How this instrument works
Psig and psia are both measured in pounds per square inch — the difference between them is entirely about where zero sits, not about the size of the unit itself. Psig, gauge pressure, is zeroed against the surrounding atmosphere, so a psig reading of 0 means “no more pressure than the air already provides,” the way a tyre gauge or a compressor dial normally reports. Psia, absolute pressure, is zeroed against a true vacuum instead, so it always reads at least atmospheric pressure under ordinary conditions and never touches zero at sea level.
Because both units are already psi, converting between them needs no scaling factor whatsoever — the entire conversion is adding or subtracting standard atmospheric pressure at sea level, fixed at 14.696 psi. That makes this the simplest pressure conversion on the site by formula, even though it trips people up more than any multiplicative one, because it's easy to assume every unit conversion multiplies through and forget that a zero-point shift needs addition instead.
The distinction matters wherever a pressure figure feeds into physics or thermodynamics rather than into equipment ratings. Boyle's and Gay-Lussac's gas laws, compressor efficiency calculations, altitude-corrected pressure work and vacuum-system specifications all require absolute pressure, because they're built on the true zero of no pressure at all — plugging a gauge reading into those formulas without first adding atmospheric pressure back in produces a systematically wrong answer, off by exactly 14.696 psi every time.
- Type your gauge reading into the PSIG (psi) field, which opens at 30 — a common tyre or compressed-air gauge pressure.
- Read the absolute pressure on the PSIA (psi, absolute) line: it's your gauge figure plus 14.696, recalculated with every keystroke.
- Going the other way? Subtract 14.696 from an absolute figure to recover the gauge reading.
- Confirm your source reading is genuinely gauge before adding the offset — an already-absolute figure run through this page will be overstated by 14.696 psi.
Worked example — a 30 psig compressor gauge reading
A shop air compressor's gauge reads 30 psig, a typical working pressure for pneumatic tools. Enter 30 into PSIG (psi) and PSIA (psi, absolute) returns 44.696 — simple addition, not a scaled conversion, since 30 plus the standard 14.696 psi atmospheric offset lands there directly.
The offset stays fixed at every reading. A gauge of 1 psig converts to 15.696 psia, and a gauge of exactly 0 psig — meaning no pressure above the surrounding air at all — still converts to 14.696 psia, because that's what the atmosphere alone weighs in at on this scale, with or without anything else pressurising the line.
Questions
Why is this the only pressure conversion here without a multiplier?
Because psig and psia are already the same unit — both measure in pounds per square inch — so there's no unit size to rescale, unlike converting bar or megapascals into psi. The only thing separating them is where their zero point sits: gauge zeroes against the surrounding atmosphere, absolute zeroes against true vacuum, and shifting between two zero points is addition or subtraction, never multiplication.
What exactly is 14.696 psi, and where does it come from?
It's standard atmospheric pressure at sea level, defined internationally as exactly 101,325 pascals and equal to about 14.6959 psi when converted, rounded here to 14.696. It's a fixed reference value, not a live measurement — actual air pressure on any given day drifts slightly above or below it with weather, but engineering and physics calculations default to this standard figure unless a specific local correction is called for.
Does altitude change the correct offset to add?
Yes, in real conditions — atmospheric pressure drops as elevation rises, down to roughly 12.2 psi at 5,000 feet and about 10.1 psi at 10,000 feet, meaning a gauge reading taken high in the mountains sits closer to its true absolute value than the same gauge reading at sea level would. This calculator always applies the fixed sea-level standard of 14.696 psi, correct for equipment calibrated at sea-level reference conditions but not for precise high-altitude work.
When do I actually need psia instead of psig?
Whenever a formula or physical law is built on true zero pressure rather than on ambient air — the ideal gas law, compressor and pump efficiency calculations, refrigerant pressure-temperature charts, and vacuum-system specifications all require absolute pressure. Feed them a gauge figure by mistake and every result comes out wrong by exactly 14.696 psi, a small-looking error that compounds badly through multi-step thermodynamic calculations.
Can psig ever be a negative number?
Yes — a gauge reading below atmospheric pressure, such as inside a partial vacuum, shows up as negative psig, sometimes called vacuum pressure. This calculator handles that correctly through simple addition: a −5 psig reading, for instance, converts to 9.696 psia, still comfortably above zero because true vacuum, not atmospheric pressure, is what absolute zero actually means.
Is 14.696 psi exact, or does it get rounded somewhere?
The defining figure, 101,325 pascals, is exact by international convention, but converting it into psi introduces the same repeating decimal that runs through every pascal-to-psi conversion: the true value is about 14.6959488 psi, and 14.696 is that figure rounded to three decimals. Precision beyond that outstrips what any practical pressure gauge or instrument can resolve.