GIFSQ Tools · Module 3 — Food Microbiology & Enumeration · Tool 056 of 500
Work out how much stock culture and how much sterile diluent to mix to reach a target concentration.
Try sample values — fills the form with the worked example below.
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Stock volume (mL)
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Diluent volume (mL)
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Dilution factor
Equation: C₁V₁ = C₂V₂, so stock volume V₁ = (C₂ × V₂) ÷ C₁; diluent = V₂ − V₁; dilution factor = C₁ ÷ C₂.
Method: Serial-dilution practice per FDA BAM Chapter 3 and ISO 7218:2007: dilutions are made in sterile diluent with a fresh pipette at each step.
Assumptions: The stock is evenly mixed, the concentrations are in the same units, and volumes add with no change on mixing.
Limitations: A stock volume below 0.1 mL is hard to pipette accurately — make an intermediate dilution first. Very large dilution factors are better done in two steps.
Rounding: Volumes to 2 decimal places (3 where below 0.1 mL); dilution factor to 1 decimal place; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Serial-dilution practice per FDA BAM Chapter 3 and ISO 7218:2007: dilutions are made in sterile diluent with a fresh pipette at each step.
Assumptions: The stock is evenly mixed, the concentrations are in the same units, and volumes add with no change on mixing.
Limitations: A stock volume below 0.1 mL is hard to pipette accurately — make an intermediate dilution first. Very large dilution factors are better done in two steps.
Rounding: Volumes to 2 decimal places (3 where below 0.1 mL); dilution factor to 1 decimal place; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Stock 1.0 × 10⁹ CFU/mL diluted to 1.0 × 10⁶ CFU/mL in 10 mL: stock volume = (10⁶ × 10) ÷ 10⁹ = 0.01 mL; diluent 9.99 mL; dilution factor 1,000.
Enter values above to see results.
Use a fresh sterile pipette for each dilution step and mix well before the next step. Tool 056 of 500 — GIFSQ
