What is an atmosfera ISA calculator?
An atmosfera ISA calculator computes standard atmospheric properties at a chosen altitude using the International Standard Atmosphere (ISA) model. Engineers, pilots, students, and simulation developers use ISA as a common reference for pressure, temperature, density, and speed of sound.
In simple terms, ISA answers this question: “If the atmosphere were perfectly standard, what would the air be like at this altitude?”
What this calculator gives you
- Standard temperature at altitude (K, °C, °F)
- Standard pressure (Pa, kPa, hPa, psi)
- Air density (kg/m³ and slug/ft³)
- Speed of sound (m/s, ft/s, knots)
- ISA ratios: θ (temperature), δ (pressure), σ (density)
- Optional ISA deviation and estimated density altitude when OAT is entered
ISA model assumptions used here
This page uses the classic ISA/US Standard Atmosphere constants for dry air and static, average sea-level conditions:
- Sea-level temperature: T₀ = 288.15 K (15°C)
- Sea-level pressure: P₀ = 101325 Pa
- Sea-level density: ρ₀ = 1.225 kg/m³
- Gravitational acceleration: g₀ = 9.80665 m/s²
- Gas constant for air: R = 287.05287 J/(kg·K)
- Heat capacity ratio: γ = 1.4
Formulas behind the calculator
Troposphere (up to 11 km)
T = T₀ + L·h, where L = -0.0065 K/m
P = P₀ · (T/T₀)^(-g₀/(L·R))
ρ = P/(R·T)
Lower stratosphere (11 km to 20 km, isothermal)
T = 216.65 K
P = P₁₁ · exp[-g₀(h-11000)/(R·T)]
ρ = P/(R·T)
20 km to 32 km layer
T = T₂₀ + L·(h-20000), where L = +0.001 K/m
P = P₂₀ · (T/T₂₀)^(-g₀/(L·R))
ρ = P/(R·T)
How to use this atmosfera ISA calculator
- Enter altitude.
- Choose feet or meters.
- Optionally enter outside air temperature (OAT) in °C.
- Click Calculate ISA.
For aviation planning, the optional OAT can help estimate density altitude quickly. This estimate is useful, but it is still an approximation.
Why ISA matters in real work
Aviation
- Aircraft performance charts are usually ISA-based.
- Takeoff and climb performance depend strongly on density altitude.
- Engine and propeller efficiency vary with pressure and temperature.
Aerospace and simulation
- Drag, lift, and Mach number calculations depend on air density and speed of sound.
- Flight simulators and trajectory models use ISA as a baseline atmosphere.
Mechanical and civil engineering
- Ventilation and combustion models often require air properties at altitude.
- Sensor calibration and environmental testing often reference ISA values.
Common mistakes to avoid
- Mixing up geometric altitude and pressure altitude.
- Using ISA values as actual weather values (real atmosphere can differ significantly).
- Forgetting unit conversion when switching between feet and meters.
- Applying simple density altitude approximations outside normal operating ranges.
Quick FAQ
Is ISA the same as live weather?
No. ISA is a standard reference atmosphere, not today’s measured weather.
Can I use this for high-altitude balloons or spaceflight?
This calculator is configured up to 32 km. For higher altitudes, use an extended atmospheric model with additional layers and composition effects.
Does humidity matter?
Yes in real life, but this ISA calculator assumes dry air. Humidity lowers air density compared with dry-air assumptions.