Subtract 14.7 from PSIA to get PSIG and convert absolute pressure to gauge pressure.

A quick moment of math can save you a lot of headaches on a job. Imagine you’re reading a pressure gauge, and you see a number. Is that number telling you how much pressure there is above a vacuum, or above the air outside? In the world of HVAC and EPA 608 certifications, that distinction matters—because it shapes how you charge, test, and service refrigerant systems.

PSIA vs PSIG: what’s the difference, really?

• PSIA stands for pounds per square inch absolute. It’s a pressure reading relative to a perfect vacuum. Think of it as “how high is the pressure if there were nothing but empty space behind the reading.”

• PSIG stands for pounds per square inch gauge. This one is relative to atmospheric pressure—the air we breathe. It’s the pressure you’d feel if you popped a balloon in the middle of a calm day.

Here’s the intuition you’ll want to keep in your toolbox: PSIA is everything you’d have if you could remove every ounce of air around you (the vacuum reference). PSIG is what you actually experience in the environment—pressure above the air around you. In most service contexts, we talk about PSIG because it lines up with our practical, day-to-day readings on gauges that are open to the atmosphere.

The conversion rule you can trust

The standard atmospheric pressure at sea level is about 14.7 psi. That number isn’t just trivia; it’s the bridge between PSIA and PSIG. To convert from PSIA to PSIG, you subtract that atmospheric baseline:

PSIG = PSIA − 14.7

That’s the simple, reliable rule you’ll use again and again. If your PSIA reads 150 psi, the PSIG is 150 − 14.7 = 135.3 psi. If PSIA is 14.7 psi, you’ve got 0 psi gauge—pressure equal to the surrounding air. If PSIA is less than 14.7, (say you’re at sea level but in a situation where the absolute pressure dips below atmospheric), PSIG would be negative. That can happen in certain vacuum or evacuation scenarios.

Why this matters in the real world

If you’re dealing with refrigerants and systems covered by EPA 608 requirements, the numbers you read on gauges translate directly into how you charge, how you test, and whether a system is safe to operate. A few practical implications:

• Charging and recovery: When you’re adding refrigerant to a system or validating that a system holds a vacuum, knowing whether you’re looking at PSIA or PSIG helps you interpret readings correctly. A misread can lead to overcharging or undercharging, both of which cause inefficiencies or potential safety concerns.

• Service diagnostics: Many service charts, pressure-temperature relationships, and refrigerant specifications are expressed in PSIG. Converting to PSIG helps you compare real-time readings against the manufacturer’s guidance.

• Safety and standardization: Technicians often work with standard references that assume PSIG. Keeping the conversion straight avoids confusion and keeps the workflow smooth.

A practical, one-page mental model

Let me put it simply: PSIA tells you “how much pressure is there in the system if you remove the air around us.” PSIG tells you “how much pressure above the air around us is present.” The sea-level atmosphere is a universal reference point, so 14.7 psi becomes the number you subtract to go from absolute to gauge pressure.

A quick, concrete example

• You measure PSIA = 120 psi on a service port that’s connected to a gauge open to ambient air.

• Subtract atmospheric pressure: PSIG = 120 − 14.7 = 105.3 psi.

• That 105.3 psi is the gauge pressure you’ll use when comparing against the system’s charging charts or manufacturer specs.

Now, a tiny digression that helps the concept stick

Altitude does play a role in the real world. The 14.7 psi baseline is the standard at sea level. If you’re working in a high-altitude location (or if you’re evaluating readings in a test lab that isn’t at sea level), the actual atmospheric pressure changes. In those cases, you’d adjust the subtracting value to the local atmospheric pressure. For everyday fieldwork at typical shop conditions, 14.7 is a handy, dependable shorthand. The key is knowing when that shorthand is a good approximation and when you need the precise local atmospheric pressure.

Common stumbling blocks—and how to avoid them

• Mixing up PSIA and PSIG: It’s easy to glance at a number and assume it’s gauge pressure. If a gauge is open to the atmosphere, you’re typically seeing PSIG. When you’re not sure, ask what reference the reading uses (absolute vs. gauge) or check the gauge labeling.

• Forgetting the reference point: If you’re comparing a chart that lists PSIG values to a measurement that’s PSIA, you’ll end up with the wrong charging value. Always confirm the reference.

• Ignoring altitude nuance: If you’re in a location with noticeably different atmospheric pressure, use the actual local atmospheric pressure rather than the standard 14.7. It’s a small adjustment that matters in tight tolerance scenarios.

• Relying on a single number: Pressure readings can fluctuate—especially during dynamic operations like charging or evacuating. Use multiple readings, note the conditions, and cross-check against your system’s spec sheet.

A few tips from the field

• Memorize the core formula: PSIG = PSIA − 14.7. It’s short, it’s reliable, and it saves you time when you’re mid-task.

• Keep a quick reference handy: A small laminated card in your toolbox with the PSIA, PSIG definitions and the conversion rule can be a lifesaver during busy days.

• Practice with real numbers: Take a few readings from your shop gauge at different times of day, at different elevations if possible, and convert to PSIG. Seeing the numbers in both forms reinforces the concept.

• Tie it to the system’s context: When you’re charging a refrigerant, always cross-check with the manufacturer’s data. Some systems specify pressures in PSIG for charging ports and service valves, so the conversion isn’t just academic—it’s practical.

Where this knowledge fits into the bigger picture

Understanding the delta between PSIA and PSIG isn’t a flashy detail; it’s a foundation for accurate diagnostics, safe handling, and compliant servicing. In EPA 608 work, where the goal is to protect the environment and ensure systems run efficiently, clean, precise measurements are part of the job. Knowing how to convert pressure readings ensures you’re interpreting data correctly, which in turn supports proper refrigerant management, leak testing, and system performance.

A final thought to tuck into your toolkit

Pressure is one of those things technicians learn to read almost by instinct. But the moment you pause to connect the numbers to the reference frame—absolute vs. gauge—you gain clarity that speeds up work and reduces mistakes. The subtraction you commit to memory is more than a math trick; it’s a small but mighty lever that keeps your assessments honest and your results reliable.

If you’re curious to explore more about pressure measurements, there are helpful resources and glossaries out there that break down terms, charts, and units with practical examples. In the end, the rule is simple: PSIG = PSIA minus 14.7. Remember it, and you’ll keep your readings grounded in real-world applicability no matter where the job takes you.