What Is a Boost Pressure Calculator?
A Boost Pressure Calculator calculates gauge boost pressure by comparing absolute intake manifold pressure with atmospheric pressure. It is useful for understanding pressure readings from turbocharged and supercharged engine systems.
If the intake manifold has an absolute pressure of 29.7 PSI and atmospheric pressure is 14.7 PSI, the calculated boost pressure is 15 PSI.
Boost Pressure Formula
Gauge boost is the pressure above the surrounding atmospheric pressure. Therefore, atmospheric pressure must be subtracted from the absolute manifold pressure.
Boost Pressure = Absolute Manifold Pressure − Atmospheric Pressure
If both pressures are entered using the same unit, the result will be expressed in that same pressure unit.
How to Use the Boost Pressure Calculator
Enter MAP Pressure
Enter the absolute intake manifold pressure measured by your data source or MAP sensor.
Enter Atmospheric Pressure
Add the atmospheric pressure applicable to the same pressure unit and conditions.
View Boost
Calculate gauge boost, pressure ratio and equivalent values in PSI, bar and kPa.
Boost Pressure Calculation Example
Assume an engine's intake manifold absolute pressure is 29.7 PSI while atmospheric pressure is 14.7 PSI.
Example
In this example, the intake manifold is approximately 15 PSI above the entered atmospheric pressure.
Absolute Pressure vs Gauge Boost Pressure
Absolute pressure and gauge pressure use different reference points. Absolute pressure is referenced to a theoretical perfect vacuum, while gauge boost is referenced to the surrounding atmospheric pressure.
Absolute Pressure
Absolute pressure includes atmospheric pressure in the reading. MAP sensor values are commonly represented as absolute pressure.
Gauge Pressure
Gauge boost represents pressure above the surrounding atmosphere and is commonly displayed by a boost gauge.
What Is Turbo Pressure Ratio?
Pressure ratio compares the absolute outlet or manifold pressure with the absolute inlet or atmospheric pressure. For the values used by this calculator, it is calculated as:
Pressure Ratio = Absolute Manifold Pressure ÷ Atmospheric Pressure
For example, 29.7 PSI absolute divided by 14.7 PSI atmospheric gives a pressure ratio of approximately 2.02.
Boost Pressure Units
Turbo boost can be expressed using several pressure units. This calculator supports PSI, bar and kilopascals and automatically converts the calculated result between them.
PSI
Pounds per square inch is a commonly used boost pressure unit in automotive applications.
Bar
One bar equals 100 kPa and is approximately 14.504 PSI.
kPa
Kilopascals are commonly used for absolute and gauge pressure readings in metric systems.
Unit Conversion
The calculator converts the result so the same boost can be compared across all three supported units.
Why Atmospheric Pressure Matters
Atmospheric pressure is not exactly the same everywhere. Elevation and weather conditions can affect ambient pressure, which means the relationship between an absolute pressure reading and gauge boost can change.
The commonly referenced value of about 14.7 PSI or 101.325 kPa represents standard atmospheric pressure near sea level, but it should not be treated as the exact local pressure in every situation.
This calculator performs pressure calculations and unit conversions only. It does not determine a safe boost level for an engine. Appropriate boost depends on the engine, turbocharger or supercharger, fuel, calibration, temperature, mechanical condition and manufacturer specifications.
Positive Boost and Vacuum
When absolute manifold pressure is greater than atmospheric pressure, the calculated gauge pressure is positive and represents boost. If manifold absolute pressure is below atmospheric pressure, the result is negative gauge pressure, commonly associated with intake manifold vacuum.
This calculator can display a negative result when the entered manifold absolute pressure is lower than the entered atmospheric pressure.
Common Boost Pressure Calculation Mistakes
- Confusing absolute manifold pressure with gauge boost pressure.
- Subtracting pressures that are expressed in different units.
- Assuming 14.7 PSI is the exact atmospheric pressure at every location and under every condition.
- Comparing a gauge-pressure reading directly with an absolute pressure reading.
- Treating pressure ratio as the same thing as boost pressure.
Boost Pressure Calculator FAQs
Subtract atmospheric pressure from absolute manifold pressure. For example, 29.7 PSI absolute minus 14.7 PSI atmospheric equals 15 PSI gauge boost.
Absolute pressure is referenced to a vacuum, while gauge boost is referenced to surrounding atmospheric pressure. Gauge boost therefore excludes the ambient atmospheric portion of the absolute reading.
If atmospheric pressure is 101.325 kPa, then 200 kPa absolute corresponds to approximately 98.675 kPa of gauge boost.
Divide the absolute manifold or compressor outlet pressure by the absolute inlet or atmospheric pressure, provided the pressures are expressed in the same unit.
Yes. The calculated boost is automatically displayed in PSI, bar and kilopascals.
Yes. If manifold absolute pressure is below atmospheric pressure, the calculated gauge pressure is negative, indicating pressure below ambient rather than positive boost.