What Is cmH2O on a CPAP?

What Is cmH2O on a CPAP?

cmH2O stands for centimeters of water, the unit CPAP machines use to measure air pressure. One cmH2O is the amount of pressure it takes to push a column of water one centimeter higher, roughly 98 pascals in standard units. So a CPAP set to 10 cmH2O is delivering air with enough gentle force to lift water 10 centimeters up a tube. Every pressure number on your machine, your prescription, and your therapy report (your set pressure, your P95, your median pressure) is expressed in this unit.

Why CPAP Uses Centimeters of Water

CPAP pressures are tiny by everyday standards, and that is exactly why this unusual unit exists. The familiar units are too coarse for them. Blood pressure is measured in millimeters of mercury (mmHg) because mercury is dense and blood pressure is comparatively high. Tire pressure is measured in psi because tires hold a lot of it. CPAP air pressure is far gentler than either, so medicine borrowed a more sensitive yardstick: a column of plain water, which rises much farther than mercury under the same small push.

Some perspective on how gentle these pressures are: the atmosphere pressing on you right now is about 1,033 cmH2O at sea level. A CPAP running at 10 cmH2O is adding roughly one percent on top of that. Even the adult maximum of 20 cmH2O is under two percent above atmospheric pressure. The machine is not inflating you; it is applying just enough extra pressure to act as an air splint that holds your upper airway open while you sleep. The same unit is used across respiratory medicine, including hospital ventilators, for the same reason.

How cmH2O Compares to Other Pressure Units

If you ever need to translate a CPAP pressure into another unit, the conversions are straightforward.

Unit	One cmH2O equals	Where you see it
cmH2O (centimeters of water)	1	CPAP, bilevel, and ventilator settings
hPa (hectopascal) / mbar (millibar)	About 0.98 hPa	European device spec sheets, weather reports
mmHg (millimeters of mercury)	About 0.74 mmHg	Blood pressure, medical gas pressures
psi (pounds per square inch)	About 0.014 psi	Tires, compressors (about 70 cmH2O per 1 psi)

The one that actually comes up for CPAP users is hPa. A hectopascal and a centimeter of water are nearly identical (1 hPa is about 1.02 cmH2O), so a machine documented in hPa is speaking essentially the same language as one documented in cmH2O. The difference is smaller than the smallest adjustment step on any CPAP, so for reading your own report you can treat them as interchangeable.

What Typical CPAP Pressures Look Like in cmH2O

Machines are built around a standard range. The American Academy of Sleep Medicine's titration guidelines set the recommended minimum starting pressure at 4 cmH2O and the recommended maximum at 20 cmH2O for adults, and mainstream machines like the ResMed AirSense 10 and 11 match that with an operating range of 4 to 20 cmH2O. Bilevel machines, which deliver separate inhale and exhale pressures, extend higher; the ResMed AirCurve line reaches a maximum of 25 cmH2O.

Where your therapy sits inside that range is individual. Your prescription might be a single fixed pressure (for example, 9 cmH2O) or a range the machine adjusts within automatically (for example, 6 to 14 cmH2O). Comfort features work in the same unit: ResMed's EPR drops the pressure by 1, 2, or 3 cmH2O each time you exhale, and ramp starts the night at a lower pressure before climbing to your prescribed level. There is no universally "good" pressure number, because the right pressure is whatever holds your particular airway open, which is exactly what a titration study or an auto-adjusting trial is designed to find. Pressure changes belong with your sleep physician, not the menu. Our CPAP pressure settings guide covers what each setting does in detail.

Some Machines Say hPa Instead

If your machine or its manual shows pressures in hPa rather than cmH2O, nothing is different about your therapy. European manufacturers often specify pressure in hectopascals; Löwenstein's prisma SMART, for example, lists its range as 4 to 20 hPa, adjustable in 0.5 hPa steps. Because 1 hPa is within about 2 percent of 1 cmH2O, a prescription written as 8 cmH2O and a machine set to 8 hPa are delivering effectively the same pressure. The number on the screen carries over; only the label changes. If you switch between machines that use different labels, your prescribed number stays the same for practical purposes.

Where to Find Your Pressure Numbers

Your set pressure or pressure range appears on the machine itself, usually on the home or settings screen, and on your prescription. Your therapy report goes further: it shows the pressures your machine actually delivered overnight, typically as a median and a 95th percentile, all in cmH2O. Manufacturer apps like myAir tend to simplify this away, while the clinician-facing reports keep it. If you want to see your delivered pressure alongside your AHI, leak, and a plain-English read of the night, you can upload your SD card data to CPAP Clarity and everything stays in your browser. For a walkthrough of every number on the report, see how to read your CPAP data.

Common Questions

What does cmH2O stand for on a CPAP machine?

cmH2O stands for centimeters of water. It is the pressure unit CPAP machines use: one cmH2O is the pressure needed to raise a column of water one centimeter. All CPAP prescriptions, settings, and report pressures are written in this unit.

Is a higher cmH2O number better or worse?

Neither. The pressure that matters is the one that keeps your airway open, and that is individual. Someone doing well at 6 cmH2O and someone doing well at 14 cmH2O are both getting appropriate therapy. What is worth a conversation with your sleep physician is a delivered pressure that keeps trending toward the top of your prescribed range, not the absolute number itself.

How much pressure is 10 cmH2O really?

Very little by everyday standards. Atmospheric pressure at sea level is about 1,033 cmH2O, so 10 cmH2O adds roughly one percent on top of the air pressure you feel all the time. It is enough to hold the soft tissue of your airway open, in the same low-pressure range respiratory medicine uses routinely. If you have questions about how pressure affects your lungs, your sleep physician is the right person to ask.

Are hPa and cmH2O the same thing on a CPAP?

Close enough to treat as the same. One hPa equals about 1.02 cmH2O, a difference smaller than the smallest adjustment step on any CPAP machine. European machines like the Löwenstein prisma line are specified in hPa; the therapy is identical, only the unit label differs.

What is the normal cmH2O range for CPAP machines?

Standard CPAP machines operate from 4 to 20 cmH2O, which matches the American Academy of Sleep Medicine's recommended titration range for adults (minimum starting pressure 4, maximum 20). Bilevel machines can go higher, up to 25 cmH2O on models like the ResMed AirCurve. Where your prescription sits within that range is determined by your sleep physician.

The Unit Behind Every Pressure Number

cmH2O is simply the ruler CPAP therapy is measured with: a water column sensitive enough to express pressures that would round to nearly nothing in tire or blood-pressure units. Once you know that 4 to 20 cmH2O is the standard adult range, that hPa means essentially the same thing, and that the whole scale sits barely above ordinary atmospheric pressure, every pressure number on your report becomes easier to place. Read yours alongside your AHI and leak, and bring questions about where your pressure should sit to your sleep physician.