Free Fan Engineering Tool

Air Density Calculator — Altitude & Temperature Correction for Fans

Fan curves are published for standard air (1.2 kg/m³, sea level, 20 °C). Thinner air at altitude or high temperature means the same fan makes less pressure, draws less power and removes less heat. Enter your site conditions to get the density, the correction factors, and the airflow you must ask for so that the job still gets done.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Altitude & Temperature Derating

ISA barometric model + ideal gas law. Optional: enter a catalog duty point to see the corrected values.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How altitude and temperature change fan performance

A fan is a constant-volume machine: at a given speed it moves the same m³/h whatever the air density. What changes is everything proportional to mass:

Airflow (volume) Q_site = Q_catalog // unchanged
Static pressure P_site = P_catalog × (ρ_site/1.2)
Shaft power W_site = W_catalog × (ρ_site/1.2)
Mass flow / cooling ṁ_site = ṁ_catalog × (ρ_site/1.2)
// ρ = p / (R_air × T), R_air = 287.05 J/kg·K, p from the ISA barometric formula

Two practical consequences. First, a system designed for 300 Pa at sea level needs a fan that makes 300 ÷ 0.82 = 366 Pa on the catalog curve if installed at 1,600 m. Second, if the purpose is cooling (removing kW), the volumetric airflow must rise by 1/ρ-ratio to carry the same mass of air — that is why Denver data centers and high-altitude inverter stations need bigger fans than their sea-level twins.

Density and derating table (20 °C, dry air)

Altitude	Barometric pressure	Density	Ratio	Fan pressure / power	Airflow needed for same cooling
0 m (sea level)	1013 hPa	1.204 kg/m³	1.00	100 %	100 %
500 m	955 hPa	1.134	0.94	94 %	106 %
1,000 m	899 hPa	1.068	0.89	89 %	113 %
1,500 m	845 hPa	1.004	0.83	83 %	120 %
2,000 m	795 hPa	0.944	0.78	78 %	128 %
2,500 m	747 hPa	0.887	0.74	74 %	136 %
3,000 m	701 hPa	0.833	0.69	69 %	145 %
4,000 m	616 hPa	0.732	0.61	61 %	164 %
5,000 m	540 hPa	0.642	0.53	53 %	188 %

Air temperature (sea level)	Density	Ratio
−25 °C	1.422 kg/m³	1.18 — motor overload risk
0 °C	1.292	1.07
20 °C	1.204	1.00
40 °C	1.127	0.94
50 °C	1.092	0.91
80 °C	1.000	0.83
150 °C	0.834	0.69
250 °C	0.675	0.56

Worked example: inverter station at 2,000 m

Given: a cabinet needs 5,000 m³/h at 300 Pa at sea level to hold its temperature. Site: 2,000 m, 35 °C ambient.

Density = 0.90 kg/m³ → ratio 0.75
Cooling airflow to order: 5,000 ÷ 0.75 = 6,670 m³/h
The fan will only make 0.75× its catalog pressure, so pick one rated for 300 ÷ 0.75 = 400 Pa at 6,670 m³/h on the standard-air curve.
Motor: derate ~10 % for altitude plus the temperature class — or choose an EC fan with electronics rated to 60 °C and a thermal model that self-protects.

Frequently Asked Questions

Does altitude reduce fan airflow?

Not the volumetric airflow (m³/h or CFM) — a fan moves the same volume at any density. It reduces the static pressure the fan can develop, the power it draws, and the mass of air (and therefore cooling) each cubic metre carries, all in proportion to the density ratio.

How do I correct a fan curve for altitude?

Multiply every pressure and power value on the curve by the density ratio (site density ÷ 1.2 kg/m³); leave the airflow axis unchanged. At 1,500 m and 20 °C the ratio is 0.83, so a 300 Pa point becomes 250 Pa.

At what altitude do I need to derate a fan motor?

Most motor standards (IEC 60034-1, NEMA MG-1) rate motors to 1,000 m. Above that, derate roughly 1 % of output per 100 m, or reduce the permissible ambient by 1 °C per 100 m, because thinner air removes less heat from the windings.

What is standard air for fans?

Standard air for fan testing (AMCA 210 / ISO 5801) is dry air at 20 °C, 101.325 kPa, density 1.2 kg/m³ (0.075 lb/ft³). All catalog curves refer to it unless stated otherwise.

Does hot exhaust air change fan selection?

Yes — at 150 °C air is 31 % lighter, so an oven or dryer exhaust fan makes 31 % less pressure. Size on the corrected curve, specify a motor out of the airstream, and check that starting the fan on cold, dense air does not overload the motor.

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