PSIG: How to Calculate Gauge Pressure

This calculator finds gauge pressure in pounds per square inch (psig) by subtracting atmospheric pressure from absolute pressure. Enter both pressures in the same unit; the result is gauge pressure in psi, which you can convert to psig because 1 psi is 1 pound-force per square inch.

Formula

Gauge pressure = absolute pressure − atmospheric pressure.

In symbols, Pg = Pabs − Patm, where Pg is gauge pressure in psi (psig), Pabs is absolute pressure in psia, and Patm is the local atmospheric pressure in psia. The calculator’s second result converts the same pressure difference to kilopascals: 1 psi = 6.894757 kPa.

Absolute pressure is measured from a perfect vacuum. Gauge pressure is measured relative to the surrounding atmosphere. The calculator expects both inputs in psia, or pounds-force per square inch absolute. It displays the difference as psig and kPa. The input ranges are broad numerical limits, not an indication that a pressure or setup is safe to use.

Worked Example

Using the calculator’s default values, absolute pressure is 14.7 psia and atmospheric pressure is 14.7 psia.

1. Subtract atmospheric pressure: 14.7 − 14.7 = 0 psi.
2. Label the difference as gauge pressure: 0.00 psig.
3. Convert to kilopascals: 0 × 6.894757 = 0.00 kPa.

At the defaults, the result is 0.00 psig and 0.00 kPa. This is the expected result when absolute pressure equals the atmospheric reference pressure.

What You’ll Need

• The absolute pressure you want to convert, expressed in psia.
• The atmospheric pressure at the measurement location, also expressed in psia. Use a local pressure reading when accuracy matters; 14.7 psia is an approximate reference near sea level.
• A calculator, or the calculator fields above. If your readings use other units, convert them before entering them.

Step-by-Step

1. Identify what the pressure reading represents. Check the instrument, data sheet, or calculation to determine whether the value is absolute pressure (psia) or gauge pressure (psig). Do not subtract atmospheric pressure from a reading that is already gauge pressure.
2. Get the absolute pressure. Enter the pressure measured relative to a vacuum in the Absolute pressure field. If your value is in another unit, convert it to psia first. For example, 100 kPa is about 14.5038 psi; if it is absolute, that is about 14.5038 psia.
3. Find the relevant atmospheric pressure. Enter the air pressure surrounding the measurement, in psia. Atmospheric pressure changes with altitude and weather. The common approximation of 14.7 psia is useful for a rough sea-level example, but it is not exact everywhere or at all times.
4. Subtract the atmospheric value. The calculator applies Pabs − Patm. For example, if absolute pressure is 30.0 psia and local atmospheric pressure is 14.7 psia, the gauge pressure is 15.3 psig.
5. Read the result and check its sign. A positive result means the pressure is above the atmospheric reference. Zero means it matches the reference. A negative result means it is below atmospheric pressure, as in a partial vacuum. Negative gauge pressure is possible; it does not mean negative absolute pressure.
6. Use the appropriate units and precision. The first output is psig, rounded to two decimal places. The second is kPa, also rounded to two places. Retain extra digits in intermediate conversions when working from a non-psi reading, then round only the final result to a precision justified by the instruments.

Common Mistakes

• Mixing absolute and gauge pressure: subtracting atmospheric pressure from an existing psig value gives an incorrect answer. First establish the reference used by the original reading.
• Using mismatched units: do not subtract a value in kPa from one in psi. Convert both readings to the same unit before subtracting.
• Assuming atmospheric pressure is always 14.7 psia: that is an approximate sea-level value. Local pressure can differ with elevation and weather, which affects the conversion.
• Reversing the subtraction: use absolute minus atmospheric, not atmospheric minus absolute. Reversing it flips the sign.
• Rounding too early: rounding each input before subtraction can distort a small pressure difference. Keep the available precision through the calculation.
• Confusing psi with psia or psig: psi describes the pressure unit, while the suffix indicates the reference. A bare psi value may be ambiguous; verify what the source means.

Tips

• Record both the numerical reading and its reference: for example, “30.0 psia” or “15.3 psig,” rather than just “30 psi.”
• For a precise conversion, use atmospheric pressure measured at the same location and time as the pressure reading when possible.
• Useful conversions include 1 psi = 6.894757 kPa and 1 bar = 14.5038 psi. Convert both inputs to psia before using this calculator, or apply the same subtraction directly in another consistent unit.
• Check that the answer is physically plausible. Near sea-level conditions, an absolute pressure close to 14.7 psia should produce a gauge pressure close to zero.
• Pressure is force per area. In SI units, pascals (Pa) and kilopascals (kPa) are common; 1 kPa = 1,000 Pa. In US customary usage, psi means pounds-force per square inch.

Troubleshooting

The answer seems too high or too low: check whether the absolute reading was entered in psia and whether the atmospheric value is appropriate. Confirm conversions and decimal placement, especially when converting from kPa, bar, or inches of mercury.

The result is negative: verify that the absolute pressure is not below zero and that the inputs are in the correct order. If absolute pressure is lower than local atmospheric pressure, a negative gauge result is mathematically correct and describes pressure below the surrounding atmosphere.

The result is not zero when you expect it to be: check that both entries refer to the same place and time and use the same pressure units. The default 14.7 psia is only approximate; a different atmospheric reading will change the answer.

Your instrument gives only psi: look up the instrument’s reference or documentation. If it is a gauge-pressure reading, it is already in psig and should not have atmospheric pressure subtracted again. If it is absolute, treat it as psia.

You need a result in another unit: calculate the pressure difference in one unit first, then convert that difference. For example, multiply psi by 6.894757 for kPa. Do not convert only one of the two input pressures.

Frequently Asked Questions

What does psig mean?
PSIG means pounds per square inch gauge. It reports pressure relative to the surrounding atmospheric pressure, not relative to a vacuum.

What is the formula for calculating psig?
Subtract atmospheric pressure from absolute pressure: psig = psia − atmospheric pressure in psia.

How do I convert psia to psig?
Subtract local atmospheric pressure in psia. Using the approximate 14.7 psia sea-level reference, 30 psia is about 15.3 psig.

Can psig be negative?
Yes. A negative gauge pressure means the pressure is below the local atmosphere. Absolute pressure itself cannot be negative.

Is 14.7 psia always the right atmospheric pressure?
No. It is a convenient approximation near sea level. Atmospheric pressure varies with elevation and weather, so use a local value for a more accurate result.

Are psi, psia, and psig interchangeable?
No. Psi names the pressure unit; psia and psig specify whether pressure is measured from a vacuum or from the atmosphere. Check the reference before calculating.

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in a | Absolute pressure | psia | 14.7 | 0 | 1000000
in t | Atmospheric pressure | psia | 14.7 | 0 | 1000000
out g | Gauge pressure | psig | a - t | 2
out k | Gauge pressure | kPa | (a - t) * 6.894757 | 2