Pressure unit conversion: psi, bar, kPa and feet of head
A field conversion table for psi, bar, kPa, inches of mercury and feet of head — plus the one conversion that changes with the fluid you are pumping.
One psi is 6.895 kPa, 0.0689 bar, 2.036 inches of mercury, 27.71 inches of water column, and — for water — 2.31 feet of head. Those five numbers cover almost everything you will meet on a plant floor. The one that catches people is the last one, because feet of head is the only conversion on the list that changes with the fluid you are pumping.
Pressure conversion table
| 1 of this | psi | kPa | bar | inH₂O | ft H₂O | inHg |
|---|---|---|---|---|---|---|
| psi | 1 | 6.8948 | 0.06895 | 27.708 | 2.3067 | 2.0360 |
| kPa | 0.14504 | 1 | 0.01 | 4.0187 | 0.33456 | 0.29530 |
| bar | 14.504 | 100 | 1 | 401.87 | 33.456 | 29.530 |
| MPa | 145.04 | 1000 | 10 | 4018.7 | 334.56 | 295.30 |
| inH₂O | 0.03609 | 0.24884 | 0.002488 | 1 | 0.08333 | 0.07349 |
| ft H₂O | 0.43353 | 2.9891 | 0.029891 | 12 | 1 | 0.88184 |
| inHg | 0.49115 | 3.3864 | 0.033864 | 13.608 | 1.1340 | 1 |
| kg/cm² | 14.223 | 98.067 | 0.98067 | 394.05 | 32.808 | 28.959 |
| atm | 14.696 | 101.325 | 1.01325 | 407.19 | 33.899 | 29.921 |
| torr (mmHg) | 0.019337 | 0.13332 | 0.0013332 | 0.53577 | 0.044603 | 0.03937 |
Water column values are referenced at about 60°F. Mercury is at 32°F. Nobody in the field will care about the difference; a metrology lab will.
Gauge versus absolute: psig, psia and the 14.7
This is where the expensive mistakes live. A gauge on a pipe reads zero when it is open to the room, because it is measuring the difference between the pipe and the atmosphere. That is gauge pressure, psig. Absolute pressure, psia, counts from a hard vacuum:
psia = psig + 14.7 (at sea level)
Steam tables, compressor ratio calculations, NPSH, vacuum work and gas law problems all want absolute. A gauge on the wall gives you gauge. Mixing them up is how a compressor gets sized wrong and how a pump that should cavitate on paper does not, or the reverse. The same split exists in metric: barg and bara, and the difference is 1.013 bar.
Vacuum adds a third convention. A vacuum gauge reading 20 inHg is telling you 20 inches below atmosphere, which is 9.9 psia, not 20 of anything absolute. Absolute vacuum instruments read in torr or millibar and count up from zero.
Field note
Atmospheric pressure is not 14.7 psia everywhere. At 3,000 feet it is about 13.2 psia, and a lot of the Inland Empire and the high desert sits well above sea level. That missing 1.5 psi comes straight off your available NPSH and off what a vacuum pump can pull. If a pump cavitates at one plant and not at another with the same setup, check the elevation before you check anything else.
psi to feet of head, and why the fluid matters
Pump curves are drawn in feet of head. Gauges read psi. Converting between them is the single most common conversion in rotating equipment work, and the formula is:
Head (ft) = psi × 2.31 ÷ specific gravity
For water, SG is 1.0 and 40 psi is 92 feet. For 66° Baumé sulfuric acid, SG is about 1.84, and that same 40 psi is only 50 feet. For a light hydrocarbon at SG 0.72, it is 128 feet.
This is why a pump curve is published in feet and not psi — the impeller does not know what it is moving. It imparts the same head to any fluid at the same speed. The discharge gauge reading changes with density; the head does not. If somebody hands you a pump problem with the discharge pressure in psi and no fluid density, you do not have enough information yet.
Where these units bite you in the field
- DP transmitters are almost always ranged in inches of water column. A transmitter ranged 0–100 inH₂O tops out at 3.61 psi — a tiny pressure that people mistakenly compare against a 0–100 psi gauge.
- Imported machinery lists everything in bar or kPa. A European machine calling for a 6 bar air supply wants 87 psi, not 60. Getting that backwards starves a clamp cylinder and everyone blames the valve.
- Hydraulics is where MPa shows up. A 21 MPa system is 3,045 psi — a normal industrial hydraulic pressure. A 210 bar system is the same thing said differently.
- Older Japanese and Mexican equipment uses kg/cm², which is close enough to bar to fool you: 1 kg/cm² is 14.22 psi against bar's 14.50. Under 2 percent apart, which is fine for a shop air reading and not fine for a relief valve setting.
- Compressed air in the US is psig on the gauge and scfm on the flow meter, but the scfm rating itself is referenced to a standard absolute pressure. Compare compressors on the same reference or the numbers mean nothing.
A shortcut worth memorizing
Two rules of thumb cover most of what you need at the machine, without a phone:
- bar to psi: multiply by 14.5. Or double it and add half again for a fast estimate — 7 bar is roughly 101 psi, actual 101.5.
- psi to feet of water: multiply by 2.3. A 60 psi city water main will push water up about 138 feet, which is why the top floors of a tall building need a booster.
Tools for the rest of it
For anything beyond those, our pressure unit converter handles all the units above in one field, including specific gravity for head conversions. If you are scaling a transmitter output, the 4–20 mA scaling calculator takes the range in whatever pressure unit you have and gives you the current. For DP flow work, the differential pressure flow calculator and the pipe velocity calculator cover the two conversions that usually come next.
When to call someone
Call if instrument readings and gauge readings disagree and you cannot find why, if a relief or safety valve setting is in doubt, or if a pump is not making the head its curve says it should. Keystone calibrates and troubleshoots pressure instrumentation on-site across Southern California — Long Beach, the South Bay, Orange County and the Inland Empire — and we support troubleshooting and repair remotely for plants nationwide.