Particle Counting: Terms and Formulas

Classification Number (Cn) - The maximum permitted concentration (particle/m3 of air) of airborne particles equal to or larger than the considered particle size). Rounded to the nearest whole number.

  • N is the ISO classification number—not to exceed the value of 9
  • Intermediate ISO classification numbers may be specified, with 0.1 the smallest permitted increment of N 
  • D (denominator) considered particle size in µm
  • 0.1 is a constant with a dimension of µm

Cleanroom - A room in which the concentration of airborne particles is controlled. Cleanrooms are constructed and operated to minimize the introduction, generation and retention of particles inside the room and control other relevant parameters (e.g., temperature, humidity, pressure controlled as necessary).

Clean Zone is a dedicated space where the concentration of airborne particles is controlled. Clean zones may be open or enclosed and may or may not be located within a cleanroom.

Counting Efficiency Test compares the performance of the particle counter under test against an instrument with a higher resolution (~0.3 microns). Both instruments are challenged with an air sample containing certified particles at the smallest calibration size (for the counter being tested, typically 0.5 microns for GMP applications). 

False Count Rate or zero-count rate is defined as the number of false counts recorded by an air particle counter with respect to time. False particles are associated with the number of intrinsic and extrinsic sources to the system such as cosmic radiation, optoelectronic noise or contamination.

Monitoring - Observations made by measurement in accordance with defined methods to provide evidence of compliance. Information may be used to detect trends in operational state and to provide process support.

Sampling locations*
ISO 14644-1:1999 calculated the number of sampling locations with the formula.
In the 2015 version, location(s) are determined by Table A.1:

Area of cleanroom (m2) less than or equal to  Minimum number of locations to be tested (NL) 
 52 10 
 56 11 
 64 12 
 68 13 
 72 14 
 76 15 
 104 16 
 108  17
 115  18
 148 19 
 156 20 
 192 21 
 232 22 
 275 23 
 352 24 
 436 25 
 500 26 
 1000 27 
 >1000 Equation A 

Equation A: N+27 (area) 1000

Sampling Flow Rate and Sample Time - Particle counts are measured by air particle counters as a function of concentration per unit volume (e.g., particles per cubic meter or cubic foot). Sample flow rate accuracy is therefore, critical to mitigating flow rate errors caused while sampling actual volume for a fixed sample time. Sample time accuracy is also critical for measuring the sample volume at a given sample rate.

Sample Volume per Location - At each sampling location, sample a volume of air such that a minimum of 20 particles would be detected if the particle concentration for the largest considered particle size were at the class limit for the specified ISO class.

Single Volume Calculation (Vs):

  • Vs - Minimum single sample volume per location (in liters) – B.4.2.2 Minimum volume sampled per location shall be at least 2 liters with minimum sampling time of one minute.
  • Cn,m - Class limit (number of particles per m3 ) for largest considered particle size noted for the relevant class
  • 20 - Number of potential samples to be counted, if particle concentration is at class limit

NOTE: When Vs is very large, sampling time can be substantial. Using the sequential sample procedure (Annex F) allows for reduced sample volume and time required to obtain samples. 

Size Resolution is a measurement of the particle counter’s ability to accurately size particles (statistical equation based on Guassian distribution). 


  • Frequent - occurs at specified intervals not exceeding 60-minutes during operation
  • 6 months - occurs at an average interval not exceeding 183 days throughout periods of operational use, subject to no interval exceeding 190 days
  • 12 months -occurs at an average interval not exceeding 366 days throughout periods of operational use, subject to no interval exceeding 400 days
  • 24 months - occurs at an average interval not exceeding 731 days throughout periods of operational use, subject to no interval exceeding 800 days

Upper Confidence Limit (95% UCL) - The 95% UCL is a measure of how accurately a result represents reality. In terms of particle counting, this is the mean of particle counts measured at various locations, which are typically means of individual samples. The accuracy of the result is calculated by specifying upper and lower confidence limits (typically a confidence of 95 percent for cleanrooms). ISO 14644-1:2015 eliminates the need for the UCL calculation provided all sample locations are within Glass/Grade requirements.

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