About the Altitude & Air Pressure Calculator
This altitude and air pressure calculator converts in both directions using the barometric formula of the International Standard Atmosphere (ISA). Enter an altitude to get the expected air pressure, or enter a barometer reading to get the altitude it corresponds to. Results come in metres and feet, and in hPa (millibars), inHg, kPa and more, along with the standard temperature, air density and the percentage of sea-level pressure (which is also the share of sea-level oxygen available).
Pilots use it for pressure altitude, hikers and climbers to check barometric altimeters, drone and weather-balloon hobbyists to turn sensor readings into height, and students to see how quickly the atmosphere thins. Leave the reference at 1013.25 hPa for pressure altitude, or enter today's local sea-level pressure (QNH) to get a true-altitude estimate the way an altimeter does.
The model follows the 1976 US Standard Atmosphere layers up to 32 km: a 6.5 °C/km temperature lapse to 11 km, an isothermal layer to 20 km and a gentle warming above. Real weather departs from the standard, so altitude from pressure is typically good to within a few per cent; temperature corrections and geometric-vs-geopotential height differences (about 0.3% at 20 km) are ignored.
How to use the altitude & air pressure calculator
- 1Choose altitude → pressure or pressure → altitude.
- 2Enter the altitude (m or ft) or the pressure reading and its unit.
- 3Keep 1013.25 hPa for pressure altitude, or enter local sea-level pressure (QNH).
- 4Read the result plus the standard temperature, density and % of sea-level pressure.
Formula and method
The barometric formula comes from balancing the weight of each thin layer of air against the pressure difference across it (hydrostatic equilibrium) and treating air as an ideal gas. In the lowest layer the standard temperature falls linearly from T₀ = 288.15 K at L = 6.5 K per km, which gives the power law above with exponent g·M/(R·L) ≈ 5.2559. Between 11 and 20 km temperature is constant at 216.65 K and pressure decays exponentially; from 20 to 32 km temperature rises by 1 K per km.
To convert pressure to altitude the calculator inverts these equations numerically. A sea-level reference other than 1013.25 hPa scales the whole profile, which mirrors how an altimeter set to local QNH reads true altitude. Standard air density is P/(R_air·T) with R_air = 287.05 J/(kg·K).
- P
- Pressure at altitude h
- P₀
- Sea-level reference pressure (101,325 Pa standard)
- T₀
- Sea-level standard temperature, 288.15 K
- L
- Temperature lapse rate, 0.0065 K/m
- g
- Standard gravity, 9.80665 m/s²
- M
- Molar mass of air, 0.0289644 kg/mol
- R
- Universal gas constant, 8.31432 J/(mol·K)
Worked examples
Pressure at 1,500 m
P = 1013.25 × (1 − 0.0065 × 1500 ÷ 288.15)^5.2559 = 845.6 hPa, 83% of sea level. The standard temperature there is 15 − 9.75 = 5.25 °C.
Summit of Mount Everest, 8,849 m
The standard atmosphere gives 314 hPa – about 31% of sea-level pressure. (Measured summit pressure is a little higher, around 330–340 hPa, because the tropical atmosphere is warmer than standard.)
Airliner cruising at 35,000 ft
35,000 ft = 10,668 m. Pressure is 238.4 hPa (7.04 inHg) and the standard temperature −54.3 °C, which is why cabins are pressurised.
Altitude from an 850 hPa reading
Inverting the formula: h = (288.15 ÷ 0.0065) × (1 − (850 ÷ 1013.25)^(1/5.2559)) = 1,457 m, the classic 850 hPa level forecasters use.
Hiker's barometer: 900 hPa with QNH 1000 hPa
With a low-pressure day (QNH 1000 hPa) the reading is compared with 1000 instead of 1013.25 hPa, giving about 880 m. Using the standard reference would overstate the altitude at 988 m.
Frequently asked questions
What is the barometric formula?+
In the lower atmosphere P = P₀ (1 − Lh/T₀)^5.2559, with P₀ = 1013.25 hPa, L = 0.0065 K/m and T₀ = 288.15 K. Above 11 km, where temperature is constant, pressure falls exponentially with height.
How much does air pressure drop with altitude?+
Near sea level pressure falls about 12 hPa per 100 m (roughly 1 inHg per 1,000 ft). It halves at about 5,500 m and is only about a quarter of sea-level pressure at airliner cruise altitudes.
What is pressure altitude?+
Pressure altitude is the altitude in the standard atmosphere that matches the current pressure, i.e. what an altimeter reads when set to 29.92 inHg (1013.25 hPa). Aircraft performance and flight levels above the transition altitude use it.
Why does my barometric altimeter drift?+
Weather systems change sea-level pressure by tens of hPa. A 1 hPa change shifts the indicated altitude by about 8 m (27 ft), so recalibrate the altimeter at a known elevation or with the local QNH regularly.
How much oxygen is available at altitude?+
Air is about 21% oxygen at every altitude, but there is less air. The oxygen partial pressure scales with total pressure: about 83% of sea level at 1,500 m, 64% at 3,600 m and 31% on Everest.