How pressure unit conversion works
Every pressure unit is just a different-sized ruler measuring the same physical thing: force per unit area. The SI unit is the pascal (1 Pa = 1 N/m²), and every other unit has a fixed factor to it — so converting between any two units means going through pascals:
The everyday units cluster around three families: SI multiples (Pa, kPa, MPa, bar), force-based imperial and metric units (psi, kg/cm²), and manometer units that state pressure as the height of a liquid column it can support (mmH₂O/mmWC, inH₂O, mmHg, inHg).
Conversion factors from 1 bar
| 1 bar equals | Value |
|---|---|
| kPa | 100 |
| MPa | 0.1 |
| psi | 14.5038 |
| kg/cm² | 1.01972 |
| atm | 0.98692 |
| mmHg (Torr) | 750.06 |
| inHg | 29.53 |
| mmH₂O (mmWC) | 10 197 |
| inH₂O | 401.46 |
Field notes — where conversions go wrong
- kg/cm² ≠ bar. They are close (1 kg/cm² = 0.9807 bar) but the 2% difference is 20 kPa at 10 bar — far outside any calibration tolerance. Old gauges marked kg/cm² paired with new transmitters ranged in bar cause endless confusion in Indian and Japanese-built plants.
- Water column units are temperature-referenced. The factors here use water at 4 °C and standard gravity (1 inH₂O = 249.089 Pa). US datasheets sometimes reference 60 °F or 68 °F, shifting the factor by up to ~0.5%. For DP transmitter ranges, check which convention the datasheet states.
- mmHg vs Torr: for engineering purposes identical (they differ by less than 0.000002%).
- Watch the g/a suffix. Converting the unit never converts the reference: 5 barg is 5.98 bara at sea level. Use the gauge ↔ absolute converter when the reference changes.
Worked example
A filter DP alarm is set at 250 mmWC, but the new transmitter must be configured in kPa:
- 250 mmH₂O × 9.80665 Pa = 2451.7 Pa
- 2451.7 ÷ 1000 = 2.452 kPa
Frequently asked questions
Is kg/cm² the same as bar?
No — they differ by about 2%. 1 kg/cm² equals 0.980665 bar. Treating them as equal is a classic error that matters at calibration tolerances.
How many psi is 1 bar?
1 bar = 14.5038 psi. Going the other way, 1 psi = 0.0689476 bar.
What is mmWC and how does it relate to bar?
mmWC (millimetres of water column) is the same unit as mmH₂O, common in India for low pressures like draft and filter DP. 1 bar equals approximately 10,197 mmWC.
Are these gauge or absolute pressures?
Unit conversion is independent of the reference — 1 barg = 14.5 psig and 1 bara = 14.5 psia. To convert between gauge and absolute you add or subtract atmospheric pressure; use our gauge-absolute converter for that.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.