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Engine Tuner's Calculator

- using absolute pressure and relative humidity -

To use the calculator, just enter the air intake temperature, absolute pressure and relative humidity, click the type of units that you are using ... then click the calculate button.

Engine Tuner's Calculator

Air Temperature deg F deg C
Absolute Pressure in Hg hPa
Relative Humidity percent

Relative Horsepower %  
Dyno Correction Factor  
Air Density lb/ft3 kg/m3
Density Altitude feet m
Relative Density %  
Virtual Temperature deg F deg C
Vapor Pressure in Hg hPa

Copyright © 1998-2023  Richard Shelquist  All Rights Reserved

The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. For tweaking the fuel/air mixture, or predicting engine power, the air density is the most important consideration.


Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.

Absolute pressure is the actual ambient air pressure (also called station pressure).

Note:  For more information about ambient air pressure measurements see the pressure measurement page.

Relative Humidity is the ratio of the partial pressure of water vapor to the saturated vapor pressure at a given temperature.

Note:  The calculator which uses Dew Point is often more accurate because the dew point is fairly constant for a given air mass and changes very little until another air mass arrives, while the relative humidity varies greatly as the ambient temperature changes.

Note: The absolute pressure and relative humidity can often be gathered from a local airport, local weather service or the national weather service.  

Calculated Values:

Relative Horsepower shows how air density alters the power output of a properly tuned engine. For example, at 85 deg F, 25.09 in-hg absolute pressure, 40% relative humidity, the engine only produces about 81% of the rated horsepower. 

Note: The relative horsepower calculations are made in accordance with  SAE J1349. The standard reference conditions for SAE J1349 are:  Air temp 77 deg F (25 deg C),   29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.

Note:  Section 5.1 of SAE J1349 AUG2004 makes it clear that the equations are not intended to provide accurate corrections over an extremely wide range, but rather that the intended range of air temperatures is 15 to 35 deg C (59 to 95 deg F), and the intended range of dry air pressures is 900 to 1050 mb (26.58 to 31.01 inches-Hg). Values outside of this range may produce inaccurate results for SAE Relative Horsepower and Dyno Correction Factor, but all other calculator results (such as Density Altitude, Air Density, etc) will still be correct.

Dyno Correction Factor, calculated according to SAE J1349 AUG2004, is simply the reciprocal of the relative horsepower value.

Air Density is the actual mass of a given volume of air. This is a key parameter for engine tuning.

Density Altitude is the altitude in dry air that would have the same density as the input conditions.

Note:  The ICAO standard conditions for zero density altitude are zero meters altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in-Hg) pressure and zero % relative humidity.

Relative Air Density is the ratio of the calculated air density to the air density at sea level using the International Civil Aviation Organization (ICAO) standard reference conditions.

Note: The ICAO standard sea level air density is 1.225 kg/m3.

Virtual Temperature is the temperature of dry air which would have the same density as the input conditions.

Vapor Pressure is the contribution of water vapor pressure to the total absolute air pressure.


For in-depth technical details, see my web page on Dyno Correction Factor and Relative Horsepower and also see the page describing Air Density and Density Altitude.

Also, there are several conversion calculators available from El Paso NWS.


Last Updated: 22-Jan-2023

https://wahiduddin.net/calc  ----   Copyright © 1998-2023 Richard Shelquist  All Rights Reserved ----- Shelquist Engineering, Colorado, USA -----  Copyright © 1998-2023 Richard Shelquist  All Rights Reserved