Bar to PSI Conversion
1 bar = 14.5 psi, so the 1.5 bar a cold heating system should read is about 21.8 psi. Here is the full conversion table, the formula, and the pressures that actually matter on a boiler.
Boiler and heating gauges in the UK are marked in bar, but plenty of gauges, pumps and imported parts are marked in psi (pounds per square inch) — so engineers and homeowners regularly need to convert between the two.
The conversion is fixed: 1 bar is exactly 14.5038 psi. This guide gives you the common values at a glance, the formula to work out any value, and a note on what a healthy system pressure looks like in either unit.
Key takeaways
- 1 bar = 14.5038 psi (and 100 kPa). To go the other way, divide psi by 14.5038.
- 1.5 bar — a typical cold fill pressure — is about 21.8 psi.
- A sealed heating system should sit around 1.0–1.5 bar (14.5–21.8 psi) cold.
- The pressure relief valve on most domestic boilers lifts at 3 bar (about 43.5 psi).
- kW is a measure of power (output), not pressure — it does not convert to bar. See the note below.
Bar to psi conversion table
The table below covers the pressures you meet on heating and plumbing work. For any value in between, use the formula in the next section.
| Bar | PSI | kPa |
|---|---|---|
| 0.5 bar | 7.3 psi | 50 kPa |
| 0.75 bar | 10.9 psi | 75 kPa |
| 1.0 bar | 14.5 psi | 100 kPa |
| 1.25 bar | 18.1 psi | 125 kPa |
| 1.5 bar | 21.8 psi | 150 kPa |
| 2.0 bar | 29.0 psi | 200 kPa |
| 2.5 bar | 36.3 psi | 250 kPa |
| 3.0 bar | 43.5 psi | 300 kPa |
| 4.0 bar | 58.0 psi | 400 kPa |
The formula
To convert bar to psi, multiply by 14.5038. To convert psi to bar, divide by 14.5038.
For example, a gauge reading 1.3 bar is 1.3 × 14.5038 = 18.9 psi. A pump rated at 30 psi is 30 ÷ 14.5038 = 2.07 bar. For a rough mental check, multiplying bar by 14.5 (or just 'about 15') gets you close enough to sanity-check a reading.
Bar, psi and kPa
1 bar = 14.5038 psi = 100 kPa = 1,000 mbar. Bar and kPa are metric; psi is imperial. Atmospheric pressure at sea level is roughly 1 bar, so a gauge reading '1.5 bar' is 1.5 bar above atmospheric (gauge pressure), which is how boiler gauges are calibrated.
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What boiler and system pressure should read
A sealed central heating system should sit between about 1.0 and 1.5 bar (14.5–21.8 psi) when cold. As the system heats up the pressure rises by 0.3–0.5 bar, so seeing 1.5–2.0 bar hot is normal.
If the gauge sits near zero the system needs topping up via the filling loop; if it climbs past 2.5–3.0 bar and the pressure relief valve starts discharging, the expansion vessel or filling loop usually needs attention. The PRV itself is set to open at 3 bar on most domestic boilers — about 43.5 psi.
'kW to bar' — a common mix-up
There is no conversion from kW to bar: kW measures a boiler's heat output (power), while bar measures pressure. If you are sizing by output, convert kW to BTU/h instead (1 kW = 3,412 BTU/h) with the unit converter.
Why it matters on site
Getting the units right avoids the two classic mistakes: over-pressurising a system because a psi-marked gauge was read as bar, and condemning a healthy system because 21 psi was mistaken for a fault. A quick conversion keeps commissioning figures and handover paperwork consistent whichever gauge you are reading.
From guidance to action
Frequently asked questions
What is 1.5 bar in psi?
1.5 bar is 21.8 psi (1.5 × 14.5038 = 21.76 psi). That is a typical cold fill pressure for a sealed heating system.
How many psi is 1 bar?
1 bar is exactly 14.5038 psi, which also equals 100 kPa or 1,000 mbar.
What pressure should my boiler be in psi?
Most sealed systems should read 1.0–1.5 bar cold, which is about 14.5–21.8 psi. It is normal for the pressure to rise to 1.5–2.0 bar (22–29 psi) once the heating is hot.
Does kW convert to bar?
No. kW is a measure of power (a boiler's heat output) and bar is a measure of pressure — they are not interchangeable. To work with a boiler's output in imperial units, convert kW to BTU/h (1 kW = 3,412 BTU/h).
Related guides
Expansion Vessel Sizing for Sealed Systems
How to size an expansion vessel from system volume, charge pressure and acceptance factor.
Why Your Boiler Keeps Losing Pressure
The common causes of a boiler losing pressure — leaks, a discharging PRV and a failed expansion vessel — and how to diagnose each.
The Benchmark Certificate and Boiler Warranties
What Benchmark is, how it protects the warranty, and completing it correctly at commissioning.
