Bar (pressure)

BIPM's SI Brochure lists the bar in its Table 8 of non-SI units, with the value "1 bar = 0.1 MPa = 10⁵ Pa": one bar is a tenth of a megapascal, or 100,000 pascals. Its footnote records that "The bar and its symbol were included in Resolution 7 of the 9th CGPM in 1948." The brochure explains why such units are tabulated at all: "It is recognized that some non-SI units are widely used and that this is expected to continue for many years. It is therefore important to recall the values of these non-SI units in terms of SI units, because the SI is the internationally agreed reference with respect to which all other units are defined."

The bar is tolerated rather than adopted. Announcing an update to the brochure on 4 June 2026, BIPM stated that the revision makes explicit that "non-SI units do not hold any special status within the SI", and reorganised the listed non-SI units into four categories.

On a watch the number appears on the caseback. Seiko's own customer-service page shows markings of 3BAR, 5BAR, 10BAR, 15BAR and 20BAR, and warns that "a watch's water resistance is not permanently guaranteed". The figure comes from a laboratory test: ISO 22810:2010, Horology - Water-resistant watches, edition 1, published August 2010, "establishes the requirements and specifies the test methods used to verify the water resistance of watches. Moreover, it indicates the marking which the manufacturer is authorized to apply to them."

What this Codex does not do: state what pressure or duration that test applies, or what a given marking permits. ISO 22810 is paywalled and only its abstract is public. The test is covered in this Codex's entry on the water-resistant watch.