One Atmosphere, Two Histories
Torricelli built the first barometer in 1643 by filling a tube with mercury and watching the column stand about 760 mm tall at sea level. Three centuries later, the unit he created still survives in weather broadcasts ("29.92 inches"), aviation altimeters, and hospital rooms — while the SI world measures the identical pressure as 101.325 kPa. The beautiful part is that both numbers are definitions: standard atmosphere is defined as 101.325 kPa, and the mercury column's height is pinned by the density of mercury and standard gravity. So 101.325 kPa converts to 760.0 mmHg — one standard atmosphere — and the conversion factor 7.500615758 is not a measurement; it is a ratio of definitions. (Strictly, the mercury-column definition lands at 759.9999 mmHg; the "exact 760" belongs to the torr, which defines 1 atm = 760 torr directly. The gap is 0.0001 mmHg — invisible to any gauge.)
mmHg = kPa × 7.500615758456563
1 kPa = 1000 / 133.322387415 mmHg
Exact — the reciprocal of the defined mmHg, from NIST SP 811.
Everyday Pressures, Both Units
| kPa | mmHg | Context |
|---|---|---|
| 101.325 kPa | 760.0 mmHg | Standard atmosphere at sea level — the anchor of both scales. |
| 100 kPa | 750.1 mmHg | Roughly sea level; the barometer's everyday neighborhood. |
| 16.0 kPa | 120 mmHg | Blood pressure systolic — the 120 of 120/80. |
| 10.7 kPa | 80 mmHg | Blood pressure diastolic — the 80 of 120/80. |
| 1 kPa | 7.50 mmHg | Rough vacuum — a strong shop vacuum pump. |
Scan the table and the pattern appears: each kPa is worth 7.5 mmHg, so the whole blood pressure range lives between 8 and 21 kPa while the atmosphere sits near 101. The hPa on the weather map is just 0.1 kPa, which is why forecast pressure "1013 hPa" is the same number family as the "101.325 kPa" in engineering tables — and both convert to 760.0 mmHg. When an anesthesia machine's monitor displays mmHg while the ventilator spec sheet quotes kPa, this single factor is the entire translation layer between them.
Engineering Context
The reverse direction — mercury to SI — is mmHg to kPa, the blood-pressure page. The same mercury scale appears as mmHg to Pa and Pa to mmHg, and its weather twin is Pa to atm. For the American-gauge side of the same pressures, kPa to psi and psi to kPa complete the set — a tire at 220 kPa reads 32 psi, and a barometer at 101.3 kPa reads 14.7 psi. Everything is one family in the pressure hub.
More: mmHg to kPa · kPa to psi · Pa to atm · Pressure Hub
Related Unit Converters
Frequently Asked Questions
How do I convert kPa to mmHg?
Multiply by 7.500615758: mmHg = kPa × 7.5006. Standard atmospheric pressure 101.325 kPa becomes 101.325 × 7.5006 = 760.0 mmHg — one standard atmosphere (the strict mercury-column figure is 759.9999 mmHg; the difference is a definitional detail between mmHg and torr). The factor is the reciprocal of 0.133322387415, so it is exact.
Why does a weather map use hPa while a barometer shows mmHg?
Meteorology standardized on the hectopascal (hPa) — identical to the millibar and to 0.1 kPa — decades ago, so forecasts quote 1013 hPa. Older aviation and marine barometers still show inches or millimeters of mercury. 1013 hPa equals 760 mmHg exactly, and every airport altimeter setting conversion is this same arithmetic.
What vacuum level do real systems use in mmHg?
Common vacuum pump ratings sit around 100 kPa to 0.1 kPa (about 750 mmHg to 0.75 mmHg). A rough vacuum — 1 kPa — is 7.5 mmHg. A laboratory rotary pump reaches about 0.1 kPa = 0.75 mmHg, and a turbomolecular pump goes below 10⁻⁵ Pa, far beyond the mmHg scale's usefulness. The mmHg scale is comfortable for the 100–760 mmHg range and gets awkward below it.
More: mmhg to kpa · kpa to psi · Pressure Hub