To convert inches of water to atmospheres, multiply by 0.002458316309. 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 low-pressure unit for duct static pressure and gas appliances in the US. Standard atmospheric pressure by definition — 101,325 pascals exactly.
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 atmospheres, so you can sanity-check whether your answer is a plausible size.
The formula
inches of water- The amount you are converting, in inches of water
0.00245831630891- The exact number of atmospheres in one inch of water
atmospheres- The converted result
How it works, step by step
- Enter the value in inches of water.
- It is multiplied by 0.00245831630891, the exact factor between the two units.
- The answer is shown in atmospheres, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
A bicycle tyre: 2490 inH₂O in atmospheres
Take a bicycle tyre, roughly 2490 inH₂O. The conversion is a single multiplication — 2490 × 0.00245832 — giving 6.12121 atm. Because the relationship is proportional, doubling the input doubles the answer to 12.2424 atm.
Sea-level air: 407 inH₂O in atmospheres
Sea-level air comes to about 407 inH₂O. Multiplying by the factor: 407 × 0.00245832 = 1.00053 atm. Dividing the answer back by 0.00245832 returns the figure you started with, which is the quickest way to check you have the factor the right way up.
How to read your score
Frequently asked questions
How many atmospheres are in one inch of water?
Exactly 0.00245831630891. 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 atmospheres?
atmospheres = inches of water × 0.00245831630891. Reversed, inches of water = atmospheres ÷ 0.00245831630891, which is the same as multiplying by 406.782478063.
Which is the bigger unit, the inch of water or the atmosphere?
The atmosphere. One inch of water is only 0.00245832 of a atmosphere, so it takes 406.782 inches of water to make one atmosphere.
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 atmosphere?
Standard atmospheric pressure by definition — 101,325 pascals exactly.
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 atm in my head?
Plenty of people do this in their head: divide by 400 instead of using the exact factor 0.002458316309. At 100 that shortcut gives 0.25 atmospheres, off by 0.004168 atmospheres or about 1.70%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Inches Of Water to atmospheres reference tables
| Inches Of Water | Atmospheres |
|---|---|
| 0.5 inH₂O | 0.00122916 atm |
| 1 inH₂O | 0.00245832 atm |
| 2 inH₂O | 0.00491663 atm |
| 3 inH₂O | 0.00737495 atm |
| 5 inH₂O | 0.0122916 atm |
| 10 inH₂O | 0.0245832 atm |
| 20 inH₂O | 0.0491663 atm |
| 25 inH₂O | 0.0614579 atm |
| 50 inH₂O | 0.122916 atm |
| 75 inH₂O | 0.184374 atm |
| 100 inH₂O | 0.245832 atm |
| 250 inH₂O | 0.614579 atm |
| 500 inH₂O | 1.22916 atm |
| 1000 inH₂O | 2.45832 atm |
| 5000 inH₂O | 12.2916 atm |
Exact factor: 1 inch of water = 0.00245831630891 atmospheres.
| Quantity | inH₂O | atm |
|---|---|---|
| Sea-level air | 406.8 inH₂O | 1 atm |
| A car tyre | 885.8 inH₂O | 2.177 atm |
| Blood pressure, systolic | 64.23 inH₂O | 0.1579 atm |
| A bicycle tyre | 2491 inH₂O | 6.124 atm |
| A domestic water main | 1606 inH₂O | 3.948 atm |
| A steam boiler | 4015 inH₂O | 9.869 atm |
Published reference values, converted with the same factor this page uses.