Find mutation frequency from mutant colonies and total viable cells plated.
Frequency is not rate: frequency measures the fraction of mutants in a population at one time, while mutation rate measures new mutations per cell division. Fluctuation tests separate the two.
Mutation frequency (per cell)
How to use this tool
The number of colonies growing under selective conditions.
The total number of viable cells plated, from a parallel non-selective count.
Frequency per cell, per million and per billion cells are shown.
Frequency is a snapshot proportion; mutation rate per generation needs a fluctuation test or sequencing design.
Formula & worked example
120 mutants from 400,000,000 viable cells: frequency 120 / 4×10⁸ = 3×10⁻⁷ per cell, i.e. 0.3 mutants per million cells.
Limitations — read this before you rely on it
Sensitive to plating efficiency, selection strength and jackpot cultures (an early mutation inflates the count). Replicate platings and a fluctuation analysis give a rate, not just a frequency.
Frequently asked questions
Frequency is mutants per cell in a population now; rate is new mutations per cell per generation. A high frequency can reflect one early mutation amplified by growth, not a high rate.
Only viable cells can form colonies. Dividing by cells plated (including dead ones) understates the frequency.
Replicate cultures started from few cells show variable mutant counts if mutations arise randomly during growth — Luria and Delbrück's classic design for estimating rate.
Strongly phenotype-dependent: antibiotic-resistance frequencies are often around 10⁻⁸ to 10⁻⁶ per cell for single-step resistance.
Raw frequencies like 3×10⁻⁷ are hard to compare at a glance; per-million or per-billion scaling makes the magnitude readable.