standard media; (3) cells requiring specific nutrients not present in the medium are missed; and (4) injured cells may fail to recover. For total viable cell count, epifluorescence microscopy with DAPI staining counts all cells; direct viable count methods (CTC reduction) count metabolically active cells.