Pressure Unit Selection Guide for HVAC Engineers (PSI, kPa, MPa, Bar, inH2O, inHg)

On a single HVAC project, an engineer might spec a chiller compressor in psi, a fan curve in inches of water, an envelope leakage test in pascals, and a refrigeration evacuation in inches of mercury. Then a European equipment submittal arrives in bar, the controls vendor wants kPa for a setpoint, and the structural engineer asks why the rooftop unit's static pressure rating “sounds so low.” Pressure has more units in active use than almost any other quantity in mechanical engineering, and using the wrong one — or doing the right conversion sloppily — is one of the easier ways to design or commission something that does not work. This guide walks through which unit fits which task, why low-pressure airflow has its own family of units, and where gauge versus absolute matters most.

Pressure is force per unit area, but units have history

The SI unit of pressure is the pascal: 1 Pa = 1 N/m². It is a very small unit. One pascal is roughly the pressure exerted by a U.S. dollar bill resting flat on a table. That is why HVAC almost never uses bare pascals for anything other than envelope and duct-leakage measurements. Practical pressures are expressed in kilopascals (kPa, 10³ Pa) or megapascals (MPa, 10⁶ Pa) when working in SI, and in psi (pound-force per square inch), bar, atmospheres, or one of the column-of-fluid units in legacy or regional contexts.

The reason there are so many is that each unit was chosen to make a specific measurement convenient. Inches of water column give readable two- and three-digit numbers for the very low pressures involved in air handling. Inches of mercury map naturally to a barometer or vacuum gauge. Psi is the conventional unit for compressed-fluid pressures in the U.S., and bar is its almost-equivalent metric counterpart popular in Europe. None of them are wrong; they are just optimized for different parts of the problem space.

When to use each unit

A practical, by-context cheat sheet:

• psi (pound-force per square inch). U.S. refrigeration and chiller work. Refrigerant suction and discharge pressures, condenser water pressures, hydronic pump heads expressed in pressure units, and ASME pressure-vessel ratings are almost always in psi (specifically psig — gauge — for field measurement, or psia for thermodynamic calculations).
• kPa (kilopascals). The standard SI unit for general HVAC pressures in international documentation and most ISO/EN-aligned equipment specs. Refrigerant pressures outside the U.S., water-side pump pressures, and pressure setpoints in BACnet objects and many BMS systems often use kPa.
• MPa (megapascals). High-pressure systems and materials engineering. Pipe wall stress, vessel design pressures, and high-side pressures of CO₂ (R-744) refrigeration systems — which routinely operate above 8 MPa — are typically quoted in MPa.
• bar. European HVAC and refrigeration equipment specs. 1 bar = 100 kPa = 10⁵Pa, conveniently close to (but not exactly) one standard atmosphere. Submittals from Daikin, Bitzer, Danfoss, and many other European-headquartered manufacturers come in bar. Note that “bar(g)” means gauge and “bar(a)” or “bara” means absolute.
• inH2O (inches of water column, also “inWC”). Low-pressure airflow design — fan static pressure, duct pressure drop, filter pressure drop, terminal box drops, diffuser pressure drops. The whole air-side world in U.S. practice runs on this unit. A typical commercial VAV system has a total external static pressure on the order of 2 to 5 inWC; an individual filter might drop 0.5 inWC clean and 1.0 inWC dirty.
• inHg (inches of mercury). Vacuum measurements. Refrigeration system evacuation is traditionally specified in inHg of vacuum, or in microns of mercury (1 inHg ≈ 25,400 µmHg). Atmospheric pressure (used in altitude corrections) is also historically in inHg in U.S. weather data — sea-level standard is 29.92 inHg.
• Pa (pascals). Building envelope and duct leakage. ASTM E779 specifies fan-pressurization testing of building envelopes at reference pressures of 50 Pa, often extrapolated across 10–60 Pa. Smoke control system commissioning per NFPA 92 likewise uses pascals. Anything where the pressure difference is just a few tens of Pa and you want a unit that puts the number in a single digit or two.
• atm (atmospheres) and torr. Rare in modern HVAC but appears in older refrigeration and laboratory texts. 1 standard atmosphere = 101.325 kPa = 760 torr = 760 mmHg. Torr is still common in laboratory vacuum work but not in field HVAC.

Conversion table

All values rounded to four significant figures. The reference rows show how each unit relates to 1 kPa, then to a few common air-side and refrigerant-side benchmarks.

Note on water-column units: the value of inH₂O depends slightly on the reference temperature (denser water, larger pressure per inch). The most common HVAC convention is 60 °F (15.56 °C), giving 1 inH₂O ≈ 248.84 Pa. Older texts and SMACNA tables sometimes use 39.2 °F (4 °C, water at maximum density) which gives 249.08 Pa, or 68 °F (20 °C) which gives 248.64 Pa. The difference is about 0.2% — meaningful only in precision flow measurement, irrelevant to typical duct design.

Why inches of water for airflow

It looks archaic, but inH₂O is not a historical accident. Air handler total static pressures, duct pressure drops, and filter pressure drops are very small compared to refrigerant pressures. A high-static commercial fan might develop 8 inWC of total pressure. In SI, that is about 1,990 Pa, or 1.99 kPa, or 0.0199 bar — small numbers with leading zeros. In psi the same pressure is 0.289 psi, which is also awkward.

In inches of water column, common HVAC values fall into the easy-to-read range of about 0.05 (a clean filter) to 8 (a maxed commercial supply fan) with one decimal of precision, which is the precision actual gauges and balance instruments offer. The unit also has an immediate physical meaning: an inclined manometer with water as the working fluid reads the pressure directly off a scale. Modern digital manometers preserve the convention.

