Convert Millimeters of Mercury to Atmospheres

Formula

atm = mmHg × 0.00131578966114

or atm = mmHg ÷ 759.9998917256

Derivation from SI units:

• 1 mmHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.001 m = 133.322387415 Pa
• 1 atm = 101 325 Pa (exact, by definition)
• 1 mmHg = 133.322387415 Pa ÷ 101325 Pa per atm = 0.00131578966114 atm

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

Worked examples

1. A reading of 80 mmHg converts to 0.1052631729 atm.
   80 mmHg × 0.001315789661 = 0.1052631729 atm
2. A value of 160 mmHg is 0.2105263458 atm.
   160 mmHg × 0.001315789661 = 0.2105263458 atm

Where it's used

Millimeters of Mercury are used for blood pressure (e.g. 120/80 mmHg), 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.

Millimeter 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, "millimeter of mercury, conventional (mmHg)".
• 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.