Air Pressure at Altitude Calculator

This calculator uses the layered barometric formula of the International Standard Atmosphere (ISA, based on the U.S. Standard Atmosphere 1976). In the troposphere (0–11 km), where temperature falls 6.5 °C per km, pressure is p = p₀ · (T/T₀)^(−g₀·M/(R·L)); in the isothermal layer (11–20 km) it is p = p₀ · e^(−g₀·M·Δh/(R·T)). Above 20 km the stratosphere uses two more gradient layers (+1.0 and +2.8 K/km) and a second isothermal layer up to 51 km. Full equations per layer are shown in the methodology section below.

Near sea level, pressure falls roughly 12 hPa (about 1.2%) for every 100 m you climb — a handy rule of thumb. The drop is exponential, not linear: pressure halves about every 5.5 km of altitude. So at 5,500 m the pressure is roughly 500 hPa (half of sea level), at 11 km about 227 hPa, and at 20 km only about 55 hPa. Enter any altitude in the calculator above for the exact ISA value.

Standard sea-level pressure is 1013.25 hPa (101,325 Pa). In other units that is 14.696 psi, 29.92 inHg, and 760 mmHg. These are the reference values this calculator compares every altitude against in the "× of sea level" readout.

At the Everest summit (8,848 m), the standard-atmosphere pressure is about 315 hPa — roughly 31% of sea-level pressure. That is why the air holds only about a third of the oxygen molecules per breath compared to sea level. Try the "Everest summit" preset above to see all units, the ISA temperature (−58 °C), and the boiling point of water there (about 69.5 °C).

Denver sits at about 1,609 m (5,280 ft), where standard pressure is roughly 834 hPa — about 82% of sea level. In other units that is about 12.1 psi, 24.6 inHg, or 626 mmHg, and water boils at about 94.4 °C. Use the "Denver" preset above for the full breakdown.

Air pressure is simply the weight of the column of air pressing down on you. At higher altitude there is less air above you, so the column weighs less and the pressure is lower. Because air is compressible, the lower layers are squeezed denser by the weight above — that is why pressure falls exponentially with height rather than in a straight line.

Yes. Warm air is less dense, so a warm column of air stretches taller and pressure falls off more slowly with height. Enter an optional sea-level temperature in the calculator to apply an ISA-deviation correction: every atmosphere layer is shifted by the difference from the standard 15 °C, and pressure is recomputed layer by layer. For example, at 5,000 m a sea-level temperature of 25 °C gives noticeably higher pressure than the standard 540 hPa.

Water boils when its vapor pressure equals the surrounding air pressure, so lower pressure means a lower boiling point. At sea level it is 100 °C, in Denver (1,609 m) about 94.4 °C, and on Everest (8,848 m) only about 69.5 °C. The calculator shows the boiling point for any altitude using the Antoine equation (NIST constants) at lower altitudes and the Clausius–Clapeyron relation higher up.

Air pressure drops to half its sea-level value (about 506.6 hPa) at roughly 5,500–5,600 m — the so-called scale height of the atmosphere is about 8.4 km, and pressure halves every ln(2) × scale height ≈ 5.82 km in an isothermal model; the real layered ISA gives about 5.57 km. Above about 6 km, pressure is less than half of sea level, matching the well-known textbook statement.

Switch to the "Pressure → altitude" tab and enter a measured pressure in hPa, psi, inHg, or mmHg. The calculator inverts the layered ISA formula numerically (bisection, accurate to better than half a metre) and returns the standard-atmosphere altitude. Note this gives pressure altitude — the altitude in a standard atmosphere — which equals true altitude only when the real atmosphere matches ISA.

Air pressure is the force the air exerts per unit area (hPa, psi); air density is the mass of air per unit volume (kg/m³). They are related — pressure, temperature, and density are linked by the ideal gas law — but they are not the same thing. This tool computes pressure from the ISA barometric formula. If you need density from temperature, pressure, and humidity, use our companion Air Density Calculator, which implements the international CIPM-2007 moist-air equation.

No. This calculator gives the static pressure of the standard atmosphere at a geometric altitude. Density altitude is an aviation concept: the altitude in the standard atmosphere where the actual air density (affected by non-standard temperature and humidity) would occur, used for aircraft performance. For that, use our Density Altitude Lab, which adds humidity correction, true airspeed, and Koch-chart performance estimates.

Airliners typically cruise near 10,700 m (35,000 ft), where outside standard pressure is only about 238 hPa — less than a quarter of sea level, far too low to breathe. The cabin pressure at cruising altitude is therefore kept at an equivalent "cabin altitude" of about 1,800–2,400 m (roughly 810–750 hPa), which is why your ears pop on descent. Try the "Cruising altitude" preset above for the outside-air numbers.

No — this tool computes pressure for the standard atmosphere, a fixed global model. Real altimeters are set with the local QNH (the current sea-level pressure corrected for today’s weather) from aviation weather reports. Using a standard-atmosphere value instead of the actual QNH would give a wrong altitude indication. Treat every result here as a reference/model value, not a flight instrument setting.

The ISA is a fixed model of how temperature, pressure, and density vary with altitude, defined from the U.S. Standard Atmosphere 1976: sea-level pressure 101,325 Pa, sea-level temperature 15 °C (288.15 K), a tropospheric lapse rate of −6.5 °C/km up to 11 km, an isothermal layer at −56.5 °C from 11–20 km, then stratospheric layers to 51 km. Real weather deviates from it daily — the ISA is a reference yardstick, not a forecast.

Yes, significantly. High- and low-pressure systems routinely shift sea-level pressure by ±30 hPa or more from the 1013.25 hPa standard, and warm or cold air masses change how fast pressure falls with height. This calculator shows the long-term standard model; actual pressure at your location and time comes from a barometer or weather station. The optional temperature input corrects for non-standard warmth or cold, but not for passing weather systems.