Altimeter setting: converting inHg to hPa when crossing into metric airspace
On a cross-border flight, ATC handed off an altimeter setting in hPa while my Kollsman window only had inHg markings — converting quickly and correctly in the cockpit was a genuinely useful drill, not just a classroom ex
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
On a cross-border flight, ATC handed off an altimeter setting in hPa while my Kollsman window only had inHg markings — converting quickly and correctly in the cockpit was a genuinely useful drill, not just a classroom exercise.
Here is the conversion I wish I had taped to the fridge before that went sideways.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
1 inHg = 33.8639 hPa
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Dialing a hPa altimeter setting number directly into an inHg-only Kollsman window as if the digits transferred over — 1013 hPa is not '10.13' or any similar-looking inHg value.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in inHg.
- Confirm the target unit is really hPa, not a similar-looking one.
- Apply the factor: 1 inHg = 33.8639 hPa.
- Round only at the end, to the precision your tool or spec supports.
Worked example
ATC issues an altimeter setting of 29.92 inHg, standard sea-level pressure, and your next sector uses hPa-only airspace.
29.92 inHg × 33.8639 = 1013.21 hPa
That's 1013.21 hPa — round only to the precision your tool or spec actually needs.
Flip the factor once as a sanity check. If you do not land near the starting number, you mixed definitions or inverted the direction.
Quick reference table
| inHg | Calculation | hPa |
|---|---|---|
| 1 | 1 × 33.8639 | 33.8639 |
| 2 | 2 × 33.8639 | 67.7278 |
| 5 | 5 × 33.8639 | 169.32 |
| 10 | 10 × 33.8639 | 338.639 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
An incorrectly set altimeter directly causes incorrect indicated altitude, which is a serious separation and terrain clearance issue — this conversion has zero tolerance for a rough guess.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Multiply inHg by 33.864 to get hPa — standard 29.92 inHg converts to almost exactly 1013.2 hPa, worth memorizing as a sanity check.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact inHg to hPa factor here?+
1 inHg = 33.8639 hPa
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Dialing a hPa altimeter setting number directly into an inHg-only Kollsman window as if the digits transferred over — 1013 hPa is not '10.13' or any similar-looking inHg value.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-25 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.