Barometric Pressure–Altitude Converter Calculator

Calculator

ISA 1976 • 0–86 km

Mode

Pressure → Altitude

Altitude → Pressure

ISA Density Altitude

Reference sea-level pressure (P₀)

Default is 1013.25 hPa. Use quick presets or set via METAR QNH.

Input pressure

Height model

Conversions between geometric and geopotential handled using Earth’s radius 6 356.766 km.

True Airspeed (optional)

If provided, Mach number uses the local speed of sound.

NOAA Station Preset

Pick a station, open METAR, copy the altimeter, and paste below.

Altimeter (QNH) from METAR

Applying sets P₀ equal to the altimeter setting.

Pressure

Output

Pa: —

hPa: —

kPa: —

atm: —

mmHg: —

inHg: —

psi: —

Altitude, Temperature, Density & More

Output

Altitude (m): —

Altitude (ft): —

Geopotential height (m): —

Standard temperature (°C): —

Standard temperature (K): —

Density (kg/m³): —

Density (slug/ft³): —

Speed of sound (m/s): —

Speed of sound (ft/s): —

Dynamic viscosity μ (Pa·s): —

Dynamic viscosity μ (μPa·s): —

Kinematic viscosity ν (m²/s): —

Kinematic viscosity ν (ft²/s): —

Mach number (if TAS given): —

Density altitude (ISA exact) (m): —

Density altitude (ISA exact) (ft): —

Pressure altitude (ft): —

Pressure altitude (m): —

Density altitude (rule-of-thumb) ft: —

Density altitude (rule-of-thumb) m: —

Altitude Range Charts

Charts update with current P₀ and show ISA profiles up to 20 km.

Performance Estimate (Rule-of-Thumb)

Scales using density ratio σ, weight-squared, surface, headwind/tailwind, slope, and optional balanced-field approximation. Always use your POH.

Baseline takeoff ground roll

Sea level, ISA, reference weight, dry paved runway.

Baseline climb rate (optional)

Reference weight (baseline)

As used for the baseline takeoff distance.

Actual takeoff weight

Used for W-squared scaling of ground roll.

Runway surface/condition

Runway slope (%)

Uphill positive. Rule-of-thumb: ≈10% distance change per 1% slope.

Runway slope sign helper Positive = uphill in takeoff direction

Departure threshold elev

Opposite threshold elev

Runway length

No slope computed yet

Enter the takeoff-direction threshold elevation in the first box. The computed sign follows that direction.

Runway heading (°)

Used to compute headwind/tailwind component.

Wind from (°)

Wind speed

Headwind reduces distance, tailwind increases. Components auto-computed.

Estimated Ground Roll

At current conditions: —

Applied factors: σ, weight², surface, wind, slope.

Estimated Climb Rate

At current conditions: —

Show balanced-field distance quick estimate

Ground roll ≈ S₀·(1/σ)·(W/W_ref)²·F_surface·F_wind·F_slope. Climb rate shown scales only with σ.

Example Data (Standard Atmosphere)

Precomputed values using ISA 1976 with P₀ = 1013.25 hPa.

Altitude (m)	Altitude (ft)	Pressure (hPa)	Pressure (inHg)	Temp (°C)	Density (kg/m³)

Formulas Used

US Standard Atmosphere 1976, piecewise layers up to 86 km (geopotential height h):

g₀ = 9.80665 m/s², R = 8.314462618 J/(mol·K), M = 0.0289644 kg/mol, R_air = 287.052 J/(kg·K)
Gradient (L ≠ 0): T = T_b + L(h - h_b), P = P_b · (T/T_b)^{-g₀·M/(R·L)}
Isothermal (L = 0): T = T_b, P = P_b · exp(-g₀·M·(h - h_b)/(R·T_b))
Geometric ↔ Geopotential: h = (R_e · H)/(R_e + H), with R_e = 6,356,766 m
Density: ρ = P / (R_air · T); Speed of sound: a = √(γ·R_air·T) (γ≈1.4)
Sutherland viscosity: μ = μ₀ · (T₀ + C)/(T + C) · (T/T₀)^{3/2} with μ₀ = 1.716×10⁻⁵ Pa·s, T₀ = 273.15 K, C = 110.4 K.
Density altitude (exact): solve ISA height where ρ_ISA(H)=ρ_actual.
Pressure altitude (rule): PA(ft)=Elev(ft)+(29.92−Alt[inHg])×1000 ≈ Elev + (1013.25−QNH[hPa])×29.53
DA rule-of-thumb: DA_rot(ft)=PA(ft)+120×(OAT−ISA_T), with ISA_T(°C)=15−1.98×Elev(kft).
Ground roll scaling: S≈S₀·(1/σ)·(W/W_ref)²·F_surface·F_wind·F_slope.
Wind factor (rule): headwind reduces ≈1% per kt (cap −40%); tailwind increases ≈5% per kt (cap +50%).
Slope factor (rule): F_slope≈1+0.10×slope(%) (downhill negative reduces distance).
Balanced-field quick estimate: BFD≈max(ASD, AGD) with ASD₀≈1.8×S₀, AGD₀≈1.4×S₀ if unspecified; both legs scaled by σ, weight², surface, wind, slope; extra tailwind penalty may apply.

How to Use

Select Mode: convert pressure to altitude, altitude to pressure, or density altitude.
Set P₀ to local sea-level pressure or apply METAR QNH.
Enter your value and choose its unit.
For density altitude, enter altitude and OAT at that altitude.
Optional: enter TAS and get the corresponding Mach number.
Enter baseline performance, weights, runway condition, wind, and slope.
Toggle balanced-field to see ASD/AGD, limiting leg, and add V-speeds.

FAQs

1) How do wind and slope affect ground roll?

Headwind shortens distance; tailwind and uphill slope increase it. We use practical rules: about 1% per kt headwind (capped), 5% per kt tailwind (capped), and 10% per 1% slope.

2) What is balanced-field distance here?

A quick approximation computing accelerate-stop and accelerate-go legs separately, then taking the maximum as the limiting distance, scaled by σ, weights, surface, wind, and slope.

3) How do I sign the slope correctly?

Use the helper: enter both threshold elevations and runway length. It fills the slope with the correct sign for your takeoff direction automatically.

4) Can I use °F or knots?

Yes. Temperature accepts °C °F or K. TAS supports m/s kt km/h ft/s and mph.

5) Are results valid above the troposphere?

The model extends to 86 km. Density altitude is most meaningful within the troposphere for flight operations and performance estimates.

6) Can I export charts?

Yes. Use the Save PNG buttons for the pressure, density, and temperature plots.

7) How accurate are these estimates?

They are coarse rules. Use your aircraft POH and performance charts. Consider runway slope, wind, contamination, engine type, and weight limits.