Altitude Air Pressure Calculator

Atmospheric pressure decreases with altitude because there is less air above you pressing down. This relationship is described by the barometric formula, one of the most practically useful equations in atmospheric science.

The barometric formula (troposphere, up to ~11km):

P(h) = P₀ × (1 − L×h/T₀)^(g×M / R×L)

Where:

• P(h) = pressure at altitude h
• P₀ = sea level pressure = 101,325 Pa (1013.25 hPa)
• L = temperature lapse rate = 0.0065 K/m
• h = altitude in meters
• T₀ = sea level temperature = 288.15 K (15°C)
• g = gravitational acceleration = 9.80665 m/s²
• M = molar mass of dry air = 0.028964 kg/mol
• R = universal gas constant = 8.314 J/(mol·K)

This simplifies to approximately:

P(h) ≈ P₀ × (1 − 0.0000226 × h)^5.256

Reference pressures at key altitudes. These are what the formula above returns, so every row is reproducible in the calculator:

Where the model and the real atmosphere part company. Climbers measure roughly 337 hPa on the summit of Everest, about 23 hPa above what the standard atmosphere predicts. That is not an error in either number. The standard atmosphere is a mid-latitude average, and the tropopause sits much higher over the subtropics, which piles more air above the mountain than the model assumes. It is also seasonal: summit pressure runs several hPa higher in July than in January, which is one of several reasons nobody climbs it in winter. Twenty-three hPa is the difference between just possible without oxygen and not possible at all, so this is a gap with consequences rather than a rounding quibble.

Where the missing pressure shows up. A pressure canner is the clearest everyday case. Its gauge reads the pressure above the surrounding air, so at 1,800 m it starts 2.9 PSI in the hole, and the USDA ladder of 11, 12, 13, 14, 15 PSI exists to hand that shortfall back. The canning altitude pressure calculator has the tested schedule, which is what to actually cook by. The result above gives the shortfall in both hPa and PSI, which is the physics underneath it.

Why altitude pressure matters:

• Cooking: Water boils at lower temperatures at altitude (1°C lower per ~295m / 960ft). Recipes designed at sea level need adjustment.
• Hiking / mountaineering: Altitude sickness becomes a risk above 2,500m (8,200ft) as oxygen partial pressure decreases.
• Aviation: Aircraft cabins are pressurized to simulate conditions at ~1,800–2,400m (6,000–8,000ft), equivalent to ~80% sea level pressure.
• Medicine: Atmospheric pressure affects dissolved gas levels in blood, which matters for diving decompression and altitude medicine.