Pressure Units Explained: PSI, Bar, kPa, MPa & Feet of Head (+ Free Converter)

Last Updated: 2026-11-06

Quick Answer: The Pressure Units Engineers Actually Use

Every pressure unit is a force spread over an area, so every unit converts to every other one by a fixed number. The seven you will meet on datasheets are psi (pounds per square inch, United States), bar (Europe and most pump catalogues), kPa / MPa (SI, used on Chinese, Japanese and Korean instruments), atm (reference only), mmHg / torr (medical and vacuum), inH2O (HVAC and low-pressure air), and feet or meters of head (pumps, tanks and level sensors). The numbers worth memorizing are:

The converter below does the arithmetic for you, including the specific gravity correction that head units need. The rest of the article explains where the numbers come from, which unit each industry expects, and how to turn a pressure in any unit into the right transmitter range.

What a Pressure Unit Actually Measures

Pressure is force divided by area: P = F / A. The SI unit is the pascal, one newton per square meter, which is tiny; a sheet of paper lying on a desk exerts about 1 Pa. That is why industry uses kilopascals (1 kPa = 1,000 Pa) and megapascals (1 MPa = 1,000,000 Pa). The bar was defined as 100,000 Pa because it is within 1.3% of standard atmospheric pressure and gives convenient numbers for process work. The psi is a pound-force over a square inch, and the imperial world has lived with it long enough that most US pump, compressor and hydraulic catalogues still quote nothing else.

Head units are different in kind. Feet of water, meters of water, inches of water column and millimeters of mercury are not force-over-area; they are the height of a liquid column that would produce that pressure. That is exactly why they are useful to pump and tank engineers, and exactly why they need a density correction for any liquid other than the reference one. Keep that distinction in mind and every conversion in this article follows from one formula.

The Head Pressure Formula: Why 1 psi = 2.31 Feet of Water

A column of liquid pushes down on its base with a pressure that depends only on the column height, the liquid density and gravity:

P = ρ · g · h
P  = pressure (Pa)
ρ  = density (kg/m³)   water at 4 °C = 999.97 kg/m³
g  = 9.80665 m/s²
h  = column height (m)

Work it in US units. Water weighs 62.4 lb per cubic foot. A 1 ft tall column standing on a 1 ft × 1 ft base therefore pushes down with 62.4 lb spread over 144 square inches:

62.4 lb/ft³ ÷ 144 in²/ft² = 0.433 psi per foot of water
1 / 0.433 = 2.31 feet of water per psi

So the two working formulas for any liquid are:

psi     = ft head × SG / 2.31
ft head = psi × 2.31 / SG

where SG is the specific gravity, the liquid density divided by water density. In metric form, bar = m head × SG / 10.2 and kPa = m head × SG × 9.807.

Temperature matters at the third decimal. Water at 4 °C has a density of 999.97 kg/m³; at 20 °C it is 998.2 kg/m³, a 0.18% difference. The 2.31 and 10.2 constants assume 4 °C. A hydrostatic level transmitter calibrated in a 20 °C lab and installed in a 4 °C well will read about 0.2% high, which is below the accuracy class of most transmitters but not below a ±0.1% FS instrument. If you are reconciling tank inventory, use the density at the actual process temperature.

Pressure Unit Conversion Chart

Read the chart left to right: one unit of the row heading equals the number shown in each column.

Water column values are at 4 °C; mmHg is at 0 °C and standard gravity, which is why 760 mmHg equals exactly 1 atm.

Quick Reference Numbers to Memorize

The kg/cm² line deserves a note. Older Japanese and some Chinese gauges are still marked in kgf/cm², which is 98.07 kPa, not 100 kPa. When a customer sends a drawing with “10 K” on it, ask whether they mean 10 kgf/cm² (0.98 MPa) or 10 bar (1.0 MPa) before you order a transmitter range.

Worked Examples

Example 1: Pump head to psi. A centrifugal pump curve shows 150 ft of total dynamic head at the duty point, pumping clean water.

psi = 150 ft × 1.0 / 2.31 = 64.9 psi  (4.48 bar)

If the same pump moves 35% brine at SG 1.20, the discharge pressure rises to 150 × 1.20 / 2.31 = 77.9 psi, which is why pump specifications quote head, not pressure; the head stays the same while the pressure changes with the liquid.

