If you’ve looked at your CPAP prescription and wondered why the pressure isn’t listed in PSI like your car tires, you’re not alone. Many new CPAP users expect to see familiar pressure units, but sleep therapy equipment uses an entirely different measurement system. Understanding this distinction can help you make sense of your treatment settings and communicate more effectively with your sleep specialist.
CPAP pressure is measured in centimeters of water pressure, written as cm H₂O or cmH₂O. This might seem unusual at first, but there’s a practical reason behind it. The pressures involved in sleep apnea therapy are extremely low compared to what PSI typically measures. Your car tires might run at 32 PSI, but your CPAP machine operates at pressures that would barely register on that scale.
To put this in perspective, 10 cm H₂O—a common CPAP setting—equals only about 0.14 PSI. Even the maximum pressure on most CPAP machines (20 cm H₂O) translates to roughly 0.28 PSI. These numbers are so small that using PSI would require awkward decimal places and make fine-tuning nearly impossible.
The centimeter of water measurement has roots in medical and respiratory science. It describes the pressure needed to push a column of water up by one centimeter. This unit became standard for lung pressure measurements because it provides meaningful numbers in a range that’s easy to work with.
When your CPAP machine is set to 10 cm H₂O, it’s delivering enough pressure to theoretically raise water 10 centimeters in a tube. While you’ll never see this demonstration on your nightstand, the concept explains why this unit was adopted across respiratory medicine, from ventilators to sleep therapy devices.
Most CPAP machines operate between 4 and 20 cm H₂O. The average prescription falls somewhere between 8 and 10 cm H₂O, though individual needs vary considerably based on the severity of sleep apnea, anatomy, and other factors.
Here’s how those numbers translate:
These conversions highlight just how gentle CPAP therapy actually is. The pressure keeping your airway open overnight is a tiny fraction of what inflates a bicycle tire.
Understanding that CPAP uses cm H₂O rather than PSI helps in several practical ways. First, it prevents confusion when reading your machine’s display or discussing settings with your healthcare provider. Second, it puts the therapy in perspective—the air pressure you’re breathing against is remarkably subtle, even at higher settings.
Some patients worry that CPAP delivers forceful, uncomfortable pressure. Knowing the actual measurements can provide reassurance. The difference between a “low” setting of 6 cm H₂O and a “high” setting of 14 cm H₂O is only about 0.11 PSI—imperceptible in most everyday contexts, though your airway certainly notices the difference.
If you use an APAP (auto-adjusting) machine, your prescription likely includes a pressure range rather than a single number. For example, your device might be set to operate between 5 and 15 cm H₂O, automatically increasing or decreasing pressure throughout the night based on your breathing patterns.
BiPAP machines, which provide different pressures for inhaling and exhaling, can sometimes exceed the standard 20 cm H₂O ceiling, reaching up to 25 or even 30 cm H₂O for patients with specific respiratory needs. Even at these higher levels, the pressure remains well under 0.5 PSI.
While it’s helpful to understand what your CPAP pressure numbers mean, adjusting them should always involve your healthcare provider. They can review your therapy data, assess whether your current pressure is effectively treating your sleep apnea, and make informed recommendations.
If you’re curious about your settings or experiencing issues like mask leaks, discomfort, or continued daytime sleepiness, that’s worth discussing at your next appointment. Your provider can explain exactly what your prescription means and whether any changes might improve your therapy experience.
The bottom line: CPAP machines use cm H₂O because it’s the right tool for the job. These small, precise measurements allow for the fine adjustments that make sleep apnea therapy effective and comfortable—something PSI simply couldn’t provide at this scale.