CFU per mL Calculator

Calculate Colony Forming Units (CFU) per mL from agar plate colony counts, volume plated, and serial dilution exponents.

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Input Parameters

Enter details for colony counting calculation.

Number of Colonies (C)

Dilution (exponent, e.g. -5 for 10⁻⁵)

Volume Plated (V)

Volume Unit

CFU per mL (Scientific Notation)

1.50 × 10⁷

Standard Integer Format

15,000,000 CFU/mL

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Countable Range Rule

Plates with 30 to 300 colonies are statistically ideal for accurate microbiological reporting.

Microbiological Plating Standards & Colony Counting Limits Reference

Organism / Assay Type	Statistically Valid Colony Range	Reporting Status if Below Min	Reporting Status if Above Max
Standard Bacterial Plate Count	30 – 300 colonies	TFTC (< 30)	TNTC (> 300)
Yeast & Mold Plate Count	10 – 150 colonies	Estimated Count (< 10)	TNTC (> 150)
Membrane Filtration Method	20 – 80 colonies	TFTC (< 20)	Confluent Growth (> 80)

Calculation Methodology & Equations

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CFU/mL Master Formula

CFU/mL = (Colonies × 10^(-exponent)) / Volume_in_mL

Multiplies colony counts by the reciprocal dilution factor and divides by plated volume in mL.

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Quality Control Rules

TFTC = "Too Few To Count" (<30 colonies, high variance). TNTC = "Too Numerous To Count" (>300 colonies, nutrient depletion bias).

Frequently Asked Questions

What is Colony Forming Unit (CFU) per mL? ▼

CFU/mL estimates the number of viable fungal or bacterial cells in a sample capable of multiplying to form a visible colony on an agar plate.

What is the acceptable colony count range per agar plate? ▼

Plates containing between 30 and 300 colonies are considered statistically valid. Below 30 introduces high sampling error; above 300 causes colony overlap and nutrient competition.

How do you calculate CFU/mL from dilution and volume? ▼

CFU/mL = (Number of Colonies × Dilution Factor) / Volume Plated (in mL), where Dilution Factor = 10^(-exponent).