Understanding CPAP Mask Leak Rate: What the Numbers Mean

Every CPAP and BiPAP machine continuously tracks the difference between the air it delivers and the air that returns. That difference is your total mask leak rate — reported in litres per minute (L/min).

Intentional Leak vs. Unintentional Leak

Not all leak is a problem. This is the distinction that most CPAP apps don't explain clearly enough.

ResMed devices (AirSense 10, AirSense 11, AirCurve series) report both figures separately: total leak (vent flow plus unintentional leak) and unintentional leak on its own. The unintentional leak figure is the more relevant one to review.

How CPAP Machines Measure Leak Rate

Your machine calculates leak by comparing the flow it delivers at the blower with the flow it detects returning through the circuit. It does this continuously during the night. The result is recorded as a time-series — not a single nightly average — so you can see how leak varied across the session.

Most apps and reports then summarise this as a 95th percentile figure: the leak rate at or below which you spent 95% of the night. This percentile approach is less sensitive to brief transient spikes (such as when you roll over) than a straight average would be. The 99th percentile figure, when available, captures those transient peaks.

Units: What L/min Means in Your Data

Leak rate is measured in litres per minute (L/min). To put it in perspective: normal tidal breathing at rest involves roughly 5–8 litres of air per minute of ventilation. An unintentional leak of 24 L/min represents a substantial fraction of that volume escaping the circuit rather than reaching your airway.

Some devices and apps display leak as a percentile without showing the raw L/min figure. When reviewing your data in tools that show the raw time-series — such as OSCAR or AirwayLab — you'll see L/min on the y-axis.

Where Leak Data Appears in Your Device Report

ResMed myAir app:Shows a “mask seal” rating derived from leak data rather than the raw L/min figure. The underlying number is in your SD card data.

OSCAR: Displays total leak and unintentional leak as separate time-series charts alongside AHI and flow data. The statistics panel shows 95th and 99th percentile figures.

AirwayLab: Reads the same SD card data and plots leak rate across the full night alongside event markers and flow limitation data, so you can see whether elevated leak periods coincide with increased breathing events.

Viewing Leak Rate Over Time

A single night of elevated leak tells you little. The more useful view is the multi-night trend: is your 95th percentile leak rate stable across weeks, worsening gradually, or variable? Consistent elevation across many nights points to a different pattern than a one-off spike on a specific night.

AirwayLab shows your leak trend across your full history so you can see whether the pattern is stable or changing over time.

Reading Leak Data with AirwayLab

Upload your ResMed SD card to AirwayLab and you can explore your leak data alongside your AHI, flow limitation scores, and pressure delivery — all in your browser. Your data never leaves your device.

Manufacturer Reference Ranges (ResMed, Philips)

Manufacturers publish reference ranges in their device documentation that many PAP users reference when reviewing their therapy data.

What these reference ranges mean for your individual situation depends on your machine model, mask type, therapy mode, and clinical context. Your clinician can help interpret what your leak data represents in context.

How Leak Rate Relates to Therapy Data Quality

Mask leak rate isn't just a comfort metric — it affects the reliability of the other numbers in your report. Your machine uses the flow signal to detect breathing events, measure flow limitation, and drive pressure adjustments on APAP and BiPAP devices. When leak is elevated, that flow signal is distorted.

Many PAP users discuss leak rate with their equipment provider or care team when reviewing therapy data, particularly when other metrics are also showing unexpected patterns.

What Happens to AHI Data During High Leak?

When unintentional leak is high, your machine's event detection becomes less accurate. This can manifest in a few ways:

This is why AHI alone, on a high-leak night, may not accurately represent what was happening with your breathing. Reviewing leak and event data together gives a more complete picture.

Unintentional leak patterns in the data are often associated with a small number of recurring factors. Understanding what these factors are helps you describe what you're seeing when discussing your data with your care team.

Mask Fit and Facial Seal

The cushion-to-face interface is the most common source of unintentional leak. Leak patterns associated with fit factors often appear in the data as persistent low-level elevation across the full night rather than intermittent spikes, because the seal is consistently imperfect rather than occasionally disrupted.

Mouth Breathing

On nasal and nasal pillow masks, mouth breathing creates a separate leak path not covered by the mask seal. In data terms, this typically appears as a sustained elevation in unintentional leak, often correlated with supine position (lying on your back) and more pronounced in the later portions of the night as sleep deepens.

Mask Wear and Cushion Condition

Silicone and gel cushions degrade with use. A cushion that sealed well when new may develop micro-perforations or lose its original shape over months of use. If your historical leak data shows a gradual worsening trend over several months, cushion condition is one pattern worth raising with your equipment provider when you discuss the data.

AirwayLab reads the full SD card data from your ResMed device — the same source that OSCAR reads — and surfaces your leak rate in context with the rest of your therapy data. Your data never leaves your browser. No account required.