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Converting inches of mercury to millibars

Remote Pilot (Part 107) practice question 1 of 25 on Weather / Effects on Performance. ACS element UA.III.B.K1c · difficulty: applied. Want to answer it in a timed quiz? Use the Remote Pilot (Part 107) practice mode.

Question bank updated 2026-09-22

QUESTIONStandard sea level pressure is 29.92 inches of mercury, or 1,013.2 millibars, and 1 inch of mercury equals approximately 34 millibars. A station reports a sea level pressure of 28.92 inches of mercury. Approximately what is that pressure in millibars?

  1. A1,047 mb
  2. B979 mb
  3. C1,003 mb
Show the answer and explanation

Correct answer: B. 979 mb

  • A — incorrect This adds 34 mb to standard instead of subtracting it; a reading below 29.92 must convert to fewer than 1,013.2 mb.
  • B — correct The reading is 1 inch of mercury below standard, and 1 inch equals about 34 millibars, so 1,013.2 minus 34 is approximately 979 mb.
  • C — incorrect This subtracts only 10 mb; the conversion factor is approximately 34 mb per inch of mercury.

BACKGROUND

Atmospheric pressure is reported both in inches of mercury and in millibars. The study guide states that 1 inch of mercury equals approximately 34 millibars, that standard sea level pressure is 29.92 inches of mercury or 1,013.2 millibars, and that typical millibar readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers and hurricane data are reported in millibars, so a remote pilot should be able to move between the two units.

STUDY TIP

Anchor on 29.92 inches equals 1,013.2 mb, then add or subtract 34 mb for every inch of difference.

KEY POINTS

  • 1 inch of mercury is approximately 34 millibars.
  • 29.92 inches of mercury equals 1,013.2 mb.
  • Typical millibar readings fall between 950.0 and 1,040.0 mb.

SOURCES

  • remote_pilot_study_guide.txt ch3b "Measurement of Atmosphere Pressure"
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MORE ON WEATHER / EFFECTS ON PERFORMANCE

SkyDrill is not affiliated with the FAA. This is an original practice item based on 14 CFR part 107, 14 CFR part 89, and the FAA Remote Pilot – Small UAS Study Guide (FAA-G-8082-22). Regulations and handbooks are revised; confirm details against the current edition.