PSIG vs PSIA in 2025: What Mechanics Should Know

When checking tire pressure, you might come across PSIG or PSIA, but what’s the difference? For garages and drivers, knowing this matters more than you think.

Tire gauges, service manuals, and even online calculators use these terms.

In this guide, we’ll break down what they mean, how they relate to atmospheric pressure at sea level, and why most gauges show PSIG, not PSIA. By the end, you’ll be able to explain it clearly to customers and use the right numbers for safe, optimal tire pressure.

What Does PSIG Mean?

PSIG stands for pounds per square inch gauge. It measures pressure relative to the surrounding atmosphere. In simple terms, PSIG ignores the weight of the air around us and only shows how much pressure is above it.

This is the unit you’ll see on a tire gauge. For example, if your tire gauge reads 32 PSI, that’s 32 PSIG; meaning 32 pounds per square inch above the current atmospheric pressure.

Because drivers only need to know how much extra pressure is in the tire (not the total pressure including the atmosphere), PSIG is the most practical measure for everyday use.

What Does PSIA Mean?

PSIA stands for pounds per square inch absolute. Unlike PSIG, it measures pressure relative to a perfect vacuum, meaning it includes the weight of the atmosphere.

At sea level, the atmospheric pressure is about 14.7 PSI. So if a tire gauge shows 32 PSIG, the absolute pressure inside the tire is actually 32 + 14.7 = 46.7 PSIA. Because PSIA = atmospheric pressure + gauge pressure, it always includes the surrounding air.

PSIA is useful in science, engineering, and when precise calculations are needed, such as testing equipment or comparing pressures at different altitudes. But for most garage and tire work, PSIG is what matters since that’s what customers see on their gauges.

Difference Between PSIA and PSIG

The terms sound similar, but the difference is clear once you know the reference point:

• PSIG (Gauge Pressure): Measures pressure above the surrounding air. What your tire gauge shows.
• PSIA (Absolute Pressure): Measures pressure against a vacuum. Always includes atmospheric pressure.

A simple way to remember it:

• PSIG = what you see on a tire gauge
• PSIA = PSIG + atmospheric pressure

Since the atmospheric pressure at sea level is 14.7 PSI, converting is easy. For example:

• Tire reads 32 PSIG
• To convert PSIG to PSIA, add 14.7 → 46.7 PSIA

This difference matters in technical fields like hydraulics or aerospace, but for everyday garage work and tire pressure measurements, PSIG is what mechanics and drivers use.

How This Affects Tire Pressure Measurements

For most garages and drivers, the choice between PSIG and PSIA comes down to practicality.

• Tire gauges use PSIG: They only show the pressure above the atmosphere, which is all a driver needs to know when inflating or checking tires.
• Service manuals list PSIG: When you read “inflate tires to 35 PSI,” it refers to PSIG, not PSIA.
• Conversions matter in technical cases. If you’re comparing systems or working with scientific values, you may need to change PSI to ATM (1 atm = 14.7 PSI at sea level).
• Why it matters for safety: A tire inflated correctly in PSIG ensures optimal tire pressure for grip, fuel economy, and safety on the road.

For garages, explaining this clearly to customers can avoid confusion. If someone asks why their gauge shows 32 PSI instead of 46.7, you can point out the difference between gauge pressure and absolute pressure.

Quick Takeaways

• PSIG measures pressure above the atmosphere, what your tire gauge displays.
• PSIA includes atmospheric pressure, total absolute pressure.
• At sea level, atmospheric pressure = 14.7 PSI.
• To convert: PSIA = PSIG + 14.7 (at sea level).
• For garages and drivers, tire pressure is always checked in PSIG.

Conclusion

The difference between PSIG and PSIA is simple once you know the role of atmospheric pressure. PSIG is what your tire gauge shows and what service manuals use. PSIA adds in the pressure of the atmosphere itself, making it more useful for engineers and scientific calculations.

For garages, understanding both helps explain results clearly to customers and ensures tires are inflated to the right levels for safety and performance. Whether you’re teaching a trainee mechanic or reassuring a driver, knowing how to read and explain these numbers builds trust and confidence.