Outside the U.S., the equivalent for low-pressure work is the pascal — exactly because it is also a small unit, putting typical air-side readings in the 10–2,000 Pa range without leading zeros.

Gauge vs absolute, and when each matters

Almost every field pressure gauge reads gauge pressure(suffix “g”: psig, bar(g), kPag), the difference between the measured pressure and local atmospheric pressure. A tire gauge that reads 32 psi is reading 32 psig; the absolute pressure inside the tire is about 46.7 psia at sea level. For most field tasks — checking that a pump is producing 30 psi of head, or that a refrigerant suction line is at 70 psig — gauge is what you want, because that is what your gauge reads.

Absolute pressure(suffix “a”: psia, bar(a), kPa absolute) is needed whenever the calculation depends on the underlying physical state of the gas — most importantly refrigerant property tables and any thermodynamic cycle analysis. If you read a refrigerant's saturation temperature off a P-T chart, the table is in absolute units; you must add the local atmospheric pressure (≈ 14.696 psi at sea level) to a gauge reading before looking it up. The same applies to compressor pressure ratios, which are calculated from absolute pressures (P_2,abs / P_1,abs), not gauge.

Vacuum readings are a third case. An evacuation reading of “29 inHg vacuum” is an absolute pressure of about 0.92 inHg, or 31 mbar, or 23,000 µmHg. Refrigeration evacuation is usually specified as a target vacuum in microns of mercury (e.g., “evacuate to 500 µm and hold”) precisely because the gauge-vacuum reading near complete evacuation is too compressed at the high end of the inHg scale to be useful.

Worked example: converting a fan curve from inH2O to Pa

Suppose you are sizing a return fan for an international project where the equipment will be specified in SI. The U.S. catalog gives the fan's rated total pressure as a curve, with these operating points:

To deliver these to the international project team, convert airflow to L/s (1 cfm = 0.4719 L/s) and pressure to Pa (1 inWC = 248.84 Pa). Doing the multiplication directly:

A few sanity checks. A typical European high-static commercial fan operates in the 500–1,500 Pa range — these numbers are right in the middle of that, as expected for a fan that develops a few inWC. The pressure-flow curve still has the same shape, since unit conversion is just multiplication by a constant.

One subtle issue: total pressure ratings per AMCA 210 / ASHRAE 51 are referenced to specific inlet air density (typically standard air at 1.20 kg/m³, 20 °C, 101.325 kPa). If your project site is at altitude or in extreme temperatures, the actual air density differs and the fan's real pressure-flow curve shifts. That correction is separate from the unit conversion. Always note on the submittal whether the pressures are at standard or actual density.

Common pitfalls

• Confusing psig with psia. An 80 psig refrigerant suction pressure is 94.7 psia at sea level. Looking up a saturation temperature on a P-T chart with 80 instead of 94.7 gives a saturation temperature low by several degrees and can send you down the wrong diagnostic path.
• Using bar where psi was meant. 14.5 bar is not 14.5 psi. They differ by a factor of about 14.5. This is usually caught by sanity-checking against operating ranges, but it has made it into commissioning reports.
• Mixing inWC reference temperatures. The difference between 4 °C and 60 °F water-column inches is about 0.1%, which is usually irrelevant. But if you are doing orifice or venturi flow measurement per ISO 5167, the standards specify the reference, and the difference does matter at the precision flow-meter level.
• Confusing pressure drop with system pressure. A duct pressure drop of 0.4 inWC across a coil is a difference, not the absolute system pressure. When calculating fan requirements you sum the pressure drops; when calculating duct wall stress (rare in HVAC, common in industrial) you need the absolute internal pressure.
• Forgetting altitude. Atmospheric pressure drops roughly 12% from sea level to 1,500 m elevation. This affects both absolute pressure references and the density used to correct fan curves. A psig reading on a high-altitude project corresponds to a different psia than at sea level.
• Using kPa for envelope leakage where Pa is the standard. ASTM E779 results are reported in pascals. 0.05 kPa is technically the same as 50 Pa, but spec sheets and test reports always use Pa to avoid leading-zero ambiguity.
• Truncating “inWC” in handwritten notes. A note that just says “0.5 in” on a field sheet is ambiguous between inches of water and inches of mercury — and they differ by a factor of about 13.6. Always include the fluid.

Quick mental conversions worth memorizing

• 1 psi ≈ 6.9 kPa ≈ 0.069 bar ≈ 27.7 inWC ≈ 2.04 inHg
• 1 bar ≈ 14.5 psi = 100 kPa ≈ 14.7 psi at sea-level absolute
• 1 inWC ≈ 249 Pa ≈ 0.25 kPa ≈ 0.036 psi
• 1 inHg ≈ 3.39 kPa ≈ 0.49 psi ≈ 13.6 inWC
• 1 atm = 101.325 kPa = 14.696 psi = 29.92 inHg = 406.8 inWC

For one-off calculations these shortcuts get you within a percent. For submittals, commissioning reports, and anything the client will sign, run the conversion with full precision and label the units explicitly — including the “g” or “a” suffix where it matters.

The takeaway

There is no single right pressure unit for HVAC. There is a right unit for each task: psi for U.S. refrigerant work, kPa for SI documentation, MPa for high-pressure piping and CO₂ systems, bar for European equipment, inWC for U.S. air-side design, Pa for envelope and duct-leakage testing, and inHg for vacuum. Knowing why each unit fits its niche makes the conversions feel less arbitrary, and labeling units clearly — gauge or absolute, and at what reference — heads off a substantial fraction of the field problems that get blamed on equipment.