Example 2: A submersible level transmitter reads 35 psi. The transmitter sits on the bottom of a water reservoir and outputs gauge pressure.

ft head = 35 psi × 2.31 / 1.0 = 80.9 ft of water  (24.6 m)

This is precisely how a submersible level transmitter works: it measures the hydrostatic pressure at the bottom and the display or PLC converts it to height using the density you enter. The YW01 is available with 4–20 mA plus HART or RS485 output over ranges from 0.5 m to 300 m of water, so a 35 psi (24.6 m) reading would sit comfortably inside a 0–30 m range.

Example 3: Diesel column, SG 0.85, 10 m tall. A tank farm wants to know the pressure a bottom-mounted sensor will see.

P = ρ · g · h = (0.85 × 999.97) × 9.80665 × 10 = 83,360 Pa = 83.4 kPa = 0.834 bar = 12.1 psi

A 0–100 kPa transmitter range is right; a 0–1 bar range is also fine. A 0–1 MPa range would use only 8% of its span and lose accuracy. Our fuel tank level monitoring guide covers the density error this introduces when the tank changes from diesel to gasoline.

Example 4: Boiler steam drum at 1.0 MPa. A Chinese datasheet lists the range as 0–1.0 MPa and the US site engineer wants psi.

1.0 MPa × 145.04 = 145 psi  (10 bar, 10.2 kgf/cm²)

Example 5: HVAC duct static pressure of 2 inH2O. The air handler specification calls for 2 inches of water column across the filter bank.

2 inH2O × 249.09 = 498 Pa = 0.498 kPa = 0.072 psi = 4.98 mbar

Pressures this small are below the useful span of a general-purpose transmitter. They are measured with a micro differential pressure transmitter whose spans are a few hundred pascals to a few kilopascals and whose display can be ordered in Pa, kPa or inH2O.

Which Unit Each Industry and Region Uses

For a buyer in Ningbo quoting a customer in Houston, the practical rule is to put both units on every document: “0–1.6 MPa (0–232 psi)”. It costs nothing and it is the single most effective way to avoid the ordering mistakes in the next section.

Five Unit Mistakes That Cost Money on Instrument Orders

1. kPa and MPa confused. They are a factor of 1,000 apart. A request for “0–100 kPa” that is typed as “0–100 MPa” buys a 14,500 psi transmitter for a 14.5 psi job. The instrument will arrive, read zero all day, and be useless. Write out the unit in full on the purchase order.
2. psig ordered when psia was needed. A gauge transmitter zeros itself to local atmosphere; an absolute transmitter zeros to vacuum. For a vacuum pump, a barometric reference or a sealed vessel that must read the same at sea level and at 2,000 m altitude, you need absolute. The difference is explained in detail in our comparison of gauge vs absolute vs differential pressure.
3. Head converted without specific gravity. A 10 m column of 98% sulfuric acid at SG 1.84 produces 1.80 bar, not 0.98 bar. Ordering a 0–1 bar transmitter for it will over-range the sensor on day one.
4. Modbus register units assumed. A digital transmitter sends a number, not a unit. Some firmware sends the value in the display unit, some always in kPa, some as a percentage of span. Read the register map and confirm with a known pressure before you trust the SCADA trend.
5. kgf/cm² taken as bar. The 2% error is small on a 10 bar line but it is enough to make a calibration certificate fail, and on a 400 bar hydraulic circuit it is 8 bar of mismatch.

How to Choose a Transmitter Range in Any Unit

Pressure transmitters are specified by a range, and the unit on the datasheet is only a label. What matters is that the working pressure sits in the right part of the span. The rule most instrument engineers use is:

Normal operating pressure ≈ 50–80% of the transmitter span
Maximum process pressure   < 100% of span
Peak / surge pressure      < overload rating (typically 1.5–2× span)

Convert the process pressure to the datasheet unit first, then pick the range.

Three details from these rows are worth a sentence each.

