Climb through the standard atmosphere and watch the air pressure fall. The gold curve is the ISA pressure profile, the cream line is your altitude, and the mercury barometer on the right reads the pressure there. Up to 11 km the model is P = P0 (1 - L h / T0)5.256; above the tropopause the air is isothermal and the fall becomes exponential.
Pressure at altitude P = P0 (T / T0)5.256
101.3 kPa
1013.3 hPa · 1.000 atm · 14.70 psi
Fraction of sea-level pressure P / P0
100.0 %
depends on the altitude alone, not on P0
Mercury barometer column h = P / (ρ g)
760.0 mm Hg
29.92 inHg
Water column equivalent
10.33 m
the same pressure holds up this much water
ISA air temperature T = T0 - L h
15.0 °C
288.15 K
Air density ρ = P M / (R T)
1.225 kg/m³
ideal gas at the ISA temperature
Altitude h0 m
Sea-level pressure P01013.25 hPa
Model: International Standard Atmosphere (1976) · g = 9.80665 m/s² · lapse 6.5 °C per km to 11 km, then -56.5 °C · mercury 13,595 kg/m³
Tip: slide P0 and the percentage does not move — P / P0 depends on altitude alone, which is why an altimeter has to be told the day's sea-level pressure.