isa temperature calculator

Calculate standard International Standard Atmosphere (ISA) temperature at a given altitude. Optionally enter outside air temperature (OAT) to get ISA deviation and a quick density altitude estimate.

What is ISA temperature?

ISA temperature is the reference temperature defined by the International Standard Atmosphere model at a specific altitude. It gives pilots, engineers, and students a consistent baseline for comparing real weather conditions against a “standard day.”

At sea level, ISA temperature is 15°C (59°F). As altitude increases in the troposphere, temperature decreases at a standard lapse rate of 6.5°C per 1,000 meters (about 1.98°C per 1,000 feet) until 11 km.

Why this calculator is useful

Quickly find standard temperature at your altitude.
Compute ISA deviation when you know actual OAT.
Estimate how “hot and high” conditions affect aircraft performance.
Support flight planning, training, and weather interpretation.

ISA temperature layers used in this tool

Primary altitude bands

0 to 11 km: temperature decreases linearly.
11 to 20 km: temperature is constant at -56.5°C.
20 to 32 km: temperature increases gradually.
32 to 47 km: temperature increases faster.
47 to 51 km: nearly isothermal.
51 to 71 km: temperature decreases.
71 to 84.852 km: temperature decreases more slowly.

This follows standard ISA layer definitions for accurate temperature lookup across a wide altitude range.

How to use the ISA temperature calculator

Enter altitude.
Select altitude unit (ft or m).
(Optional) Enter outside air temperature in °C.
Click Calculate to view results.

You’ll receive ISA temperature in °C, °F, and K, plus ISA deviation if OAT is provided.

Formula overview

Troposphere shortcut (common pilot rule)

For many day-to-day aviation calculations below 11 km: ISA Temp (°C) ≈ 15 − 1.98 × (altitude in thousands of feet).

Layer model (used by calculator)

The calculator uses piecewise ISA equations by altitude layer: T = T_base + L × (h − h_base), where L is lapse rate for that layer.

Worked examples

Example 1: 5,000 ft

ISA temperature is approximately 5°C. If actual OAT is 20°C, ISA deviation is +15°C, indicating significantly warmer-than-standard conditions.

Example 2: 10,000 ft

ISA temperature is about -5°C. If actual OAT is -2°C, deviation is +3°C. This is slightly warmer than standard and may modestly increase density altitude.

ISA deviation and performance impact

ISA deviation is simply: Actual OAT − ISA temperature. Positive values mean warmer air than standard; negative values mean colder than standard.

Positive deviation: reduced air density, longer takeoff rolls, lower climb performance.
Negative deviation: denser air, generally improved aerodynamic and engine performance.
High elevation + high temperature: strongest density altitude penalties.

Important notes

This is an ISA reference tool, not a replacement for certified performance charts.
The quick density altitude estimate shown is approximate and based on ISA deviation.
Always use your aircraft POH/AFM and official weather data for operational decisions.

FAQ

Is ISA temperature the same as actual weather temperature?

No. ISA is a standardized model. Real temperatures vary by location, season, and weather systems.

What altitude should I enter?

For aviation calculations, pressure altitude is often preferred. For general reference, field elevation or geometric altitude can still be useful.

Can I use feet and meters?

Yes. This calculator accepts both units and handles conversion automatically.