inHg Vacuum to atm Converter
Standard atmospheric pressure at sea level supports a mercury column 29.92 inches high. This converter expresses vacuum (in inches of mercury) as a fraction of standard atmosphere....
Formula
Source: Engineering Toolbox, NIST | Last reviewed: June 8, 2026
Examples
29.92 in Hg
= 1 atm
Standard atmospheric pressure
15 in Hg
= 0.501 atm
15 inHg = ~0.5 atm (half atmosphere)
0 in Hg
= 0 atm
0 inHg = 0 atm (perfect vacuum)
25 in Hg
= 0.836 atm
Typical process vacuum level
Quick Reference Table
| inHg | atm |
|---|---|
| 0 | 0 |
| 5 | 0.167 |
| 10 | 0.334 |
| 15 | 0.501 |
| 20 | 0.669 |
| 25 | 0.836 |
| 29.92 | 1 |
Where is this used?
Process vacuum requirements: chemical processes specifying vacuum in atmospheres or torr.
Condenser vacuum ratings in steam power plants.
Frequently Asked Questions
Is this inches of mercury absolute or vacuum?
When used as a vacuum measurement, inHg typically refers to inches of mercury below atmospheric. A reading of 25 inHg vacuum means the absolute pressure is 29.92 − 25 = 4.92 inHg absolute. This converter assumes inHg absolute for direct conversion to atm.
How does this relate to perfect vacuum?
Perfect vacuum is 0 atm = 0 inHg absolute. On a vacuum gauge, perfect vacuum would read 29.92 inHg vacuum. The converter handles both absolute and vacuum measurements — adjust your input accordingly.
What vacuum level do I need for my application?
Rough vacuum (0–15 inHg vacuum) for material handling and filtration. Medium vacuum (15–28 inHg vacuum) for vacuum forming and distillation. High vacuum (>28 inHg vacuum) for specialized processes requiring multi-stage pumps.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.