What Is PSI? Pressure Unit Definition & Spec-Sheet Guide
PSI stands for pounds per square inch, the imperial unit of pressure used on most North American spec sheets. One PSI equals 6.894757 kilopascals (kPa), or about 0.0689 bar. The suffix after it (PSIA, PSIG, PSID) tells you the reference point. This guide covers what PSI means, how to read it on a transmitter spec sheet, when to use it instead of SI units, and how to match a PSI range to the right transmitter.
PSI Defined: Pounds-Force per Square Inch
PSI is an imperial pressure unit. The symbol is written in lowercase as psi, and the spec-sheet form looks like 0–100 psi, 0–150 psi, or 0–1500 psi depending on the application. The unit comes from the US customary system and was codified in early twentieth-century American boiler practice, before SI units became the international engineering default.
The physical idea is force divided by area: a one-pound load spread evenly over one square inch sits at 1 psi, and halving the area or doubling the load both double the reading. The next section returns to what psi literally stands for and how engineers usually write the abbreviation; this section focuses on the numbers.
The exact SI conversion is 1 psi = 6.894757 kPa. Most engineering work rounds this to 6.895 kPa, but legal metrology (for example a Chinese JJG 52-2013 verification report) uses the six-decimal value to avoid traceability errors when converting test points between units. The reverse is also worth memorising: 1 MPa = 145.038 psi, the number you will see on every Chinese-origin transmitter spec sheet that ships to a North American customer.
1 PSI in Other Units: A Conversion Reference Table
PSI needs to convert in both directions: read a PSI value into SI, or write a North American spec from an SI design number. The table below uses the NIST SP 811 exact factor (6.894757 kPa per psi), also required by China’s JJG 52-2013 verification regulation.
Forward: 1 PSI expressed in other units
| Target unit | Value | Common use |
|---|---|---|
| Pascal (Pa) | 6,894.757 Pa | SI base unit, scientific work |
| Kilopascal (kPa) | 6.894757 kPa | HVAC, low-pressure transmitter spec |
| Megapascal (MPa) | 0.00689476 MPa | Chinese GB/T 1226 industrial gauges |
| Bar | 0.0689476 bar | European spec sheets |
| Atmosphere (atm) | 0.0680460 atm | Reference / textbook calculations |
| Millimetres of mercury (mmHg) | 51.7149 mmHg | Vacuum, medical, barometric |
| kgf/cm² | 0.0703070 kgf/cm² | Legacy Asian industrial gauges |
Reverse: how many PSI in 1 unit (memorise these five)
| Source | PSI equivalent |
|---|---|
| 1 MPa | 145.038 psi |
| 1 bar | 14.5038 psi |
| 1 atm | 14.6959 psi |
| 1 kPa | 0.145038 psi |
| 1 inH₂O (60 °F) | 0.0360912 psi |
The six-decimal value matters for traceable calibration: rounding 1 PSI to ≈ 6.9 kPa stacks a 0.07% error per conversion, past the tolerance band of a 0.25-class transmitter. Day-to-day reading is fine with 6.895 kPa; calibration certificates need the full factor.
For repeated conversions use the HMK online pressure unit converter, or jump to the MPa to PSI, bar to PSI, or PSI to bar tool pages directly. The inches of water column reference covers HVAC duct work where PSI numbers turn awkward.
What Does “PSI” Actually Stand For?
PSI stands for pounds per square inch. The more technically correct expansion is pound-force per square inch (lbf/in²), because pressure is force-over-area, and pound on its own can mean either a unit of mass or a unit of force in older US texts. On modern engineering documents the two are interchangeable, and psi is treated as lbf/in².
The abbreviation reads as three letters: P-S-I, not as a single word. It is written lowercase in the unit form (100 psi) and may appear capitalised when used as an acronym in headings or sentence starts.
A quick note on disambiguation: in non-engineering contexts the same three letters refer to other things, including the Greek letter Ψ in physics and statistics, the Pneumonia Severity Index in medicine, Police Sub-Inspector in South Asian policing, and Pre-Sentence Investigation in US courts. This guide is about the pressure unit only. If you arrived here looking for one of the others, those topics live elsewhere.
