What is wind "CALM"?
Wind &quot;CALM&quot; means a wind speed less than 0.5m/s (1 kt) according to ICAO Annex 3, Appendix 3:

4.1.5.2    In local routine and special reports and in METAR and SPECI:
...
d)  when a wind speed of less than 0.5m/s (1 kt) is reported, it shall be indicated as calm;

Before Amendment 75 of ICAO Annex 3 this read:

d)  when a wind speed of less than 2 km/h (1 kt) is reported, it shall be indicated as calm;

See also Annex 3:

Appendix 5, recommendation 1.2.1
Attachment C


The FAA, in their Order JO 7110.65X (Air Traffic Control), defines a limit of 3 knots.

2−6−3.  REPORTING WEATHER CONDITIONS
....
b. Describe  the  wind  as  calm  when  the  wind velocity is less than three knots.