Dilution Plating Calculator
Calculate colony-forming units per mL (CFU/mL) from colony count, dilution factor, and volume plated.
About this calculator
Dilution plating estimates how many viable, culturable organisms were in an original sample by counting colonies on a plate that received a known, diluted fraction of that sample — and then mathematically reversing the dilution. This calculator's core relationship is CFU/mL = colony count × dilution factor ÷ volume plated (mL), where the dilution factor is entered as the full dilution ratio (e.g., 1,000,000 for a 10⁻⁶ dilution series), not the exponent alone. Because a countable plate can only hold so many distinguishable colonies before they merge into an unreadable lawn, microbiologists serially dilute a sample by factors of 10 and plate several dilutions in parallel, then pick the plate that landed in a reliable counting window — conventionally 30 to 300 colonies — to do the back-calculation from, which is why the input's helper text flags that range explicitly.
Below 30 colonies, statistical noise from a small sample size dominates; above 300, overcrowding causes colonies to compete for space and merge, undercounting the true population. The result is also broken into scientific-notation mantissa and exponent, since microbial counts routinely span many orders of magnitude and reporting "6.4 × 10⁷ CFU/mL" is both more standard and easier to compare across samples than a long string of digits. One limitation worth remembering: CFU/mL only counts organisms capable of forming a visible colony under your specific growth conditions (medium, temperature, atmosphere), so it will systematically undercount viable-but-non-culturable cells or organisms that need different conditions than the ones you plated under.
Inputs
Results
CFU/mL
1,500,000,000
Log₁₀ CFU/mL
9.18
How to Use This Calculator
- Enter Colony Count, Dilution Factor, and Volume Plated (mL).
- Review CFU/mL and Log₁₀ CFU/mL.
- Use Scientific Notation Mantissa and Scientific Notation Exponent to inform your decision.
- Use the chart to visualize the results and explore different scenarios by adjusting inputs.
How the result changes with Volume Plated (mL)
| Volume Plated (mL) | CFU/mL | Log₁₀ CFU/mL |
|---|---|---|
| 0.05 | 3,000,000,000 | 9.48 |
| 0.08 | 2,000,000,000 | 9.3 |
| 0.15 | 1,000,000,000 | 9 |
| 0.25 | 600,000,000 | 8.78 |
What each input means
- Colony Count
- Number of colonies counted on the plate. Ideal range is 30–300 for accuracy.
- Dilution Factor
- Total dilution factor (e.g., enter 1000000 for a 10⁻⁶ dilution).
- Volume Plated (mL)
- Volume of diluted sample spread on the plate in milliliters.
How this is calculated
Formula
CFU/mL = colony count × dilution factor / volume plated (mL)Worked example, using the default values
- Identify Input ParametersColony Count = 150, Dilution Factor = 1000000, Volume Plated (mL) = 0.1 = 3 input(s) provided
- Calculate CFU/mLCFU/mL1500000000 = 1500000000
- Calculate Log₁₀ CFU/mLLog₁₀ CFU/mL9.18 = 9.18
- Calculate Scientific Notation MantissaScientific Notation Mantissa1.5 = 1.5
- Calculate Scientific Notation ExponentScientific Notation Exponent9 = 9
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why do I enter the full dilution factor like 1,000,000 instead of just the exponent 6?
The calculator's formula is CFU/mL = colony count × dilution factor ÷ volume plated, and it expects the dilution factor as the actual ratio the sample was diluted by, not the exponent from a 10⁻⁶ notation. Entering 6 instead of 1,000,000 would understate the back-calculated concentration by six orders of magnitude, so the input's helper text spells out the full number explicitly to avoid that mistake.
Why does the 30–300 colony count range matter for accuracy?
Below 30 colonies, random variation in exactly which cells happened to land where dominates the count, making the back-calculated CFU/mL statistically unreliable. Above 300, colonies begin to compete for space on the plate and can merge into indistinguishable clumps, causing the calculator's back-calculation to understate the true population since some colonies never got counted as separate. Picking whichever dilution in your series landed in that window gives the most trustworthy result.
What do the scientific notation mantissa and exponent outputs mean?
Microbial counts routinely span many orders of magnitude, so the calculator also expresses your CFU/mL as a mantissa (a number between 1 and 10) times 10 raised to the exponent — for example, a result of 6.4 × 10⁷ CFU/mL would show mantissa 6.4 and exponent 7. This is the standard way microbiologists report and compare counts across samples with very different concentrations.
Does CFU/mL count every organism that was actually alive in the sample?
No — CFU/mL only counts organisms that were both alive and able to form a visible colony under the specific growth conditions (medium, temperature, atmosphere) you plated under. Viable-but-non-culturable cells, or organisms that simply need different conditions than the ones provided, won't form colonies and will be systematically missing from this count even though they were present and alive in the original sample.
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