This page converts inches of water into millitorr using the factor 1868.320395, and shows the reverse figure alongside it. Both units measure pressure, which is why one number does the whole job.
One thousandth of a torr, the working unit of vacuum deposition. 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. Below the calculator there is a step table for the values people look up most, and a second table putting real pressure figures into millitorr so the number you get has something to be compared against.
The formula
inches of water- The amount you are converting, in inches of water
1868.32039477- The exact number of millitorr in one inch of water
millitorr- The converted result
How it works, step by step
- Enter the value in inches of water.
- It is multiplied by 1868.32039477, the exact factor between the two units.
- The answer is shown in millitorr, with the reverse conversion beneath it.
- Check the reference table if you want a figure of comparable size.
Worked examples
A steam boiler: 4010 inH₂O in millitorr
At 4010 inH₂O (a steam boiler), the same quantity in millitorr is 4010 × 1868.32 = 7.49196e+06 mTorr. One inch of water is 1868.32 millitorr, so the numbers grow as expected.
A domestic water main: 1610 inH₂O in millitorr
Take a domestic water main, roughly 1610 inH₂O. The conversion is a single multiplication — 1610 × 1868.32 — giving 3.008e+06 mTorr. Because the relationship is proportional, doubling the input doubles the answer to 6.01599e+06 mTorr.
How to read your score
Frequently asked questions
How many millitorr are in one inch of water?
Exactly 1868.32039477. 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 millitorr?
millitorr = inches of water × 1868.32039477. Reversed, inches of water = millitorr ÷ 1868.32039477, which is the same as multiplying by 0.000535240102715.
Which is the bigger unit, the inch of water or the millitorr?
The inch of water. One inch of water equals 1868.32 millitorr.
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 millitorr?
One thousandth of a torr, the working unit of vacuum deposition.
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 mTorr in my head?
Plenty of people do this in their head: multiply by 1900 instead of using the exact factor 1868.320395. At 100 that shortcut gives 190,000 millitorr, off by 3168 millitorr or about 1.70%. Fine for a rough estimate, not fine for anything you are cutting to size or dosing.
Inches Of Water to millitorr reference tables
| Inches Of Water | Millitorr |
|---|---|
| 0.5 inH₂O | 934.16 mTorr |
| 1 inH₂O | 1868.32 mTorr |
| 2 inH₂O | 3736.64 mTorr |
| 3 inH₂O | 5604.96 mTorr |
| 5 inH₂O | 9341.6 mTorr |
| 10 inH₂O | 18,683.2 mTorr |
| 20 inH₂O | 37,366.4 mTorr |
| 25 inH₂O | 46,708 mTorr |
| 50 inH₂O | 93,416 mTorr |
| 75 inH₂O | 140,124 mTorr |
| 100 inH₂O | 186,832 mTorr |
| 250 inH₂O | 467,080 mTorr |
| 500 inH₂O | 934,160 mTorr |
| 1000 inH₂O | 1.86832e+06 mTorr |
| 5000 inH₂O | 9.3416e+06 mTorr |
Exact factor: 1 inch of water = 1868.32039477 millitorr.
| Quantity | inH₂O | mTorr |
|---|---|---|
| Sea-level air | 406.8 inH₂O | 7.6e+05 mTorr |
| A car tyre | 885.8 inH₂O | 1.655e+06 mTorr |
| Blood pressure, systolic | 64.23 inH₂O | 1.2e+05 mTorr |
| A bicycle tyre | 2491 inH₂O | 4.654e+06 mTorr |
| A domestic water main | 1606 inH₂O | 3e+06 mTorr |
| A steam boiler | 4015 inH₂O | 7.501e+06 mTorr |
Published reference values, converted with the same factor this page uses.