# PSI vs. PSIG: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 11, 2024
PSI (Pounds per Square Inch) is a unit of pressure relative to the surrounding atmosphere, while PSIG (Pounds per Square Inch Gauge) measures pressure relative to atmospheric pressure.

## Key Differences

PSI, or Pounds per Square Inch, is a unit that measures pressure by calculating the force in pounds exerted on one square inch of area. PSIG, or Pounds per Square Inch Gauge, is a similar measurement but it specifically denotes pressure measurements that are above atmospheric pressure, effectively discounting atmospheric pressure from its reading.
PSI is used to express absolute pressure, which includes the pressure exerted by the atmosphere. In contrast, PSIG is the pressure reading given by most pressure gauges, as it subtracts atmospheric pressure and focuses on the pressure exerted by the system itself.
For example, a tire pressure gauge reading in PSIG does not include atmospheric pressure; if it reads zero, it means the pressure inside the tire is the same as the atmospheric pressure. However, if a tire's pressure were measured in PSI (absolute pressure), the reading would be higher, including the atmospheric pressure.
Understanding the difference between PSI and PSIG is crucial in fields like engineering, where precise pressure measurements are essential. For instance, in a closed system where external atmospheric conditions do not affect the pressure, PSI may be more relevant, whereas PSIG is more applicable in open systems interacting with atmospheric pressure.
Atmospheric pressure at sea level is approximately 14.7 PSI. So, a reading of 30 PSIG is actually equivalent to 44.7 PSI (30 + 14.7). This distinction is important in applications such as compressed air systems, hydraulic systems, and refrigeration.

## Comparison Chart

### Definition

Measures absolute pressure, including atmospheric
Measures pressure above atmospheric pressure

### Atmospheric Pressure

Included in the measurement
Not included, measured against atmospheric

### Typical Use

In scenarios requiring absolute pressure readings
Common in everyday pressure gauges

Higher than PSIG for the same condition
Lower than PSI due to atmospheric pressure exclusion

### Application

Scientific research, closed systems
General use, open systems interacting with atmosphere

## PSI and PSIG Definitions

#### PSI

PSI is crucial for accurate readings in closed systems.
The vessel was pressurized to 100 PSI for the experiment.

#### PSIG

PSIG measures pressure relative to atmospheric pressure.
The car tire was inflated to 32 PSIG.

#### PSI

PSI includes the atmospheric pressure in its reading.
At sea level, atmospheric pressure is about 14.7 PSI.

#### PSIG

PSIG is used in systems where atmospheric pressure affects readings.
The pool's filtration system operates at 20 PSIG.

#### PSI

PSI is a standard unit for measuring pressure in various fields.
The hydraulic system operates at a maximum of 2500 PSI.

#### PSIG

PSIG readings are lower than PSI for the same condition due to atmospheric pressure exclusion.
The gauge showed a drop in pressure to 15 PSIG.

#### PSI

PSI measures the absolute pressure exerted on a square inch of area.
The laboratory equipment was calibrated to 150 PSI.

#### PSIG

PSIG is commonly used in everyday applications like tire pressure.
The air compressor shut off automatically at 120 PSIG.

#### PSI

PSI is used in scientific and engineering contexts for precise pressure measurement.
The boiler's safety valve is set to release at 200 PSI.

#### PSIG

PSIG is important for measurements in open systems.
The boiler's operating pressure was maintained at 50 PSIG.

#### PSI

The 23rd letter of the Greek alphabet. See Table at alphabet.

#### PSI

Parapsychological phenomena or abilities considered as a group.

#### PSI

(countable) The twenty-third letter of Classical and Modern Greek and the twenty-fifth letter of Old and Ancient Greek.

#### PSI

A form of psychic energy.

#### PSI

A unit of pressure

#### PSI

The 23rd letter of the Greek alphabet

## FAQs

#### How does PSI differ from PSIG?

PSI includes atmospheric pressure in its measurement, while PSIG does not.

#### Is PSI or PSIG more common in car tire pressure?

Car tire pressure is typically measured in PSIG.

#### What does PSI stand for?

Pounds per Square Inch.

#### What is the main use of PSIG?

PSIG is mainly used in pressure gauges for everyday applications.

#### Do pressure cookers use PSI or PSIG?

Pressure cookers commonly use PSI for pressure measurement.

#### In what scenario would PSIG read zero?

PSIG would read zero when the pressure inside a system equals atmospheric pressure.

#### How do I convert PSIG to PSI?

Add the atmospheric pressure (approximately 14.7 at sea level) to the PSIG value.

#### Is atmospheric pressure always considered in PSI?

Yes, PSI measurements always include atmospheric pressure.

#### What is the atmospheric pressure in PSI at sea level?

Approximately 14.7 PSI.

#### Can PSI and PSIG have the same numerical value?

Yes, but only if the atmospheric pressure is zero, which is practically impossible.

#### Can PSIG readings be negative?

No, PSIG readings represent pressure above atmospheric and cannot be negative.

#### Why is PSI used in hydraulic systems?

PSI is used for its accuracy in measuring absolute pressure in closed systems.

#### Is PSIG dependent on altitude?

Yes, as atmospheric pressure changes with altitude, affecting PSIG readings.

#### How does temperature affect PSI and PSIG?

Temperature changes can affect the volume and pressure of gases, impacting both PSI and PSIG readings.

#### Is PSI used in the aviation industry?

Yes, for measuring cabin pressure and other critical systems.

#### How is PSIG useful in automotive applications?

It's used for measuring tire pressure, oil pressure, and fuel pressure, among others.

#### Is PSI important in scientific research?

Yes, PSI is crucial for precise pressure measurements in scientific research.

#### Why is PSI used in scientific equipment calibration?

For its precision and ability to provide absolute pressure readings.

#### Is PSIG used in weather forecasting?

No, meteorologists typically use other units like millibars or inches of mercury.

#### Are PSI and PSIG interchangeable?

They are not interchangeable as they measure different types of pressure.