mmWC to bar — the conversion
mmWC — millimetres of water column, also written mmH₂O or mmWG — is a manometer unit. It states pressure as the height of a water column that pressure would support, which is exactly how the original instruments measured it. One millimetre of water is a very small pressure, which is the point: it gives usable numbers for measurements that would be awkward fractions of a bar.
Reference table
| mmWC | bar | kPa | mbar | psi |
|---|---|---|---|---|
| 10 | 0.000981 | 0.0981 | 0.981 | 0.01422 |
| 25 | 0.002452 | 0.2452 | 2.452 | 0.03556 |
| 50 | 0.004903 | 0.4903 | 4.903 | 0.07112 |
| 100 | 0.009807 | 0.9807 | 9.807 | 0.14223 |
| 150 | 0.014710 | 1.4710 | 14.710 | 0.21335 |
| 200 | 0.019613 | 1.9613 | 19.613 | 0.28447 |
| 250 | 0.024517 | 2.4517 | 24.517 | 0.35558 |
| 300 | 0.029420 | 2.9420 | 29.420 | 0.42670 |
| 400 | 0.039227 | 3.9227 | 39.227 | 0.56893 |
| 500 | 0.049033 | 4.9033 | 49.033 | 0.71117 |
| 750 | 0.073550 | 7.3550 | 73.550 | 1.06675 |
| 1,000 | 0.098067 | 9.8066 | 98.066 | 1.42233 |
| 1,500 | 0.147100 | 14.7100 | 147.100 | 2.13350 |
| 2,000 | 0.196133 | 19.6133 | 196.133 | 2.84467 |
| 3,000 | 0.294199 | 29.4199 | 294.199 | 4.26700 |
| 5,000 | 0.490333 | 49.0333 | 490.332 | 7.11167 |
| 10,000 | 0.980665 | 98.0665 | 980.665 | 14.22334 |
Where these ranges come from
If you are converting mmWC, you are almost certainly working on one of these measurements — and the typical ranges are narrow enough to be worth knowing:
| Measurement | Typical range | In bar |
|---|---|---|
| Furnace / boiler draft | −50 to +50 mmWC | ±0.0049 |
| Clean room pressurisation | 1 to 15 mmWC | to 0.0015 |
| Filter differential | 0 to 250 mmWC | to 0.0245 |
| Fan discharge | 0 to 1 000 mmWC | to 0.098 |
| Low-range DP transmitter | 0 to 500 mmWC | to 0.049 |
| Duct / ventilation | 0 to 100 mmWC | to 0.0098 |
Field notes
- Useful approximations. 1 mbar ≈ 10.2 mmWC, and 100 mmWC is just under 1 kPa. Both are close enough for a sanity check, though not for setting calibration limits — use the exact factor for those.
- mmWC is temperature-referenced in principle. The 9.80665 Pa figure assumes water at 4 °C and standard gravity. Nobody states this on a datasheet, but it is the convention every instrument manufacturer uses, so the factor is effectively fixed.
- The unit says nothing about the reference basis. A 250 mmWC range is almost always gauge or differential, but mmWC itself is neither — see the gauge to absolute converter when the basis matters.
- inWC is the US equivalent at 25.4 mmWC each, so 1 inWC = 249.089 Pa. American-specified equipment uses it for exactly the same measurements.
- Ranging a low-range DP transmitter? Those ranges are small enough that static pressure effect and mounting position matter — see the hydrostatic pressure calculator and the pressure units guide.
Frequently asked questions
How many mmWC are in 1 bar?
10,197.2 mmWC make up 1 bar. The factor comes from 1 mmWC = 9.80665 Pa, so 100,000 Pa ÷ 9.80665 = 10,197.2.
What is 250 mmWC in bar?
250 mmWC equals 0.024517 bar, which is 2.4517 kPa or 24.517 mbar. Ranges like 250 mmWC are typical of filter differential and furnace draft measurement.
Is mmWC the same as mmH2O?
Yes. mmWC (millimetres of water column) and mmH₂O are the same unit, and both equal 9.80665 Pa using water at 4 °C and standard gravity. Some datasheets write mmWG for water gauge, which is also the same.
How do I convert mmWC to psi?
Divide by 703.07. One psi equals 703.07 mmWC, so 1,000 mmWC is 1.42233 psi. Going the other way, multiply psi by 703.07.
Is mmWC gauge or absolute?
Neither by itself — mmWC is a unit, not a reference basis. Low-range instrumentation almost always uses it for gauge or differential pressure, so the figure is usually gauge or a differential, but the unit itself carries no reference.
Provided for reference and education. For every pressure unit at once, use the full pressure unit converter. See our disclaimer.