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Pascals to Inches of Water Converter

Type a value in either box to convert between pascals (Pa) and inches of water column (in WC). One inch of water column is 249.0889 Pa, so -0.02 in WC is about -4.98 Pa.

Type in either box; the other one follows. A minus sign means negative pressure (the space is lower than the reference side).

Result

-4.98 Pa

Inches of water column
-0.0200 in WC
Millimeters of water
-0.51 mm WC

Containment pressure targets

WhereTargetIn pascalsSource
Negative pressure enclosure for asbestos work At least -0.02 in WC, relative to outside the enclosure -4.98 Pa OSHA 29 CFR 1926.1101(g)(5)(i)(A)(3), which also calls for at least 4 air changes per hour in the enclosure
Temporary negative pressure isolation room About -0.01 in WC (negative 2.5 Pa) -2.49 Pa Minnesota Department of Health, Airborne Infectious Disease Management: Methods for Temporary Negative Pressure Isolation
Protective environment room (positive pressure) At least 0.01 in WC (2.5 Pa) 2.49 Pa CDC Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B

Quick conversion table

1 in WC = 249.0889 Pa. Values rounded.
Pascalsin WCin WCPascals
1 Pa0.00400.005 in1.25
2 Pa0.00800.01 in2.49
2.5 Pa0.01000.02 in4.98
5 Pa0.02010.03 in7.47
10 Pa0.04010.04 in9.96
12.5 Pa0.05020.05 in12.45
25 Pa0.10040.1 in24.91
50 Pa0.20070.25 in62.27
100 Pa0.40151 in249.09

How the conversion works

An inch of water column is the pressure of a column of water one inch tall. NIST lists the conventional inch of water as 249.0889 pascals, the factor used here.

Pa = in WC × 249.0889
in WC = Pa ÷ 249.0889
mm WC = Pa ÷ 9.80665

Example: OSHA's minimum of -0.02 in WC for an asbestos enclosure is -0.02 × 249.0889 = -4.98 Pa. A pressure monitor that reads ±25 Pa covers about ±0.10 in WC.

Sources

NIST Special Publication 811, Appendix B.9: inch of water, conventional = 249.0889 Pa; centimeter of water, conventional = 98.0665 Pa. OSHA, 29 CFR 1926.1101(g)(5)(i)(A). Minnesota Department of Health, Airborne Infectious Disease Management: Methods for Temporary Negative Pressure Isolation. CDC, Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B.

Pressure monitors and negative air

Frequently asked questions

How many pascals are in an inch of water?

249.0889 Pa, using NIST's conventional inch of water. For quick math, 1 in WC is about 249 Pa and 1 Pa is about 0.004 in WC.

What is -0.02 inches of water in pascals?

About -4.98 Pa. It is the minimum pressure differential OSHA requires inside a negative pressure enclosure for asbestos work, measured against the pressure outside the enclosure.

Is in WC the same as in H2O?

Yes. Inches of water column, in WC, in H2O and in w.g. (water gauge) all name the same unit.

What does a minus sign mean on a pressure monitor?

The space being measured is at a lower pressure than the reference side, usually the hallway or room outside. That negative reading is what makes air flow in through gaps rather than out.

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