Free CPAP tool
CPAP Distilled Water Usage Calculator
Enter how much water your humidifier uses on a typical night (the difference between what you pour in and what is left in the morning) and how many nights a week you use it. You get monthly liters, gallons and the number of jugs to keep on hand.
Formula
Nights per month = nights per week x 365 / 7 / 12 (about 30.4 for nightly use). Liters per month = mL per night x nights per month / 1000. US gallons = liters / 3.78541. Jugs = liters / jug size, rounded up.
The default of 380 mL is the full capacity of ResMed's HumidAir 11 standard tub, which the AirSense 11 manual says lasts more than 8 hours at 23 degrees C. Most people use less than a full tub, so measure your own nightly use for a better number. ResMed specifies distilled water for the standard tub.
Worked example
380 mL a night, 7 nights a week, 1 gallon (3.785 L) jugs. Nights per month = 7 x 365 / 7 / 12 = 30.42. Liters = 380 x 30.42 / 1000 = 11.56 L. Gallons = 11.56 / 3.78541 = 3.05. Jugs = ceil(11.56 / 3.785) = ceil(3.05) = 4.
Assumptions and limits
- Uses an average month; a 31-day month uses a little more.
- Dry winter air and high humidity settings use more water.
- Empty and refill the tub with fresh water daily rather than topping up old water.
Supplies that can help
Distilled water is what most CPAP humidifier chambers are meant to be filled with, because it leaves little or no mineral scale behind.
The removable water tank of a CPAP humidifier; it wears and needs replacing, especially if it cracks, clouds or leaks.
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Frequently asked questions
How much distilled water does a CPAP use per night?
It depends on your humidity setting, room air and pressure. The AirSense 11 standard tub holds 380 mL and is rated to last more than 8 hours, so one full tub is a reasonable upper bound for that machine. Measure what is left in the morning to find your own number.
Can I use tap water in my CPAP?
ResMed specifies distilled water for its standard HumidAir 11 tub, because minerals in tap water build up as scale. Its separate cleanable tub is rated for drinking quality water. Follow the manual for your own machine and tub.
How many gallons of distilled water do I need for a month of CPAP?
At a full 380 mL tub every night it is about 3 gallons a month, so buy 4 one-gallon jugs. At 250 mL a night it is about 2 gallons.
Sources
Last reviewed October 11, 2026. How we research · Report an error