PSI, PSIA, PSIG, PSID: Why Spec Sheets Use Different Suffixes
The suffix after psi tells you the reference point. Mistaking one for another is the single most common spec-sheet misread, and it can put a transmitter out by 14.7 psi (one atmosphere), enough to ruin a vacuum reading or break a pressure-vessel calculation. The deep breakdown lives in the dedicated PSIA vs PSIG vs PSI cheat sheet; this section gives the working summary.
| Suffix | Name | Reference | Use case |
|---|---|---|---|
psi | Generic / unspecified | Implied gauge in most industrial spec | Catalog text, general engineering |
psia | Absolute | Perfect vacuum (0 psia) | Vacuum systems, altitude work, thermo calcs |
psig | Gauge | Local atmosphere (0 psig) | Most process, hydraulic, HVAC spec |
psid | Differential | One port relative to another | Flow ΔP, filter monitoring |
The two are linked by a constant: psia = psig + atmospheric pressure (14.696 psi at sea level). At sea level, 0 psig = 14.696 psia and 1 atm = 0 psig = 14.696 psia. See the absolute pressure formula and the matching gauge pressure formula for worked examples, or the absolute vs gauge pressure breakdown for the decision logic on which one to spec.
How to Read PSI on Industrial Spec Sheets Without Mistakes
In the field most PSI errors are not arithmetic; they are mis-reads of the spec sheet itself. Five patterns repeatedly cause range mismatches, calibration disputes, and burst incidents.
1. A bare psi is almost always psig. Industrial spec sheets (including the format set by China’s GB/T 1226-2017, General pressure gauges) treat gauge as the implied reference when the suffix is omitted. Confirm in writing before a vacuum or sealed-system order; the wrong reference shifts every reading by one atmosphere (14.696 psi at sea level).
2. A negative PSI number is gauge, not absolute. A vacuum-gauge reading of -12 psi means 12 psi below local atmosphere, not 12 psia (which would be physically impossible). True absolute spec sheets always carry the explicit psia suffix.
3. Compound gauges cover both sides of atmosphere. A range like -14.7 to +30 psi covers full vacuum through 30 psig, typical on refrigeration, gas-evacuation, and pump-suction transmitters.
4. % FS is full-scale accuracy, not accuracy of reading. A ±0.5% FS figure on a 0–100 psi transmitter is ±0.5 psi at every test point; at the 10 psi end that is 5% of the reading. China’s metrological verification regulation JJG 52-2013 (spring-tube pressure gauges) uses the same full-scale convention, which is why a Chinese-export transmitter and its US-side calibration certificate will agree on accuracy even when expressed in different units.
5. Burst pressure is not working pressure. Vendor sheets often quote a burst figure several multiples above the rated working range, survivable as a one-off spike, not as a duty point. Spec against the working range, not the burst number.
PSI Ranges in Common Industrial Applications
Anchor PSI in memory by application. Each range below maps to a class of transmitter you would actually spec. The HMK ranges are pulled from live product pages and converted using the exact 6.894757 factor.
| Application | Typical PSI range | HMK match (curl-verified) |
|---|---|---|
| HVAC duct static pressure / cleanroom DP | 0.07 to 14 psi (≈ 2 inH₂O to 100 kPa) | HM30 micro-DP transmitter |
| HVAC supply / refrigeration low side | up to 15 psi | HM23 HVAC/R compressor transmitter |
| General process gauge / compressed air | 1.5 to 290 psi | HM31 differential pressure transmitter |
| Industrial smart DP / SCADA loop | 0.018 to 5,800 psi | HM3051 smart DP transmitter |
| OEM hydraulic / pump skid | 3 to 8,700 psi | HE10 diffused-silicon OEM transmitter |
| Anti-corrosive / chemical process | 14.5 to 8,700 psi | HM50 anti-corrosive pressure transmitter |
| Oilfield wellhead / mining | up to 31,900 psi (220 MPa) | HM23Y oilfield-mining transmitter |
| Sapphire ultra-high-pressure | up to 37,700 psi (260 MPa) | HM28 sapphire transmitter |
| Vacuum / absolute | 0 to 145 psia (5 kPa to 1 MPa) | HM27 vacuum-absolute transmitter |
| Car / truck tire (consumer) | 30 to 35 / 100 to 120 psi | Consumer tire gauge territory, not an industrial transmitter use case |
| Pressure washer (consumer) | 1,500 to 3,000 psi | Short-burst pump, not a long-duty transmitter spec |
A note on the Chinese oil & gas convention: wellhead and pipeline operators write spec in dual units (for example 0–35 MPa / 0–5,075 psi), so a North American buyer reads the same datasheet without converting. Most export-grade HMK SKUs follow this dual-unit format whenever installation and operations teams sit in different regions.