• The YL805 covers -100 kPa to 70 MPa, which is roughly -14.5 to 10,150 psi, in ±0.5% FS accuracy with 0–5 V, 0–10 V, 4–20 mA or RS485 output. That one family answers most “what range do I need” questions in water, air and general machinery.
• The YL0500 and YL2088 add HART and higher accuracy classes (down to ±0.1% FS on the YL0500) and reach 100 MPa, which is the hydraulic and high-pressure process territory where psi numbers get large and unit mistakes get expensive.
• A pressure switch does not transmit a value; it closes a contact at a setpoint. The YLK6710 shows the live pressure on its display, so you can set the cut-in and cut-out points in psi, bar or kPa directly on the device instead of converting on paper.

If you are not sure whether a gauge, absolute or differential version is right, our types of pressure transmitters guide walks through each, and the 8-step pressure transmitter selection guide covers accuracy class, process connection and wetted materials once the range is settled. The full pressure sensors and transmitters category lists every model with its range in both kPa and MPa.

Pressure, Flow and the Units in Between

Pressure units show up in one more place that catches people out: flow calculations. Pipe friction loss is quoted in psi per 100 ft in the US and in bar per 100 m or m head per 100 m in Europe; orifice plate sizing works in differential pressure, usually kPa or inH2O; and pump curves plot head against flow. If you need to go from a pressure reading to a flow rate, or to size a differential pressure transmitter across an orifice, our guide to flow rate vs pressure in pipes carries the same unit discipline through Bernoulli and Hazen-Williams.

Sourcing Note

SIBXSUN supplies pressure transmitters, switches and hydrostatic level sensors sourced from partner factories in China, and we quote every range in both the SI unit the factory uses and the unit your site uses. If your drawing says psig, we convert it to kPa on the order; if your PLC expects inH2O, we confirm the display unit before the instrument ships. When you send an enquiry, include the normal and maximum process pressure with its unit, the liquid or gas and its specific gravity, and whether you need gauge or absolute. With those three facts we can confirm a range within 24 hours. The pressure transmitter supplier page lists lead times and MOQ for the YL series.

Frequently Asked Questions

How do you convert feet of head to psi?

Divide feet of head by 2.31 for water: psi = ft head / 2.31. For any other liquid multiply by its specific gravity first: psi = ft head × SG / 2.31. A 100 ft column of water is 43.3 psi; the same column of diesel (SG 0.85) is 36.8 psi. The 2.31 constant comes from water weighing 62.4 lb per cubic foot spread over 144 square inches.

What is the difference between psi, psia and psig?

psi is just the unit, pounds per square inch. psig is gauge pressure, measured relative to the local atmosphere, so an open tank reads 0 psig. psia is absolute pressure, measured relative to a perfect vacuum, so the same open tank reads about 14.7 psia at sea level. Most industrial transmitters are gauge; order an absolute version only for vacuum, barometric or altitude-independent work.

Is bar the same as atm?

No, but they are close. 1 atm is defined as 101.325 kPa, while 1 bar is exactly 100 kPa, so 1 atm = 1.01325 bar. The 1.3 percent difference does not matter for a rough estimate, but it does matter for calibration certificates and for absolute transmitters, where the atmosphere is the zero reference.

How many psi is 1 MPa?

1 MPa equals 145.038 psi, or 10 bar. Chinese and Japanese datasheets quote ranges in MPa and kPa; US datasheets quote psi. A common ordering error is mixing kPa and MPa, which are a factor of 1,000 apart. Always write the unit next to every number on a purchase order, for example 0–1.6 MPa (0–232 psi).

What is 1 bar in meters of water head?

1 bar equals 10.197 meters of water column at 4 °C, which is usually rounded to 10.2 m. Going the other way, 1 meter of water is 9.807 kPa or 0.098 bar. For liquids other than water, multiply the head by specific gravity: 10 m of diesel at SG 0.85 produces 0.83 bar, not 0.98 bar.

Have a pressure in one unit and a datasheet in another? Send us the process pressure, the liquid and the output you need and we will confirm the transmitter range in both units, with calibration data included.