Convert Inches of Mercury to Atmospheres

Formula

atm = inHg × 0.03342105739295

or atm = inHg ÷ 29.92125557975

Derivation from SI units:

• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 atm = 101 325 Pa (exact, by definition)
• 1 inHg = 3386.388640341 Pa ÷ 101325 Pa per atm = 0.03342105739295 atm

Both units are defined exactly, so the factor is exact. Inch of Mercury definition: inch of mercury; Atmosphere definition: atmosphere.

Worked examples

1. A reading of 15 inHg converts to 0.5013158609 atm.
   15 inHg × 0.03342105739 = 0.5013158609 atm
2. A value of 29 inHg is 0.9692106644 atm.
   29 inHg × 0.03342105739 = 0.9692106644 atm

Where it's used

Inches of Mercury are used for aircraft altimeter settings (standard 29.92 inHg), while atmospheres appear in chemistry and gas laws (STP and NTP conditions). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Inch of Mercury to other pressure units

Frequently asked questions

Sources

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "inch of mercury, conventional (inHg)".
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "atmosphere, standard (atm)".

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.