This worksheet sets the two numbers that drive a cleanroom classification: how many locations to sample, and how much air to draw at each. It gives the formulas and the method; you look up the location count from the current standard and plug in your own class limit. The calculation mechanics here are physical formulas and conventions, not reproduced standard text; confirm every value against ISO 14644-1:2015 before you rely on it. A worked example follows. This content is educational and general.
Step 1: define the classification basis
| Field | Entry |
|---|---|
| Room / area ID | <<FILL>> |
| Target ISO class (state) | <<FILL: e.g. ISO 5, at rest>> |
| Cleanroom area, m2 | <<FILL>> |
| Considered particle sizes | <<FILL: e.g. 0.5 um; 5.0 um where required>> |
| Class concentration limit Cn,m for the most stringent considered size, particles/m3 | <<FILL: from the standard>> |
Step 2: number of sampling locations
Under ISO 14644-1:2015 the minimum number of sampling locations is read from the standard’s Annex A lookup table by cleanroom area (the 2015 revision replaced the older square-root-of-area rule with a fixed table tied to a 95% upper confidence approach). Do not use the pre-2015 square-root shortcut.
| Field | Entry |
|---|---|
| Area band from ISO 14644-1:2015 Table A.1 | <<FILL>> |
| Minimum number of locations, N | <<FILL: read from the table>> |
| Locations laid out to represent the whole space, including process-critical points (over the filling zone, near returns, at work height)? | Yes / No |
Step 3: minimum single-sample volume per location
The minimum single-sample volume ensures you would expect to count at least 20 particles at the class limit for the most stringent considered size:
Vs = (20 / Cn,m) x 1,000 litres
where Cn,m is the class concentration limit (particles/m3) for that size. Apply two floors: never less than 2 litres, and never less than a one-minute sample.
| Field | Entry |
|---|---|
| Cn,m used (most stringent considered size), particles/m3 | <<FILL>> |
| Vs = (20 / Cn,m) x 1000, litres | <<FILL>> |
| Applied minimum (>= 2 L and >= 1 minute) | <<FILL>> |
| Practical sample volume chosen, litres | <<FILL: often 1,000 L / 1 m3, justified by the largest considered size and the need to resolve low counts>> |
| Basis for the practical volume | <<FILL>> |
Note: for ISO 5 at 0.5 um alone (Cn,m = 3,520) the formula yields only about 5.7 litres, so the common 1 m3 per point comes from the largest considered size (a lower concentration limit) and the practical need to resolve very low counts reliably, not from the 0.5 um limit.
Step 4: sampling time per location
| Field | Entry |
|---|---|
| Counter flow rate, L/min | <<FILL: e.g. 28.3 (1 cfm)>> |
| Time per location = volume / flow rate, minutes | <<FILL>> |
| Total classification time (N x time per location), plus setup | <<FILL>> |
Step 5: sign-off
| Role | Name | Signature | Date |
|---|---|---|---|
| Prepared by | <<FILL>> | ||
| QA reviewed | <<FILL>> |
References
ISO 14644-1:2015 (classification, location count and sample volume method). EU GMP Annex 1 (2022) for the grade-to-class mapping and where a 5.0 um figure applies.
Worked example
A 24 m2 Grade B background, classified at rest, target ISO 5 at 0.5 um.
| Field | Value |
|---|---|
| Area | 24 m2 |
| Locations N (from the 2015 Annex A lookup for that area band) | 6 |
| Cn,m at 0.5 um for ISO 5 | 3,520 particles/m3 |
| Vs = (20 / 3,520) x 1,000 | about 5.7 litres |
| Applied minimum | 5.7 L exceeds the 2 L and 1-minute floors |
| Practical volume chosen | 1,000 L (1 m3) per location, to resolve low 5.0 um counts reliably |
| Counter flow | 28.3 L/min |
| Time per location | about 35 minutes |
| Total (6 locations) | about 3.5 hours plus setup |
The example shows why the 0.5 um formula alone (5.7 L) is not what drives the sample size; the practical 1 m3 comes from the largest considered particle size and the need to count reliably at very low concentrations.
Common inspection findings this worksheet prevents
- Using the pre-2015 square-root-of-area rule for the location count.
- A sample volume too small to detect particles at the class limit for the most stringent size.
- Locations that do not represent the room or miss the process-critical points.
- No record of how the sample volume was justified.
How to adapt this worksheet
- Read the location count from the current ISO 14644-1 Annex A table for your exact area.
- Use the class limit for your most stringent considered size in the Vs formula.
- Record the practical volume and the basis, so the choice is defensible.
- Reuse the location layout across classification, EM, and requalification for consistency.