CFU Calculator
Colonies on a plate, dilution in the chain, volume in the pipette — the three numbers that price the original culture, with the 30–300 window enforced.
CFU Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Spread-plate and pour-plate arithmetic only
- No MPN or membrane-filtration methods
In short: The default plate counted 100 colonies from a 1:1,000 dilution with 0.1 mL plated: 100 colonies ÷ (0.1 mL × 1/1,000) = 1,000,000 CFU/mL — the textbook one-million culture. The count sits inside the 30–300 countable window, which is what makes the answer statistics rather than luck: below 30 a plate counts anecdotes, above 300 colonies merge and the number quietly undercounts.
Formula
CFU/mL = colonies × dilution ÷ volume plated
Every colony on the plate is the descendant of one viable unit — a cell or a clump — in the drop you plated. The plate saw only a diluted slice of the culture, so the count is multiplied back through the dilution chain and normalized to a millilitre. The arithmetic is exact; the 30–300 window is what makes the inputs worth trusting.
Worked Example
- Count the colonies (average replicates first).
- Enter the fold-dilution of the plate and the volume plated.
- Check the countable-window card — 30 to 300 colonies.
- Read the culture density and the chain that produced it.
Defaults: 100 colonies, 1:1,000, 0.1 mL → 1,000,000 CFU/mL. The roboculator check: 85 colonies at 1:100,000 and 0.1 mL → 85,000,000 CFU/mL. A pour plate at 1 mL divides the plated volume out directly.
Strengths & Limits Of This Model
Where this engine is strong
- 30–300 window enforced, not assumed
- Chain card shows the back-multiplication
Where it stops
- No replicate statistics or confidence intervals
- No OD-to-CFU calibration
Practical Use Cases
Quality control
bioburden and sterility checks
Inoculum prep
standardize tomorrow’s culture
Teaching
the plate as a sampling instrument
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
CFU Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why must a countable plate carry 30–300 colonies?
Below 30, each colony carries too much statistical weight — moving one colony shifts the answer by more than 3%, and plate-to-plate noise is bigger than that. Above 300, colonies crowd, merge and are missed against the lawn, so the count quietly reads low — the classic ‘too numerous to count’ failure. The window is the compromise where a plate count behaves like a measurement.
I counted the 1:1,000 plate. Is the dilution factor 1,000 or 1/1,000?
Enter the fold-dilution — 1,000, meaning the sample on that plate is a thousand times thinner than the original culture. The page multiplies by it: colonies × dilution ÷ volume plated — the count goes UP the chain, back toward the stock. Entering 0.001 (the fraction) would undercount by a factor of a million, which is why the chain card prints the whole path back to the bottle.
What about colonies that merged or tiny satellite colonies?
They are exactly why the window exists and why replicates are averaged before this page is used. Count what you can defend, average honest plates, and let a plate outside the window disqualify itself — the card says so rather than absorbing a bad plate into a confident number.
My plate had zero colonies.
Zero is data: the culture is below the plate’s detection limit, which for your dilution and volume the card states as a ‘less than’ bound rather than a fake zero density. Plate a lower dilution — or more volume — if you need a real number.
Is one colony always one cell?
No — that is the CF in CFU. A colony-forming unit is the viable unit that founded the colony: a single cell, a chain of cells that stayed stuck together, a clump on a streak. Staphylococcus chains and clumped suspensions read low for true cell numbers while reading exactly right for infectious-unit purposes. The page reports CFU/mL because that is what the plate actually measures.
Should I average plates before or after this page?
Before — the page takes one colony number, so average the counts of your replicate plates first (the arithmetic mean for well-behaved spread plates). Averaging the CFU/mL of plates at different dilutions instead weights bad plates unevenly, and one plate outside the window should disqualify itself rather than hide inside an average.
How many dilutions should I plate?
Enough tenfold steps to straddle the window — three or four dilutions is standard practice, then count the plates that landed between 30 and 300 colonies and let the others go. Plating a single dilution is a bet that your density estimate was right; the chain exists so you do not have to bet.
Is CFU/mL the same as cells per mL?
No — a colony-forming unit is a viable unit, which can be a single cell, a chain, or a clump. Clumped organisms read low against true cell counts while reading exactly right for infective-unit purposes. Optical-density correlations are culture-specific calibrations, not conversions, and this page does not pretend otherwise.