The 120 ft per degree rule against the full formula
Pilots learn a shortcut: take pressure altitude, then add about 120 ft for every degree Celsius the air is warmer than standard. It's quick, and close enough for a mental check. This page compares it with the National Weather Service formula the calculator uses, at a 5,000 ft field on a standard pressure day.
Not for flight planning. Use your aircraft's POH or flight manual and an official weather briefing.
Open these numbers in the calculator and change them to match your own conditions.
| Temperature | NWS formula | 120 ft rule | Difference |
|---|---|---|---|
| 0 C | 4,407 ft | 4,389 ft | -18 ft |
| 10 C | 5,593 ft | 5,589 ft | -4 ft |
| 20 C | 6,728 ft | 6,789 ft | 61 ft |
| 30 C | 7,817 ft | 7,989 ft | 172 ft |
| 35 C | 8,345 ft | 8,589 ft | 244 ft |
Standard temperature falls about 2 C per 1,000 ft, so standard at the field is 15 C minus 2 C for each 1,000 ft of pressure altitude. The rule ignores humidity, which the formula can include through the dew point.
How these figures are worked out
Each figure uses the National Weather Service method: station pressure from the altimeter setting and elevation, vapour pressure from the dew point, then virtual temperature and density altitude. The calculator runs the same code. Figures are rounded to the nearest foot and are estimates.