This CFU calculator converts plate colony counts into colony-forming units per mL of the original sample — colonies × dilution factor ÷ volume plated — the microbiology workhorse formula.
CFU/mL = Colonies × Dilution ÷ mL Plated
The formula inverts the dilution chain built by our Serial Dilution Calculator: each colony testifies that at least one viable unit survived the trip through the dilution series onto the plate. Part of our Biology Calculators collection.
Worked Example (Verified)
- 87 colonies at 10⁻⁶ dilution, 0.1 mL plated (defaults): 87 × 10⁶ ÷ 0.1 = 8.7 × 10⁸ CFU/mL — a classic overnight E. coli density, which is no coincidence: the example mirrors the textbook lab
- Volume matters ten-fold: the same plate poured with 1 mL (pour plate) reads 8.7 × 10⁷ — identical biology, different divisor; record the plated volume or the number is meaningless
- Duplicate discipline: plates at 92 and 82 colonies average 87 — averaging replicates before the formula tightens Poisson and pipetting noise together
- Membrane filtration variant: colonies on the filter × (total volume ÷ filtered volume) — same inversion principle, different plumbing (water microbiology’s standard)
Reading Plates Like a Microbiologist
- 30–300 rule: below it, noise; above it, crowding. Design dilution series (six 1:10 tubes reach anywhere) to land a plate inside.
- Spreaders (Bacillus, Proteus swarms) invalidate a plate’s count — note and rerun with drier plates/agar adjustments.
- Incubation time is part of the method: slow growers appear late; counting early is a biased census.
- Report with method (spread vs pour), medium, temperature and time — CFU/mL is only reproducible as a package.
Related Tools
- Serial Dilution Calculator — build the dilution ladder
- Bacterial Growth Calculator — how the culture got that dense
- Biology Calculators — the full collection
Frequently Asked Questions
How do you calculate CFU per mL?
Colonies × dilution factor ÷ volume plated in mL. Example: 87 colonies on a 10⁻⁶ plate spread with 0.1 mL = 87 × 10⁶ ÷ 0.1 = 8.7 × 10⁸ CFU/mL. Use plates with 30–300 colonies and state the dilution alongside the result.
Why count only plates with 30–300 colonies?
Statistics below, physics above: under 30, Poisson counting noise exceeds ±18% and single contaminants matter; over 300, colonies merge and compete, undercounting the living. The rule standardizes accuracy across labs — the calculator flags which side of the range your count falls on.
Is CFU/mL the same as cells per mL?
No — CFU counts units that formed colonies: a chain of 50 streptococci yields 1 CFU, dead and VBNC cells yield 0. Direct counts (hemocytometer) usually read higher. CFU is the regulatory/clinical standard because it measures what can actually grow — but never confuse it with total cell census.
What dilution should I plate?
Bracket the unknown: plate two or three consecutive dilutions (e.g. 10⁻⁵, 10⁻⁶, 10⁻⁷ for an overnight culture expected near 10⁹/mL) so at least one lands in 30–300. Expected density ÷ target colonies ≈ dilution volume logic — the serial dilution planner makes the ladder.
Spread plate vs pour plate — does the formula change?
No: only the plated volume changes (typically 0.1 mL spread vs 1 mL pour). Same colonies × dilution ÷ volume. Pour plates additionally stress heat-sensitive organisms in molten agar — a biological footnote the formula politely ignores.
Is this CFU calculator free?
Yes — free, instant, no sign-up.