CFU Calculator
Calculate cfucalculator with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Calculator
Adjust values & calculateExpected Colonies at Each Dilution
Formula
Colony Forming Units per milliliter is calculated by dividing the average number of colonies counted on replicate plates by the product of the dilution factor (e.g., 10^-6) and the volume of diluted sample plated (typically 0.1 mL for spread plates or 1.0 mL for pour plates). The dilution factor accounts for the serial dilutions performed before plating.
Last reviewed: December 2025
Worked Examples
Example 1: Standard CFU/mL Calculation
Example 2: Water Sample Analysis
Background & Theory
The CFU Calculator applies the following established principles and formulas. Biology is the scientific study of life, encompassing the structure, function, growth, evolution, and distribution of living organisms. At the cellular level, all life is composed of cells, the basic structural and functional units of organisms. Prokaryotic cells lack a membrane-bound nucleus, while eukaryotic cells possess a nucleus and membrane-bound organelles including mitochondria, which generate ATP through oxidative phosphorylation, and ribosomes, which synthesize proteins. Genetics quantifies the inheritance of traits. Gregor Mendel's laws describe how alleles segregate during gamete formation and assort independently for genes on different chromosomes. Punnett squares provide a visual method for calculating the probability of offspring genotypes and phenotypes from known parental genotypes. For a monohybrid cross of two heterozygotes (Aa × Aa), the expected phenotypic ratio is 3 dominant to 1 recessive. The Hardy-Weinberg equilibrium principle states that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces. If p and q are the frequencies of two alleles at a locus, then p + q = 1 and genotype frequencies are p², 2pq, and q² for the three possible genotypes. Deviations from equilibrium signal the action of natural selection, genetic drift, mutation, migration, or non-random mating. Population growth follows two primary models. Exponential growth, N = N₀eʳᵗ, describes unlimited growth where N₀ is the initial population, r is the intrinsic rate of increase, and t is time. Logistic growth incorporates carrying capacity K, describing how growth slows as population approaches the environment's maximum sustainable size: dN/dt = rN(1 − N/K). Enzyme kinetics describes the rate of enzyme-catalyzed reactions. The Michaelis-Menten equation, v = Vmax[S]/(Km + [S]), relates reaction velocity v to substrate concentration [S], maximum velocity Vmax, and the Michaelis constant Km, which equals the substrate concentration at half-maximal velocity. DNA replication relies on complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and guanine with cytosine (three hydrogen bonds), ensuring faithful copying of genetic information.
History
The history behind the CFU Calculator traces back through the following developments. The systematic study of living things began with Aristotle (384–322 BCE), who classified over 500 animal species and wrote foundational texts on anatomy, reproduction, and animal behavior. His scala naturae ranked organisms in a hierarchy from simple to complex and influenced biological thought for two millennia. Theophrastus, his student, applied similar methods to plants. Carl Linnaeus established modern taxonomy in Systema Naturae (1735), introducing the binomial nomenclature system that assigns each organism a genus and species name. His hierarchical classification system — species, genus, family, order, class, phylum, kingdom — provided the organizational framework that biologists still use, now extended to seven ranks and supplemented by cladistics. Charles Darwin and Alfred Russel Wallace independently developed the theory of evolution by natural selection, which Darwin published in On the Origin of Species in 1859. Darwin argued that heritable variation exists within populations, that organisms with advantageous traits survive and reproduce at higher rates, and that this differential reproduction gradually changes the character of populations over generations. This unified all of biology under a single explanatory framework. Gregor Mendel's meticulous pea plant experiments, conducted from 1856 to 1863 and published in 1866, established the particulate nature of inheritance and the laws of segregation and independent assortment. Overlooked until 1900, when three botanists independently rediscovered his work, Mendel's laws laid the foundation for the science of genetics. James Watson and Francis Crick, building on Rosalind Franklin's X-ray crystallography data, determined the double-helix structure of DNA in 1953, revealing the physical basis of heredity and the mechanism by which genetic information is stored and copied. The Human Genome Project, a 13-year international collaboration, published the complete sequence of the human genome in 2003, comprising approximately 3.2 billion base pairs. The development of CRISPR-Cas9 gene editing by Jennifer Doudna, Emmanuelle Charpentier, and colleagues from 2012 onward opened an era of precise genome modification with transformative implications for medicine, agriculture, and basic research.
Frequently Asked Questions
Formula
CFU/mL = Average Colony Count / (Dilution Factor x Volume Plated in mL)
Colony Forming Units per milliliter is calculated by dividing the average number of colonies counted on replicate plates by the product of the dilution factor (e.g., 10^-6) and the volume of diluted sample plated (typically 0.1 mL for spread plates or 1.0 mL for pour plates). The dilution factor accounts for the serial dilutions performed before plating.
Frequently Asked Questions
What does CFU stand for and what does it measure?
CFU stands for Colony Forming Unit, which is a measure of viable (living) bacterial or fungal cells in a sample. Unlike direct cell counting methods, CFU measurement only counts cells that are able to multiply and form visible colonies on agar plates. One CFU may originate from a single cell or from a clump of cells, which is why the term 'colony forming unit' is used rather than 'cell.' CFU/mL is the standard unit for expressing the concentration of microorganisms in liquid samples. It is the gold standard for quantifying viable microbes because only living organisms capable of reproduction will form colonies.
What is the countable range for CFU plates?
The generally accepted countable range for standard pour plates and spread plates is 25 to 250 colonies per plate (FDA BAM method). Some references use 30 to 300 colonies (ISO methods). Plates with fewer than 25 colonies have high statistical variability and results are reported as 'estimated' or 'TFTC' (too few to count). Plates with more than 250 colonies are reported as 'TNTC' (too numerous to count) because colonies may merge, compete for nutrients, or be obscured. For membrane filtration methods, the countable range is typically 20-80 colonies. Always plate multiple dilutions to ensure at least one falls within the countable range.
How do you perform serial dilutions for CFU counting?
Serial dilutions are performed by transferring a fixed volume of sample into a fixed volume of diluent in a stepwise fashion. The most common scheme is 1:10 (ten-fold) serial dilutions. Add 1 mL of sample to 9 mL of sterile diluent (or 0.1 mL to 0.9 mL), vortex thoroughly, then transfer 1 mL of this dilution to another 9 mL tube, and repeat. Each step reduces concentration by 10-fold. Label tubes clearly as 10^-1, 10^-2, 10^-3, etc. Use a fresh pipette tip for each transfer to prevent carryover. Plate 0.1 mL (spread plate) or 1.0 mL (pour plate) from each dilution. The dilution with 25-250 colonies is used for calculation.
How do you calculate CFU/mL from multiple plates?
When multiple plates are used (which is recommended for accuracy), average the colony counts from replicate plates at the same dilution before calculating CFU/mL. The formula is: CFU/mL = Average colony count / (Dilution factor x Volume plated in mL). For example, if duplicate plates at 10^-5 dilution show 142 and 158 colonies with 0.1 mL plated: CFU/mL = ((142+158)/2) / (10^-5 x 0.1) = 150 / 10^-6 = 1.5 x 10^8 CFU/mL. If plates at different dilutions are in the countable range, you can average the calculated CFU/mL values from each dilution. The coefficient of variation between replicates should ideally be less than 15%.
What inputs do I need to use CFU Calculator accurately?
Each field is labelled with the required unit (metric or imperial). Gather your source values before starting — for example, a weight measurement in kilograms, a distance in metres, or a dollar amount — and enter them exactly as measured. The formula section on this page lists every variable and explains what each represents.
How do I verify CFU Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy