GIFSQ Tools · Module 3 — Food Microbiology & Enumeration · Tool 057 of 500
Turn a swab plate count into CFU per square centimetre, corrected for the swab's recovery efficiency.
Try sample values — fills the form with the worked example below.
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Total CFU recovered
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CFU per cm²
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Recovery-corrected CFU/cm²
Equation: Total CFU = colonies × (diluent volume ÷ plated volume); CFU/cm² = total CFU ÷ area swabbed; corrected CFU/cm² = CFU/cm² ÷ (recovery % ÷ 100).
Method: Surface sampling and recovery per ISO 18593:2018 (surface sampling techniques) with enumeration per FDA BAM Chapter 3 / ISO 4833-1:2013; recovery efficiency comes from the laboratory's own swab validation.
Assumptions: The swabbed area is evenly sampled, the diluent is evenly mixed, and the stated recovery efficiency applies to this surface and organism.
Limitations: Recovery varies by surface, swab type and organism, so the corrected value is only as good as the validation behind the efficiency figure. Counts below 30 colonies carry high counting variation.
Rounding: CFU values 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: Surface sampling and recovery per ISO 18593:2018 (surface sampling techniques) with enumeration per FDA BAM Chapter 3 / ISO 4833-1:2013; recovery efficiency comes from the laboratory's own swab validation.
Assumptions: The swabbed area is evenly sampled, the diluent is evenly mixed, and the stated recovery efficiency applies to this surface and organism.
Limitations: Recovery varies by surface, swab type and organism, so the corrected value is only as good as the validation behind the efficiency figure. Counts below 30 colonies carry high counting variation.
Rounding: CFU values 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: 45 colonies from 1 mL plated out of 10 mL diluent, 25 cm² swabbed, 80% recovery: total = 45 × 10 = 450 CFU; 450 ÷ 25 = 18.0 CFU/cm²; corrected = 18 ÷ 0.80 = 22.5 CFU/cm².
Enter values above to see results.
Use the recovery efficiency from your own swab validation for this surface — a borrowed figure can mislead. Tool 057 of 500 — GIFSQ
