To convert inches of mercury to centimeters of water, multiply by 34.53155767. That factor is the whole relationship: both units measure pressure, so the conversion is a proportion with no offset and no substance-specific constant.
US aviation and weather altimeter setting unit. 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. Below the calculator there is a step table for the values people look up most, and a second table putting real pressure figures into centimeters of water so the number you get has something to be compared against.
The formula
inches of mercury- The amount you are converting, in inches of mercury
34.5315576675- The exact number of centimeters of water in one inch of mercury
centimeters of water- The converted result
How it works, step by step
- Enter the value in inches of mercury.
- It is multiplied by 34.5315576675, the exact factor between the two units.
- The answer is shown in centimeters of water, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
A domestic water main: 118 inHg in centimeters of water
A domestic water main comes to about 118 inHg. Multiplying by the factor: 118 × 34.5316 = 4074.72 cmH₂O. Dividing the answer back by 34.5316 returns the figure you started with, which is the quickest way to check you have the factor the right way up.
A steam boiler: 295 inHg in centimeters of water
At 295 inHg (a steam boiler), the same quantity in centimeters of water is 295 × 34.5316 = 10,186.8 cmH₂O. One inch of mercury is 34.5316 centimeters of water, so the numbers grow as expected.
How to read your score
Frequently asked questions
How many centimeters of water are in one inch of mercury?
Exactly 34.5315576675. 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 mercury to centimeters of water?
centimeters of water = inches of mercury × 34.5315576675. Reversed, inches of mercury = centimeters of water ÷ 34.5315576675, which is the same as multiplying by 0.0289590179982.
Which is the bigger unit, the inch of mercury or the centimeter of water?
The inch of mercury. One inch of mercury equals 34.5316 centimeters of water.
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 mercury?
US aviation and weather altimeter setting unit.
What is a centimeter of water?
The unit ventilator and CPAP pressures are set in.
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 inHg to cmH₂O in my head?
Plenty of people do this in their head: multiply by 35 instead of using the exact factor 34.53155767. At 100 that shortcut gives 3500 centimeters of water, off by 46.84 centimeters of water or about 1.36%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Inches Of Mercury to centimeters of water reference tables
| Inches Of Mercury | Centimeters Of Water |
|---|---|
| 0.5 inHg | 17.2658 cmH₂O |
| 1 inHg | 34.5316 cmH₂O |
| 2 inHg | 69.0631 cmH₂O |
| 3 inHg | 103.595 cmH₂O |
| 5 inHg | 172.658 cmH₂O |
| 10 inHg | 345.316 cmH₂O |
| 20 inHg | 690.631 cmH₂O |
| 25 inHg | 863.289 cmH₂O |
| 50 inHg | 1726.58 cmH₂O |
| 75 inHg | 2589.87 cmH₂O |
| 100 inHg | 3453.16 cmH₂O |
| 250 inHg | 8632.89 cmH₂O |
| 500 inHg | 17,265.8 cmH₂O |
| 1000 inHg | 34,531.6 cmH₂O |
| 5000 inHg | 172,658 cmH₂O |
Exact factor: 1 inch of mercury = 34.5315576675 centimeters of water.
| Quantity | inHg | cmH₂O |
|---|---|---|
| Sea-level air | 29.92 inHg | 1033 cmH₂O |
| A car tyre | 65.15 inHg | 2250 cmH₂O |
| Blood pressure, systolic | 4.724 inHg | 163.1 cmH₂O |
| A bicycle tyre | 183.2 inHg | 6328 cmH₂O |
| A domestic water main | 118.1 inHg | 4079 cmH₂O |
| A steam boiler | 295.3 inHg | 1.02e+04 cmH₂O |
Published reference values, converted with the same factor this page uses.