How to Convert Inches of mercury to Atmospheres (inHg to atm)
Pressure conversions are used for tyre pressure (psi or bar), blood pressure readings in mmHg and weather maps in millibars. This guide shows how to convert inches of mercury into atmospheres by hand, with examples and a quick-reference table.
The conversion
To convert inches of mercury to atmospheres, multiply the value by 0.0334210609. To go the other way, divide by the same number (or multiply by 29.9212524019). For a quick estimate, multiply by about 0.033.
Atmospheres = inches of mercury × 0.0334210609
Worked examples
- 3 inHg = 0.100263 atm
- 12.5 inHg = 0.417763 atm
- 75 inHg = 2.5066 atm
Inches of mercury to Atmospheres table
| Inches of mercury | Atmospheres |
|---|---|
| 1 inHg | 0.033421 atm |
| 2 inHg | 0.066842 atm |
| 5 inHg | 0.167105 atm |
| 10 inHg | 0.334211 atm |
| 20 inHg | 0.668421 atm |
| 25 inHg | 0.835527 atm |
| 50 inHg | 1.6711 atm |
| 100 inHg | 3.3421 atm |
| 250 inHg | 8.3553 atm |
| 500 inHg | 16.7105 atm |
| 1,000 inHg | 33.4211 atm |
Common questions
Is inHg to atm exact?
The factors used here are the internationally defined values where they exist; results are rounded for display only.
How do I convert back?
Use the Atmospheres to Inches of mercury converter.
Mistakes to avoid
Rounding the factor too early
Using 3 instead of 3.28 for feet per metre introduces an error of almost 9% before you have started.
Dropping the units
A bare number without a unit is easy to misread later. Always write the unit with the result.
Forgetting squared and cubed units
Area and volume factors are the length factor squared or cubed, so 1 m² is about 10.76 sq ft, not 3.28.
Background
Unit conversion is simply multiplication by a fixed ratio. The ratio between two units of the same quantity never changes, so once you know it you can convert any value in either direction.
For everyday use, three or four significant figures are usually enough. Engineering, science and trade documents may need more, and rounding should happen only at the end of a calculation.
Writing the units next to every number while you work is the simplest way to avoid mistakes. If the units do not cancel to leave the one you want, the calculation is set up wrongly.
For any value, use the Inches of mercury to Atmospheres Converter or browse all pressure converters.
Run your own numbers: Inches of mercury to Atmospheres Converter