Altimeter Setting Calculator

An altimeter is a barometer with a height scale. Air pressure falls as you climb, by about 1 hPa for every 27 ft near sea level. The setting in the little Kollsman window tells the altimeter which pressure counts as zero. Set the QNH and it reads altitude above sea level. Set the QFE and it reads height above the airfield. Set 1013.25 hPa (29.92 inHg) and it reads pressure altitude, which is what flight levels use.

The formula

pressure altitude of a pressure P = 145,442 × (1 − (P ÷ 1013.25)^0.190263) ft
pressure altitude of the field = field elevation + pressure altitude of the QNH
QFE = the pressure whose pressure altitude is (field elevation + pressure altitude of the QNH)
rule of thumb: QFE ≈ QNH − elevation ÷ 27 (or ÷ 30) hPa
rule of thumb: pressure altitude ≈ elevation + (1013.25 − QNH) × 27 ft, or + (29.92 − inHg) × 1,000 ft
true altitude with the wrong setting = shown + pressure altitude of the setting − pressure altitude of the right setting
flight level altitude on QNH = FL × 100 − pressure altitude of the QNH
1 inHg = 33.8639 hPa · 1 mmHg = 1.33322 hPa · 1 kPa = 10 hPa · 1 psi = 68.9476 hPa · 1 mb = 1 hPa

The formulas use the International Standard Atmosphere (15 °C and 1013.25 hPa at sea level, cooling 1.98 °C per 1,000 ft). An altimeter subtracts the pressure altitude of its setting from the pressure altitude of the air around it, and these sums do the same.

Worked example

QNH 1003 hPa at an airfield 1,000 ft above sea level.

• Units: 1003 ÷ 33.8639 = 29.62 inHg. It is also 752.3 mmHg, 100.30 kPa and 14.547 psi.
• QFE: the exact formula gives 967.2 hPa, 35.8 hPa less than the QNH. The 27 ft rule gives 1003 − 1,000 ÷ 27 = 1003 − 37.0 = 966.0 hPa, and the 30 ft rule gives 1003 − 33.3 = 969.7 hPa.
• Pressure altitude: the pressure altitude of 1003 hPa is 281 ft, so the field is at 1,000 + 281 = 1,281 ft. The rule gives 1,000 + 10.25 × 27 = 1,277 ft (1,308 ft with 30 ft per hPa).
• Wrong setting: with 1013.25 left in the window, the altimeter shows 3,000 ft when you are really at 3,000 − 281 = 2,719 ft. The setting is too high, so you are lower than shown.
• Cold temperature at −20 °C, published altitude 3,000 ft: the height above the field is 2,000 ft. t0 = −20 + 0.00198 × 1,000 = −18.02 °C. Correction = 2,000 × (15 − (−18.02)) ÷ (273 + (−18.02) − 0.5 × 0.00198 × (2,000 + 1,000)) = 66,040 ÷ 252.01 = 262 ft, so fly about 3,262 ft on the altimeter (round up as your operator says). The 4% rule gives 2,000 × 4% × 3.302 = 264 ft.
• Flight levels: FL50 on QNH 1003 is 5,000 − 281 = 4,719 ft. A 3,000 ft transition altitude is FL32.8 (3,000 + 281 = 3,281 ft pressure altitude), so the lowest flight level above it is FL35, which is 3,219 ft.

hPa and inHg side by side

Cold temperature correction

Altimeters assume standard temperatures. In colder air, the air is denser, the pressure levels sit closer together and you are lower than the altimeter shows. This matters most for minimum altitudes near terrain. The calculator uses the ICAO Doc 8168 approximate formula:

t0 = airfield temperature + 0.00198 × airfield elevation
correction = H × (15 − t0) ÷ (273 + t0 − 0.5 × 0.00198 × (H + airfield elevation))
H = height above the airfield (published altitude − elevation)

It is an approximation, assumes the altimeter setting comes from the airfield, and gives no correction when the air is warmer than ISA. Many states publish their own tables, cold-temperature airports and rules for which altitudes to correct. Check your AIP and operator procedures, and tell ATC when you fly a corrected altitude.

Notes

Transition altitudes and levels vary by country and airport, so use the published values. QFE is used for some military and gliding flying and in a few countries. For take-off performance in hot or high conditions use the density altitude calculator, and decode the full report with the METAR & TAF decoder.

For planning and education. Always set the altimeter from official reports and ATC, and follow your AIP and operator procedures.