Inches of mercury and hectopascals
The conventional inch of mercury is 25.4 mmHg: about 3,386.39 pascals. US weather reports and aviation altimeter settings (29.92 inHg) use inches of mercury.
A hectopascal is 100 pascals, identical to one millibar. Weather forecasts and barometers report air pressure in hPa (about 1013 hPa at sea level).
Worked example
Convert 29.92 inHg: 29.92 × 33.8638864 = 1,013.207481, so 29.92 inches of mercury is 1,013.207481 hectopascals. Likewise 20 inHg = 677.2777281 hPa.
Estimating in your head
Multiply by 34 for a quick figure; it is off by about 0.4%. For 29.92 inHg that gives 1,020 hPa, against 1,013.21 exactly.
Going the other way
To convert hectopascals to inches of mercury, divide by 33.8638864 (or multiply by 0.029529983301).
How this works
hPa = inHg × 33.8638864
1 inHg = ≈ 3,386.38864 Pa and 1 hPa = 100 Pa, both by definition, so the factor is their ratio: 33.8638864 (rounded here; the page uses the full value). Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many hectopascals are in a inch of mercury?
1 inch of mercury = 33.8638864 hectopascals. The factor is exact by definition but has endless decimals, so it is shown rounded; the page uses the full value.
How do I convert inches of mercury to hectopascals?
Multiply by 33.8638864. For example, 20 inHg × 33.8638864 = 677.2777281 hPa.
What is 100 inHg in hectopascals?
100 inHg = 3,386.38864 hPa. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- NIST SP 811 Appendix B.9 — Factors for units listed by kind of quantity
- Wikipedia — Pound per square inch
Reviewed Oct 17, 2026 · test vectors fixtures/units/pressure/inhg-to-hpa/