One inch of water is exactly 2.490889 millibars, so converting inches of water to millibars is a single multiplication. Enter any figure and the result appears as you type — the arithmetic runs in your browser, so nothing is sent anywhere.
The meteorologist’s unit — sea-level pressure averages 1013.25 mbar. The low-pressure unit for duct static pressure and gas appliances in the US.
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. Two tables follow: rounded steps for quick lookup, and a set of real reference quantities converted into millibars, which is usually more useful than the bare number.
The formula
inches of water- The amount you are converting, in inches of water
2.490889- The exact number of millibars in one inch of water
millibars- The converted result
How it works, step by step
- Enter the value in inches of water.
- It is multiplied by 2.490889, 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
Sea-level air: 407 inH₂O in millibars
At 407 inH₂O (sea-level air), the same quantity in millibars is 407 × 2.49089 = 1013.79 mbar. One inch of water is 2.49089 millibars, so the numbers grow as expected.
A steam boiler: 4010 inH₂O in millibars
At 4010 inH₂O (a steam boiler), the same quantity in millibars is 4010 × 2.49089 = 9988.46 mbar. One inch of water is 2.49089 millibars, so the numbers grow as expected.
How to read your score
Frequently asked questions
How many millibars are in one inch of water?
Exactly 2.490889. 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 inches of water to millibars?
millibars = inches of water × 2.490889. Reversed, inches of water = millibars ÷ 2.490889, which is the same as multiplying by 0.401463092093.
Which is the bigger unit, the inch of water or the millibar?
The inch of water. One inch of water equals 2.49089 millibars.
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 inch of water?
The low-pressure unit for duct static pressure and gas appliances in the US.
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 inH₂O to mbar in my head?
Plenty of people do this in their head: multiply by 2.5 instead of using the exact factor 2.490889. At 100 that shortcut gives 250 millibars, off by 0.9111 millibars or about 0.37%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Inches Of Water to millibars reference tables
| Inches Of Water | Millibars |
|---|---|
| 0.5 inH₂O | 1.24544 mbar |
| 1 inH₂O | 2.49089 mbar |
| 2 inH₂O | 4.98178 mbar |
| 3 inH₂O | 7.47267 mbar |
| 5 inH₂O | 12.4544 mbar |
| 10 inH₂O | 24.9089 mbar |
| 20 inH₂O | 49.8178 mbar |
| 25 inH₂O | 62.2722 mbar |
| 50 inH₂O | 124.544 mbar |
| 75 inH₂O | 186.817 mbar |
| 100 inH₂O | 249.089 mbar |
| 250 inH₂O | 622.722 mbar |
| 500 inH₂O | 1245.44 mbar |
| 1000 inH₂O | 2490.89 mbar |
| 5000 inH₂O | 12,454.4 mbar |
Exact factor: 1 inch of water = 2.490889 millibars.
| Quantity | inH₂O | mbar |
|---|---|---|
| Sea-level air | 406.8 inH₂O | 1013 mbar |
| A car tyre | 885.8 inH₂O | 2206 mbar |
| Blood pressure, systolic | 64.23 inH₂O | 160 mbar |
| A bicycle tyre | 2491 inH₂O | 6205 mbar |
| A domestic water main | 1606 inH₂O | 4000 mbar |
| A steam boiler | 4015 inH₂O | 1e+04 mbar |
Published reference values, converted with the same factor this page uses.