ISA Calculator (Metric — metres & hPa)
The standard atmosphere in SI: temperature, pressure and density at any altitude in metres — for meteorologists, engineers and the metric flying world.
Metric anchors worth keeping: 5,500 m is the half-pressure level; the 0.0065 K/m lapse; ρ₀ = 1.225 kg/m³. Mountaineers' 'death zone' (8,000 m) sits at 36% of sea-level pressure — ISA arithmetic explains alpinism too.
Formula
⚠️ For planning and education only. Weight & balance must be computed from YOUR aircraft's actual empty weight, arm and current equipment list, and verified against the POH/AFM envelope before flight.
The standard atmosphere in SI: temperature, pressure and density at any altitude in metres — for meteorologists, engineers and the metric flying world.
About ISA Calculator (Metric — metres & hPa)
The standard atmosphere was defined in SI — 288.15 K, 1013.25 hPa, 0.0065 K/m — and only later dressed in feet for cockpits. This calculator keeps it native: metres in; Celsius, hectopascals and kg/m³ out, with the troposphere/stratosphere regimes both modeled. Built for meteorology students, engineers, balloonists and mountaineers, for whom 'half the atmosphere below 5,500 m' is the natural phrasing of the same physics pilots meet in feet.
How to use ISA Calculator (Metric — metres & hPa)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula T = 288.15 − 0.0065·h (K); p = 1013.25(T/T₀)^5.2559 hPa; ρ = p/RT — pure SI throughout substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use ISA Calculator (Metric — metres & hPa)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula T = 288.15 − 0.0065·h (K); p = 1013.25(T/T₀)^5.2559 hPa; ρ = p/RT — pure SI throughout with sources cited on the page
- ✓Metric anchors worth keeping: 5,500 m is the half-pressure level; the 0.0065 K/m lapse; ρ₀ = 1.225 kg/m³. Mountaineers' 'death zone' (8,000 m) sits at 36% of sea-level pressure — ISA arithmetic explains alpinism too.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
At what altitude is air pressure half of sea level?+
About 5,500 m (18,000 ft) — handily, also roughly where 50% of the atmosphere's mass lies below. The next halving takes a similar distance (to ~11 km), the exponential character of hydrostatic decay. Mountaineers on Denali's summit (6,190 m) breathe 47% sea-level pressure; this tool gives the exact curve.
Why 1013.25 hPa exactly for sea level?+
Definition by international agreement — one standard atmosphere, equal to 760 mm of mercury at 0 °C under standard gravity, converted to SI. It is a calibration constant, not a claim about average weather (real sea-level pressures span roughly 870–1085 hPa). Every altimeter's standard setting and every δ ratio is referenced to it.
How does this connect to weather-map pressure levels?+
Synoptic charts live on constant-pressure surfaces whose ISA heights this tool returns: 850 hPa ≈ 1,460 m, 700 hPa ≈ 3,010 m, 500 hPa ≈ 5,570 m, 300 hPa ≈ 9,160 m. When the real 500 hPa surface sits below its ISA height, the column beneath is cold — geopotential anomalies on charts are exactly this comparison, mapped.
What's the density at altitude good for in engineering?+
Anything that flies, falls or burns: drone propulsion sizing (thrust ∝ ρ), wind-turbine output at hub height and site elevation, projectile drag, HVAC at mountain cities, even cooling design for high-altitude electronics. ρ = p/RT from this page's outputs is the standard input; deviations from ISA add the weather on top.
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