🧫 CFU Calculator
Calculate Colony Forming Units (CFU) per mL or per gram from your plate count data. Enter the number of colonies, the dilution factor, and the volume plated to get the concentration of viable microorganisms in your original sample.
What Are Colony Forming Units (CFU)?
Colony Forming Units (CFU) are a measure of viable microbial cells in a sample. Each CFU represents a single viable microorganism (or a cluster of microorganisms) capable of forming a visible colony on a solid culture medium. Unlike direct microscopic counts, CFU counts only living organisms that can grow and divide, making it a critical metric in microbiology for assessing the viability and concentration of bacteria and fungi.
How to Calculate CFU
The standard formula for calculating CFU is:
CFU/mL = (Colonies Counted × Dilution Factor) ÷ Volume Plated (mL)
For solid samples: CFU/g = CFU/mL × Volume (mL) ÷ Sample Weight (g)
Step-by-step example:
- Count colonies: 150 colonies on a plate
- Dilution factor: 1:100 dilution → Factor = 100
- Volume plated: 100 µL = 0.1 mL
- Calculate: CFU/mL = (150 × 100) ÷ 0.1 = 150,000 CFU/mL
Why Use a CFU Calculator?
- Instant Results: Get accurate CFU/mL or CFU/g calculations in seconds.
- Countable Range Validation: Automatically checks if your colony count falls within the standard countable range (typically 30–300 colonies).
- Multiple Formats: View results as standard concentration, scientific notation, and Log₁₀ value.
- Liquid & Solid Samples: Supports both CFU/mL for liquid samples and CFU/g for solid samples.
- Free & Private: No registration, no data storage. All calculations are performed in your browser.
Countable Range Guidelines
For reliable results, it is recommended to count plates with 30–300 colonies. Plates with fewer than 30 colonies are considered "Too Few To Count" (TFTC) and may not be statistically reliable. Plates with more than 300 colonies are "Too Numerous To Count" (TNTC) and may have overlapping colonies that make counting inaccurate.
- < 30 colonies: TFTC , results may be unreliable
- 30–300 colonies: ✓ Optimal countable range
- > 300 colonies: TNTC , results may be unreliable
Dilution Factor Explained
The dilution factor is the reciprocal of the dilution used. For example, if you plated from a 10⁻⁴ dilution tube, the dilution factor is 10⁴ = 10,000. When performing serial dilutions, you multiply the individual dilution factors.
Clinical Applications
- Urine culture: > 10⁵ CFU/mL indicates a urinary tract infection
- Blood culture: Any growth is considered significant
- Wound infections: > 10⁵ CFU/g indicates infection
- Food safety: Enumeration of pathogens in food products
- Pharmaceutical sterility testing: Quality control of medical devices and products
❓ CFU Calculator FAQ
What does CFU stand for?
CFU stands for Colony Forming Unit. It is a measure of viable microbial cells capable of forming colonies on a culture medium.
What is the formula for calculating CFU?
The standard formula is: CFU/mL = (Colonies Counted × Dilution Factor) ÷ Volume Plated (mL).
What is the countable range for CFU plates?
The standard countable range is 30 to 300 colonies per plate. Counts outside this range may be unreliable.
What is the difference between CFU/mL and CFU/g?
CFU/mL is used for liquid samples (e.g., urine, broth cultures). CFU/g is used for solid samples (e.g., food, soil, tissue).
What is a dilution factor?
The dilution factor is the reciprocal of the dilution. For a 10⁻⁴ dilution, the factor is 10⁴ = 10,000.
How do I convert µL to mL?
Divide by 1000. For example, 100 µL = 0.1 mL.
What does TNTC mean?
TNTC stands for "Too Numerous To Count". It means there are more than 300 colonies on the plate, making counting unreliable.
What does TFTC mean?
TFTC stands for "Too Few To Count". It means there are fewer than 30 colonies on the plate, making results statistically unreliable.
How do I calculate CFU for a serial dilution?
Use the formula: CFU/mL = (Colonies × Total Dilution Factor) ÷ Volume Plated. The total dilution factor is the product of all individual dilutions.
What is a good CFU count for urine culture?
For urine cultures, > 10⁵ CFU/mL is typically considered indicative of a urinary tract infection.
What is the Log₁₀ CFU value used for?
The Log₁₀ CFU value is used for reporting and statistical analysis, especially when concentrations span several orders of magnitude.
How accurate is this CFU calculator?
This calculator provides accurate results based on the standard CFU formula. However, accuracy depends on proper laboratory technique, correct dilution series, and accurate colony counting.
Can I use this calculator for fungal cultures?
Yes, the CFU method applies to both bacteria and fungi that form visible colonies on solid media.
What if I have multiple plates with different colony counts?
Use the plate with a countable number of colonies (30–300) for your calculation. If multiple plates are countable, you can average the results.
Is this calculator free?
Yes, this calculator is completely free to use. No registration or personal data storage is required. All calculations are performed in your browser.