To convert centimeters of water to millibars, multiply by 0.980665. That factor is the whole relationship: both units measure pressure, so the conversion is a proportion with no offset and no substance-specific constant.
The meteorologist’s unit — sea-level pressure averages 1013.25 mbar. The unit ventilator and CPAP pressures are set in.
Gauge versus absolute. A tyre gauge reading 32 psi means 32 psi above atmospheric, which is about 46.7 psi absolute. Converting the number is straightforward; remembering which zero it was measured from is where the mistakes happen. N/mm² and MPa are the same size, so a drawing quoting either is quoting the same stress. The tables underneath give both a plain step-by-step lookup and a list of real quantities expressed in millibars, so you can sanity-check whether your answer is a plausible size.
The formula
centimeters of water- The amount you are converting, in centimeters of water
0.980665- The exact number of millibars in one centimeter of water
millibars- The converted result
How it works, step by step
- Enter the value in centimeters of water.
- It is multiplied by 0.980665, the exact factor between the two units.
- The answer is shown in millibars, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
A car tyre: 2250 cmH₂O in millibars
Take a car tyre, roughly 2250 cmH₂O. The conversion is a single multiplication — 2250 × 0.980665 — giving 2206.5 mbar. Because the relationship is proportional, doubling the input doubles the answer to 4412.99 mbar.
A steam boiler: 1.02e+04 cmH₂O in millibars
At 1.02e+04 cmH₂O (a steam boiler), the same quantity in millibars is 1.02e+04 × 0.980665 = 10,002.8 mbar. One centimeter of water is 0.980665 millibars, so the numbers shrink as expected.
How to read your score
Frequently asked questions
How many millibars are in one centimeter of water?
Exactly 0.980665. Every figure on this page is that one number multiplied by your input, so the only rounding is at the display step.
What is the formula to convert centimeters of water to millibars?
millibars = centimeters of water × 0.980665. Reversed, centimeters of water = millibars ÷ 0.980665, which is the same as multiplying by 1.01971621298.
Which is the bigger unit, the centimeter of water or the millibar?
The millibar. One centimeter of water is only 0.980665 of a millibar, so it takes 1.01972 centimeters of water to make one millibar.
What does this quantity actually measure?
Both units measure pressure. That is why a single factor is enough: there is no temperature offset and no material property involved, unlike conversions that cross between different quantities.
What is a centimeter of water?
The unit ventilator and CPAP pressures are set in.
What is a millibar?
The meteorologist’s unit — sea-level pressure averages 1013.25 mbar.
Why does my answer differ from another converter?
Almost always because the other tool rounded the factor before multiplying. This page keeps full double precision through the calculation and rounds only for display, and it prints the factor it used so you can see where any difference came from.
Is there a quick way to do cmH₂O to mbar in my head?
Plenty of people do this in their head: divide by 1 instead of using the exact factor 0.980665. At 100 that shortcut gives 100 millibars, off by 1.933 millibars or about 1.97%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Centimeters Of Water to millibars reference tables
| Centimeters Of Water | Millibars |
|---|---|
| 0.5 cmH₂O | 0.490333 mbar |
| 1 cmH₂O | 0.980665 mbar |
| 2 cmH₂O | 1.96133 mbar |
| 3 cmH₂O | 2.94199 mbar |
| 5 cmH₂O | 4.90332 mbar |
| 10 cmH₂O | 9.80665 mbar |
| 20 cmH₂O | 19.6133 mbar |
| 25 cmH₂O | 24.5166 mbar |
| 50 cmH₂O | 49.0333 mbar |
| 75 cmH₂O | 73.5499 mbar |
| 100 cmH₂O | 98.0665 mbar |
| 250 cmH₂O | 245.166 mbar |
| 500 cmH₂O | 490.332 mbar |
| 1000 cmH₂O | 980.665 mbar |
| 5000 cmH₂O | 4903.32 mbar |
Exact factor: 1 centimeter of water = 0.980665 millibars.
| Quantity | cmH₂O | mbar |
|---|---|---|
| Sea-level air | 1033 cmH₂O | 1013 mbar |
| A car tyre | 2250 cmH₂O | 2206 mbar |
| Blood pressure, systolic | 163.1 cmH₂O | 160 mbar |
| A bicycle tyre | 6328 cmH₂O | 6205 mbar |
| A domestic water main | 4079 cmH₂O | 4000 mbar |
| A steam boiler | 1.02e+04 cmH₂O | 1e+04 mbar |
Published reference values, converted with the same factor this page uses.