Convert Atmospheres to Inches of Mercury

Formula

inHg = atm × 29.92125557975

Derivation from SI units:

• 1 atm = 101 325 Pa (exact, by definition)
• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 atm = 101325 Pa ÷ 3386.388640341 Pa per inHg = 29.92125557975 inHg

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

Worked examples

1. A reading of 1.5 atm converts to 44.88188337 inHg.
   1.5 atm × 29.92125558 = 44.88188337 inHg
2. A value of 10 atm is 299.2125558 inHg.
   10 atm × 29.92125558 = 299.2125558 inHg

Where it's used

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

Atmosphere 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, "atmosphere, standard (atm)".
• 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)".

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