How this instrument works
A pound per square inch is exactly what its name says: one avoirdupois pound of force, 4.4482216152605 newtons under the standard gravity fixed in 1901, spread across one square inch, 0.00064516 square metres by the 1959 inch. Multiply those together and one psi comes out to 6894.75729317 Pa, a terminating decimal built entirely from defined quantities — nothing about it was ever measured. American residential water systems have used psi for pressure switches, tanks and gauges since long before metrication was even a conversation, and the unit persists there today.
Kilopascals answer the same question in SI terms, and every plumbing code, pressure-tank datasheet and switch specification written outside psi's home territory uses them. A submersible well pump's discharge, the pre-charge in a bladder tank, and the cut-in and cut-out points on a pressure switch are all genuinely the same kind of quantity whichever unit labels them — the conversion carries no ambiguity, just the fixed 6.89475729317 kPa per psi that falls out of the pound and inch definitions above.
Residential well systems in North America are almost always specified by a pressure-switch pairing: the pump's cut-in pressure, where it switches on, and its cut-out pressure, where it switches off — commonly 20/40, 30/50 or 40/60 psi. A pressure tank imported from a European manufacturer, or a replacement switch sourced from an international supplier, often lists its own working range and pre-charge specification in kPa or bar instead, leaving a homeowner or well contractor to convert one system's psi figures into the other's metric ones before anything gets installed correctly.
- Enter your reading into the PSI (psi) field; it opens at 30, the typical cut-in pressure on a '30/50' well-pump switch.
- Read the Kilopascals (kPa) field below — recalculated instantly as you type.
- Setting a switch from a metric datasheet? Convert both the cut-in and cut-out figures separately, then confirm the gap between them matches your original psi pairing.
- Checking a bladder tank's pre-charge? Set the tank's air pre-charge about 2 psi below the pump's cut-in pressure, and convert that same margin in kPa if working from a metric gauge.
- Going the other way, from kPa to psi, divide your figure by 6.89475729317, or multiply by 0.14503773773.
Worked example — a 30 psi cut-in on a metric-labelled switch
A homeowner replacing a failed well-pump pressure switch buys one from an overseas supplier whose packaging lists working pressures only in kilopascals, while the old switch — and every reference chart for '30/50' systems — is marked in psi. Enter 30 into PSI (psi) and Kilopascals (kPa) reads 206.842718795, the figure to look for on the new switch's kPa scale or adjustment dial.
The matching cut-out figure, 50 psi, converts the same way to 344.737864659 kPa, so the target working range becomes roughly 207 to 345 kPa rather than 30 to 50 psi. Setting the switch to that converted range reproduces the original 30/50 psi behaviour exactly, since nothing about the pump or plumbing changed — only the scale printed on the adjustment screw did.
Questions
Is 6.89475729317 an exact conversion factor?
It is exact, to the eleven decimal places carried here. One psi is defined as precisely 6894.75729317 Pa — a pound-force of 4.4482216152605 N divided by a square inch of 0.00064516 m², both fixed by treaty rather than measured — so dividing by 1000 to reach kilopascals loses nothing at all. The figure never needs revising; only how many of its digits get displayed changes from one calculator to the next.
What do 20/40, 30/50 and 40/60 actually mean on a well-pump switch?
They're the cut-in and cut-out pressures, in psi, at which a pressure switch turns a well pump on and off. At 30/50, the pump starts when tank pressure drops to 30 psi and stops once it climbs back to 50 psi, giving a 20 psi working range — 137.895 to 344.738 kPa on a metric gauge. Higher pairings like 40/60 push more pressure through the household plumbing and usually need a correspondingly rated pressure tank and pump to match.
Why do American well systems still use psi when most of the world uses bar or kPa?
Because domestic well-water plumbing developed almost entirely within the United States' imperial-unit tradition, and pressure switches, tanks and gauges have been manufactured and stocked in psi for generations — changing now would mean re-tooling an entire supply chain with no functional benefit. Imported components increasingly print both scales, but psi remains the default on hardware-store shelves and in the reference charts most well contractors use daily.
How does psi relate to the height of a column of water?
Directly, since a static column of water exerts pressure proportional to its height: one psi supports roughly 2.31 feet of water column. A pressure tank cutting out at 50 psi is therefore capable of pushing water about 115 feet straight up before running out of pressure, a useful sanity check when a well system needs to serve a building well above the pump.
Does elevation change what pressure switch setting is needed?
Not the switch's psi numbers themselves, but it can change what setting actually delivers usable pressure at the tap. A fixture 100 feet higher than the pressure tank loses about 43 psi of static pressure just from elevation, using the 2.31 feet-per-psi relationship, so a system feeding upper floors or a hillside property sometimes needs a higher-pressure switch pairing than one serving a single-storey home at the same source pressure.
How many decimal places matter when setting a pressure switch?
Usually none — most mechanical pressure switches adjust in whole psi or a few kPa at best, and their own repeatability runs a few percent, so 206.842718795 kPa and 207 kPa describe an identical setting in practice. Full precision earns its keep only when converting several figures in sequence, such as an entire cut-in/cut-out pairing plus a tank pre-charge, so small roundings don't stack into a setting that no longer matches the intended psi range.