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 |
psig to barg — the conversion engineers search for most
Plants built to American specifications state pressure in psig while the rest of the instrumentation runs in barg, so this conversion turns up constantly during commissioning and ranging work:
The gauge reference passes straight through. Converting a unit does not convert the reference basis — 100 psig is 6.895 barg, both still measured relative to atmosphere. Only adding or removing atmospheric pressure changes gauge to absolute.
| psig | barg | kPa (gauge) |
|---|---|---|
| 10 | 0.690 | 68.9 |
| 15 | 1.034 | 103.4 |
| 30 | 2.068 | 206.8 |
| 50 | 3.447 | 344.7 |
| 100 | 6.895 | 689.5 |
| 150 | 10.34 | 1034 |
| 300 | 20.68 | 2068 |
mmWC to bar — low-range instrumentation units
Millimetres of water column (mmWC, identical to mmH₂O) is the working unit for the smallest pressures in a plant: furnace draft, filter differential, room pressurisation and low-range DP transmitters. It is especially common on Indian and Japanese-specified equipment.
| mmWC | bar | kPa | Typical use |
|---|---|---|---|
| 50 | 0.00490 | 0.490 | furnace draft |
| 100 | 0.00981 | 0.981 | room pressurisation |
| 250 | 0.02452 | 2.452 | filter DP |
| 500 | 0.04903 | 4.903 | low-range DP |
| 1000 | 0.09807 | 9.807 | fan discharge |
| 10 197 | 1.000 | 100.0 | — |
A useful field shortcut: 1 mbar ≈ 10.2 mmWC, and 100 mmWC is a shade under 1 kPa. Both are close enough for sanity checks, though not for setting calibration limits.
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.
How do I convert psig to barg?
Divide by 14.5038: barg = psig ÷ 14.5038. So 100 psig is 6.895 barg, and 1 barg is 14.504 psig. The gauge reference carries across unchanged — converting the unit never converts the reference.
What is 250 mmWC in bar?
250 mmWC equals 0.02452 bar, or 2.452 kPa. The factor is 1 mmWC = 9.80665 Pa, so 1 bar = 10,197 mmWC. Low ranges like 250 mmWC are typical of draft, filter differential and furnace pressure measurements.
How many mmWC are in 1 bar?
10,197 mmWC (millimetres of water column) make up 1 bar. For quick field estimates, 1 mbar is very close to 10.2 mmWC, and 100 mmWC is just under 1 kPa.
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.