What m³ means

The cubic metre of air that gives ONE CUBIC METRE its name and every issue its number.

1 m × 1 m × 1 m = 1,000 litres of air

m³ stands for cubic metre: the volume of air that cleanroom limits for particles and microorganisms refer to. That is why every issue carries a number: m³ 00, m³ 01 and so on.

See all issues →

How big is a cubic metre

1,000litres of air in 1 m³
28.3litres in 1 cubic foot (ft³), the unit many counters use
35cubic feet in 1 m³, roughly

Why everything is counted per cubic metre

Airborne particles are counted per cubic metre both in the ISO 14644-1 classification and in the EU GMP Annex 1 limits. Active air sampling for microorganisms also has its limits in CFU (colony-forming units) per cubic metre. The same volume makes results from different rooms, instruments and days comparable.

Total particles per m³

Grade	≥ 0.5 µm		≥ 5.0 µm
Grade	at rest	in operation	at rest	in operation
A	3 520	3 520	not specified	not specified
B	3 520	352 000	not specified	2 930
C	352 000	3 520 000	2 930	29 300
D	3 520 000	not predetermined	29 300	not predetermined

Maximum total particle concentration per m³ of air. Source: EU GMP Annex 1, C(2022) 5938 final, 22 August 2022, Table 5. For ≥ 5.0 µm the limit is not specified in Grade A (at rest and in operation) and in Grade B at rest, unless set by the contamination control strategy. The official text of the standard is binding.

Microorganisms: action limits

Grade	Air, CFU/m³	Settle plates Ø 90 mm, CFU/4 hours	Contact plates Ø 55 mm, CFU/plate	Glove print, 5 fingers on both hands, CFU/glove
A	no growth	no growth	no growth	no growth
B	10	5	5	5
C	100	50	25	–
D	200	100	50	–

Maximum action limits for viable particle contamination. Source: EU GMP Annex 1, C(2022) 5938 final, 22 August 2022, Table 6. Only air is expressed per m³; plates are counted per time, plate or glove. The official text of the standard is binding.

How long it takes to sample 1 m³

Microbial air sampler at 100 L/min10 min
Particle counter at 28.3 L/min (1 ft³/min)about 35 min

Typical flow rates: the actual time depends on the instrument. Diagram: ONE CUBIC METRE.

From sample to value per m³

1Samplea known volume, for example 500 litres.
2Countthe colonies on the plate: 2 CFU.
3Scale to 1 m³2 × 1,000 ÷ 500 = 4 CFU/m³.
4Comparewith the limits for the room’s grade.
m³Trend itthe value is read together with the others, in the trend.

How a count becomes a value per cubic metre. Example with test numbers. Diagram: ONE CUBIC METRE.

Try the numbers

Two quick calculations with the numbers on this page. They help you see the order of magnitude: official values come from validated procedures and the text of the standard.