When to Use PSI vs SI Units (Pa, kPa, MPa, bar)
PSI is the right choice on a spec sheet when the operator will read in PSI. Outside North America that is rarely the default; the BIPM SI brochure defines the metric units that most non-US engineering documents prefer. The convention by destination market is consistent enough that a spec engineer can decide before drafting:
| Spec destination | Default unit | Recommended secondary |
|---|---|---|
| North America (US, Canada, Mexico) | psi (psig / psia distinguished) | kPa or bar |
| Europe (incl. UK) | bar | kPa |
| China and most domestic projects | MPa (per GB/T 1226 convention) | psi for export |
| Middle East and Southeast Asia | dual MPa + psi | bar |
| Africa and parts of Latin America | kPa | bar |
The cleanest rule is dual labelling: a transmitter intended for export reads 0–1 MPa / 0–145 psi on its faceplate and calibration certificate, so installation and operations teams in different countries never argue about a converted number. Most export-grade HMK SKUs follow this practice. For mixed-region projects, agree on dual labelling in the requisition document; a single mis-read PSI on a high-pressure run costs far more than a clearer label.
Choosing a Pressure Transmitter by PSI Range
Once the operating PSI band is known, two more decisions follow: how much headroom to allow, and which sensing technology fits the chemistry and temperature of the medium.
For headroom, size the transmitter so normal operation sits in the middle two-thirds of full scale. A pump running at 80 psi belongs on a 0–150 psi unit, not a 0–100 psi one; the extra range absorbs surges. Spec sheets quote both an over-pressure limit (survivable) and a burst pressure (one-time only); never spec either as a duty point.
For technology, the choice tracks the PSI tier: capacitive diaphragm for sub-1-psi work, piezoresistive diffused-silicon for the broad 1 to 8,700 psi industrial band, and sapphire for ultra-high and corrosive service above 8,700 psi. The full technology-vs-application matrix lives in the pressure transmitter types pillar guide; the SKU links per tier are in the application table above. Map the PSI number to the right tier first, then pick the technology, an order that avoids most field selection mistakes.
FAQ
What is the PSI meaning?
PSI means pounds per square inch (more precisely pound-force per square inch, lbf/in²). It is the standard imperial unit of pressure on North American spec sheets, equal to 6.894757 kPa.
What does 30 PSI mean?
30 PSI is 30 pounds-force per square inch of contact area, about 207 kPa or 2.07 bar. The value appears in passenger-tire inflation, low-side refrigeration, plumbing supply lines, and light industrial equipment.
Is PSI a metric or imperial unit?
PSI is imperial / US customary, not metric. The matching SI units are pascal (Pa), kilopascal (kPa), and megapascal (MPa). Chinese and most non-US documents prefer MPa or bar; PSI usually appears as a secondary unit on export-grade spec sheets.
What is PSI in medical terms?
In medicine, PSI refers to the Pneumonia Severity Index, unrelated to the pressure unit covered here. Other same-acronym uses (Police Sub-Inspector, Pre-Sentence Investigation) are similarly out of scope.
Does HMK label its transmitters in PSI?
Yes, on request. HMK export-grade transmitters typically ship with dual-unit labelling (for example 0–1 MPa / 0–145 psi) on faceplate, datasheet, and calibration certificate. Single-unit labelling is available when the requisition document specifies it.
Match the PSI on Your Spec Sheet to a Transmitter
If you still need a unit conversion, the HMK online pressure unit converter handles every pair in this guide. If the operating PSI band is already set, the application table above links verified SKUs by tier; pick the row matching your service medium and range, then check for over-pressure headroom. For mixed-region projects, ask your HMK engineer for the dual-unit faceplate option.
— Lin Jun, Pressure Application Engineer, HMK
