GUIDES
Microbiology calculation guides
Practical explanations for the three calculators on this site. Each guide covers the formula, a worked example, and the limits of the calculation, and links back to the tool that performs it.
CFU
- CFU/mL vs CFU/g: formulas, units, and sample preparationWhen to report CFU/mL and when to report CFU/g, how homogenization volume and sample mass enter the formula, and how to plan dilutions for solid samples.
- How to calculate CFU/mL from colony countsStep-by-step CFU/mL calculation from colony counts, dilution fraction, and plated volume, with worked examples, countable-range guidance, and common mistakes.
- What one CFU represents — and what the count does not tell youWhy a colony-forming unit is not the same as a cell, how clumping and non-culturable cells affect plate counts, and what a CFU result can and cannot support.
OD600
- OD600 and CFU/mL: why the relationship needs validationWhy OD600 cannot be converted to CFU/mL with a universal constant, what shifts the ratio, and how to build a paired calibration for your own system.
- OD600 during bacterial growth: growth curves and log phaseHow to read lag, exponential, and stationary phases from an OD600 growth curve, calculate doubling time, and know when OD stops tracking viability.
- OD600 to cells/mL: build and use a calibrationHow to build a calibration from paired OD600 and cell-count data, choose between a factor, linear fit, or log model, and apply it with the OD600 calculator.
- What OD600 measures and how to take a readingWhat optical density at 600 nm represents, how spectrophotometer settings affect the reading, and practical steps for reproducible OD600 measurements.
Serial dilution
- How to plan a 10-fold serial dilutionTransfer and diluent volumes for a 10-fold serial dilution at any final tube volume, with a worked four-tube example and cumulative dilution tracking.
- How to plan a 2-fold serial dilutionTransfer and diluent volumes for a 2-fold (1:2) serial dilution, how cumulative factors grow more slowly than 10-fold series, and when to choose finer steps.
- Serial dilution factor vs cumulative dilutionThe difference between a single-step dilution factor and the cumulative dilution of a series, how to compute each, and how both enter the CFU calculation.
- Serial dilution: definition, principle, and purposeWhat a serial dilution is, the principle behind stepwise dilution, and why laboratories dilute in a series instead of one large dilution.
Formulas at a glance
The calculation methods page collects every formula the calculators use in one place, with the terms defined. The tools index lists all three calculators with what each one needs as input.