Atmospheric Pressure vs Altitude
Standard-atmosphere (ISA) pressure, temperature and air density from sea level to 10,000 m — plus the boiling point of water at each altitude, computed from the IAPWS-IF97 saturation curve. The same barometric formula drives the altitude option in the NPSH calculator.
ISA standard atmosphere table
| Altitude (m) | Altitude (ft) | Pressure (kPa abs) | Pressure (psia) | ISA temp (°C) | Air density (kg/m³) | Water boils at (°C) |
|---|---|---|---|---|---|---|
| 0 | 0 | 101.325 | 14.70 | 15.0 | 1.2250 | 100.0 |
| 250 | 820 | 98.358 | 14.27 | 13.4 | 1.1959 | 99.1 |
| 500 | 1,640 | 95.461 | 13.85 | 11.8 | 1.1673 | 98.3 |
| 750 | 2,461 | 92.634 | 13.44 | 10.1 | 1.1392 | 97.5 |
| 1,000 | 3,281 | 89.875 | 13.04 | 8.5 | 1.1116 | 96.6 |
| 1,250 | 4,101 | 87.182 | 12.64 | 6.9 | 1.0846 | 95.8 |
| 1,500 | 4,921 | 84.556 | 12.26 | 5.3 | 1.0581 | 95.0 |
| 1,750 | 5,741 | 81.994 | 11.89 | 3.6 | 1.0320 | 94.1 |
| 2,000 | 6,562 | 79.495 | 11.53 | 2.0 | 1.0065 | 93.3 |
| 2,500 | 8,202 | 74.683 | 10.83 | -1.3 | 0.9569 | 91.6 |
| 3,000 | 9,843 | 70.109 | 10.17 | -4.5 | 0.9091 | 90.0 |
| 3,500 | 11,483 | 65.764 | 9.54 | -7.8 | 0.8632 | 88.3 |
| 4,000 | 13,123 | 61.640 | 8.94 | -11.0 | 0.8191 | 86.6 |
| 4,500 | 14,764 | 57.728 | 8.37 | -14.3 | 0.7768 | 84.9 |
| 5,000 | 16,404 | 54.020 | 7.83 | -17.5 | 0.7361 | 83.3 |
| 6,000 | 19,685 | 47.181 | 6.84 | -24.0 | 0.6597 | 79.9 |
| 8,000 | 26,247 | 35.600 | 5.16 | -37.0 | 0.5252 | 73.1 |
| 10,000 | 32,808 | 26.436 | 3.83 | -50.0 | 0.4127 | 66.2 |
ISA model: 15 °C at sea level, lapse rate 6.5 K per 1,000 m. Air density is at the ISA pressure AND ISA temperature of each altitude — a site at 25 °C is ~3.4% less dense than the tabulated value (correct with ρ_site = ρ_ISA · T_ISA/T_site in absolute temperatures). Day-to-day weather moves real pressure roughly ±3 kPa around the standard values.
The barometric formula
Through the troposphere (0–11,000 m) the ISA pressure follows:
- — altitude above sea level (m)
- — absolute pressure (kPa); 101.325 kPa = 1 atm = 14.696 psia
Air density then follows the ideal-gas law at the local pressure and temperature, with R = 287.05 J/(kg·K), and the boiling point of water is where the IAPWS-IF97 saturation pressure equals the local P. Two practical rules of thumb near sea level: pressure falls about 1.2 kPa per 100 m, and water's boiling point falls about 1 °C per 300 m.
Data source and basis
- Pressure is the ISA barometric formula (ISO 2533 / ICAO standard atmosphere), evaluated by the same function the NPSH calculator's altitude option uses — anchors: 84.56 kPa at 1,500 m and 70.11 kPa at 3,000 m match published ISA tables to 0.02%.
- Temperature is the ISA troposphere profile (15 °C − 6.5 K/km); density is ideal-gas at the ISA P and T, reproducing the ISA sea-level 1.225 kg/m³ exactly.
- The water boiling point is the IAPWS-IF97 Region 4 saturation temperature at the local pressure — the same steam formulation behind the steam tables calculator and the water properties table. (At 101.325 kPa it gives 99.97 °C — the modern IAPWS value; the historical definition was exactly 100.)
- All columns are computed at build time from the application's own libraries, not transcribed — so the table stays consistent with what SimuPipe's tools evaluate.
Frequently Asked Questions
How much does atmospheric pressure drop with altitude?
Roughly 11–12% per 1,000 m near sea level: from 101.325 kPa at sea level to 89.9 kPa at 1,000 m, 84.6 kPa at 1,500 m, 70.1 kPa at 3,000 m, and 54.0 kPa at 5,000 m. The decline is close to exponential — each additional 1,000 m removes a similar fraction (not a similar amount) of the remaining pressure. The values here are the ISA (International Standard Atmosphere) model; actual weather moves the local pressure by roughly ±3 kPa around them.
Why does water boil below 100 °C at altitude?
Water boils when its vapour pressure equals the surrounding pressure. With only 84.6 kPa available at 1,500 m, that happens at 95.0 °C; at 3,000 m it is 90.0 °C, and at 5,000 m 83.3 °C. The table's boiling-point column is computed from the IAPWS-IF97 saturation curve at the ISA pressure of each altitude. This matters for engineering, not just cooking: any open vessel, flash tank, or deaerator at altitude operates against the lower local pressure.
How does altitude affect pumps (NPSH)?
The atmospheric-pressure head is usually the largest term in NPSH available for open-tank systems, and it shrinks with altitude: the drop from 101.3 to 84.6 kPa at 1,500 m removes about 1.7 m of water head. A suction arrangement with a comfortable margin at sea level can cavitate when the same design is installed at altitude. The NPSH calculator has a built-in altitude option using exactly this table's ISA formula.
How does altitude affect compressed-air systems?
A compressor is a volume machine: it inhales the same volumetric flow, but at 3,000 m each cubic metre of intake air holds about 26% less mass than at sea level, so the delivered mass flow (and the FAD rating referenced to sea level) drops accordingly. Pressure ratios also shift — reaching 7 bar(a) from 70 kPa intake is a ratio of 10 instead of 6.9, increasing discharge temperature and power per delivered kilogram. Free-air ratings should always be corrected to site conditions.
What is the International Standard Atmosphere (ISA)?
A reference model of the atmosphere: 15 °C and 101.325 kPa at sea level, temperature falling 6.5 K per 1,000 m through the troposphere (to 11,000 m), and pressure following the barometric formula P = 101.325·(1 − 2.25577×10⁻⁵·h)^5.25588 kPa. It is a standard reference state — real conditions vary with weather and season — but it is the accepted basis for altitude corrections in engineering, aviation and compressor rating.
Does altitude change gas density at line pressure?
Only through the intake. Once gas is compressed to a controlled absolute pressure, its density at that pressure is the same at any altitude (for the same temperature). What altitude changes is the ambient reference: gauge instruments read relative to the lower local pressure, so 7 bar(g) at 3,000 m is 7.70 bar(a) instead of 8.01 bar(a) — about a 4% difference worth accounting for in accurate compressible-flow calculations.
Design for your site conditions
Check pump suction margins with the altitude-aware NPSH calculator, or model the whole network in SimuPipe with site-correct pressures.