ISA Standard Atmosphere Calculator
Enter an altitude and instantly compute standard atmosphere properties using the ICAO/US Standard Atmosphere model (valid up to 84,852 m geopotential altitude).
What this ISA standard atmosphere calculator does
This calculator estimates key atmospheric properties at a selected altitude according to the International Standard Atmosphere (ISA). It is useful for pilots, aerospace students, engineers, drone operators, and anyone who needs a reliable baseline model of the air.
The calculator returns:
- Temperature (K and °C)
- Pressure (Pa, hPa, kPa, inHg, and psi)
- Air density (kg/m³)
- Speed of sound (m/s and knots)
- Non-dimensional ratios: δ (pressure), θ (temperature), and σ (density)
What is the ISA model?
The ISA model defines a reference atmosphere so performance numbers can be compared consistently. Real weather changes hour by hour, but ISA gives a fixed baseline. Aircraft performance charts, engine maps, and aerodynamic calculations often assume ISA conditions unless otherwise noted.
At sea level, ISA reference values are:
- Temperature: 288.15 K (15 °C)
- Pressure: 101,325 Pa (1013.25 hPa)
- Density: 1.225 kg/m³
Atmospheric layers used in this calculator
The atmosphere is modeled with piecewise linear temperature gradients (lapse rates). Each layer uses either a constant lapse rate or an isothermal assumption:
- 0 to 11 km: -6.5 K/km
- 11 to 20 km: 0 K/km (isothermal)
- 20 to 32 km: +1.0 K/km
- 32 to 47 km: +2.8 K/km
- 47 to 51 km: 0 K/km (isothermal)
- 51 to 71 km: -2.8 K/km
- 71 to 84.852 km: -2.0 K/km
Equations behind the results
1) Geometric to geopotential altitude
ISA equations are applied in geopotential altitude:
h = (Re · z) / (Re + z)
where z is geometric altitude and Re is Earth's effective radius.
2) Pressure in a layer with lapse rate
For non-zero lapse rate L:
P = Pb · (T / Tb)-g₀/(L·R)
3) Pressure in an isothermal layer
If L = 0:
P = Pb · exp[-g₀(h - hb)/(R·Tb)]
4) Density and speed of sound
ρ = P/(R·T) and a = √(γ·R·T)
How to use the calculator
- Enter altitude (for example, 3000 or 10000).
- Select the unit (meters or feet).
- Click Calculate Atmosphere.
- Read the full output table for pressure, temperature, density, and speed of sound.
Example use case
Suppose you want atmosphere properties at 10,000 ft. Click the example button to load that altitude, then calculate. You will see lower pressure and density than sea level, plus a reduced speed of sound. These differences directly affect lift, drag, propeller performance, engine behavior, and true airspeed relationships.
Why ISA matters in flight and engineering
- Flight planning: Better estimates for climb, cruise, and fuel behavior.
- Performance comparisons: Standardized baseline for aircraft and engine specs.
- Aerodynamics: Density is needed for dynamic pressure and Reynolds number estimates.
- Education: ISA is foundational in meteorology and aerospace coursework.
Limitations to keep in mind
ISA is a reference model, not a weather forecast. Real atmosphere values can deviate significantly due to humidity, fronts, temperature inversions, pressure systems, and local microclimates. For operational decisions, always use current METAR/TAF data and official flight planning tools.
Frequently asked questions
Is this the same as pressure altitude?
Not exactly. Pressure altitude is an altitude indication based on standard pressure settings. This tool computes atmosphere properties from geometric altitude under ISA assumptions.
Can I use this for drones and UAVs?
Yes. It is especially useful when estimating density effects on thrust, climb, and power requirements.
Does this include humidity?
No. ISA assumes dry air for standard reference calculations.