CPAP leak rate is air escaping anywhere other than the mask's built-in vent, usually at the mask edge or an open mouth, in litres per minute. ResMed AirSense devices report this unintentional leak separately from vent flow.
Does it matter? Sometimes, high leak can hide events and disturb sleep, while moderate leak often does not. The per-night pattern tells you which.
What is a normal CPAP leak rate?
Some leak is always present, because masks vent air on purpose. A common rule of thumb treats unintentional leak above about 24 L/min as high. It is a working cut-off, not a clinical reference range.
Source: ResMed AirSense 11 user guide, Mask Fit section [M1]. Checked 21 October 2026.
What Is Mask Leak Rate in CPAP Data?
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.
Intentional leak (vent flow)
Engineered exhaust vents built into your mask to continuously flush exhaled CO₂. Without them, you would be re-breathing spent air. This flow is constant, expected, and varies by mask model.
Unintentional leak
Air escaping from an imperfect seal between the mask and your face — around the cushion, from the frame, or through an open mouth on a nasal mask. This is what elevated leak rates reflect.
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).
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.
How to Read a CPAP Mask Leak Rate Chart
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. Open your SD card in AirwayLab to see leak per night next to the flow waveform.
Reading Leak Rate in OSCAR: Step by Step
OSCAR (Open Source CPAP Analysis Reporter) is a desktop tool for detailed PAP data review. Here is how to read leak data in it:
- 1Import your CPAP data: From your SD card or ResMed data folder using the import wizard.
- 2Open the daily view: Look at the "Leak Rate" chart. By default, OSCAR displays unintentional (residual) leak after subtracting the vent flow curve for your detected mask model.
- 3Check the statistics panel: On the left, review the 95th percentile, median, and maximum leak values for the session.
- 4Overlay with AHI or flow limitation: Look for correlations between leak rate spikes and event clusters. Clusters that coincide with high-leak episodes are worth noting and discussing with your clinician.
One important note: confirm OSCAR has detected the correct mask model. The vent flow curve used to calculate unintentional leak depends on the specific mask — if OSCAR is using the wrong profile, the residual leak figure will be off. Set your mask manually in OSCAR's settings if the auto-detected mask doesn't match what you use.
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.
Explore your leak rate data in AirwayLab
Free, built on an open-source engine, and 100% private — your data never leaves your browser.
Upload Your SD Card DataWhat Do Different Leak Rate Values Indicate?
How ResMed and Philips Report Leak
ResMed and Philips record leak differently in the data on the SD card.
ResMed (AirSense 10, AirSense 11, AirCurve)
ResMed devices report unintentional leak in L/min. Total leak runs higher because it includes the intentional vent flow [M3].
Philips (DreamStation, System One)
Philips DreamStation and System One devices record large leak as an event flag on the SD card, as decoded by the open-source OSCAR Philips loader [M4].
What your leak figures mean 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:
Underreported events
Real apneas or hypopneas may go undetected if the flow signal is too noisy to distinguish a genuine event from leak-related airflow disruption.
False events
Conversely, some machines may flag leak artefacts as respiratory events, inflating the AHI count on high-leak nights.
Pressure mistitration on APAP
Auto-titrating devices may respond to leak artefacts by raising or lowering pressure inappropriately, since the algorithm relies on the same flow signal.
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.
Understanding the "Large Leak" flag
A "Large Leak" flag in your device data or analysis software is a data-quality signal, not a diagnosis. It marks a stretch of the night where unintentional leak was high. Your AHI on a high-leak night should be read with a caveat: the figure may be less reliable not because your breathing was better or worse, but because the underlying signal quality was lower. Your clinician can help interpret recurring Large Leak flags in the context of your full therapy data.
Common Sources of Mask Leak
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.
Analyzing Your Leak Rate Data with AirwayLab
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.
Leak trend across nights
See whether your 95th percentile mask leak rate is stable, worsening, or variable across your history.
Nightly leak timeline
View leak rate plotted across the full night alongside events — see whether elevated leak periods coincide with increased AHI or flagged breathing events.
H1/H2 split
Compare first-half vs second-half of the night. Leak that worsens in H2 often correlates with positional changes as sleep deepens.
Cross-metric view
Review leak alongside flow limitation scores, AHI, and pressure to understand whether a high-leak night also had less reliable event data.
Explore your CPAP mask leak rate data
Upload your ResMed SD card to AirwayLab. Free, built on an open-source engine, and 100% private — your data never leaves your browser.
Frequently Asked Questions
What is a normal CPAP leak rate?
Some leak is always present, because masks vent air on purpose. A common rule of thumb treats unintentional leak above about 24 L/min as high. It is a working cut-off, not a clinical reference range.
Does leak affect AHI?
High leak can make event detection less reliable, so AHI on a high-leak night is less trustworthy.
How do I see leak next to events?
Upload your SD card to AirwayLab and open the night, leak and events are charted together.
Is air supposed to leak from a CPAP mask?
Yes, some air is meant to leave a CPAP mask. ResMed's AirFit F20 mask guide says masks used with CPAP have venting to allow continuous flow of air out, and new air from the device flushes your exhaled air out through the mask holes. This vent flow is intentional leak. Ask your equipment supplier how much vent flow to expect from your mask.
Is air supposed to leak from the CPAP hose?
ResMed's AirSense 11 guide tells you to check the CPAP air tubing and replace it if there are any holes, tears or cracks. The same guide lists air tubing that is not connected properly as a cause of its "High leak detected. Connect your tubing." message. If air is escaping from your hose, ask your equipment supplier.
Should my CPAP mask leak around the edges?
ResMed's AirSense 11 and AirFit F20 guides both list air leaking around the CPAP mask as a problem to troubleshoot. The AirSense 11 guide says its Mask Fit check reads leak under 24 L/min as a good mask seal and leak over 24 L/min as "Adjust the mask". For mask size and fit, talk to your clinician or equipment supplier.
What does "High leak detected" mean on my CPAP?
The ResMed AirSense 11 CPAP guide lists two "High leak detected" messages. For "Check your humidifier or side cover", the listed cause is a humidifier tub or side cover not inserted properly. For "Connect your tubing", the listed causes are air tubing not connected properly or a mask fitted incorrectly. If the message keeps coming back, ask your equipment supplier.
Should I block the vent holes on my CPAP mask?
No. ResMed's AirSense 11 guide says to make sure the CPAP mask vent holes are kept clear and unblocked to maintain the flow of fresh air into the mask. ResMed's AirFit F20 mask guide says the passive venting is there to protect against rebreathing. If vent noise or airflow bothers you, ask your equipment supplier.
How much air comes out of a CPAP mask vent?
ResMed's AirFit F20 full face mask guide lists vent flow that rises with pressure, for example 19 L/min at 3 cm H2O and 37 L/min at 10 cm H2O. The guide says the vent flow rate may vary as a result of manufacturing variations. This vent flow is intentional CPAP leak. For another mask, check its own user guide.
A note on interpreting your data
AirwayLab analysis is informational only and does not constitute medical advice, a diagnosis, or a recommendation to change your therapy settings. Mask leak rate thresholds vary by machine model, mask type, and individual clinical context. Your clinician can help interpret these findings in context.
Related reading
Understanding Your CPAP Data: A Plain-Language Guide — a full walkthrough of the metrics in your nightly report.
How to Read Your CPAP Data (And Why AHI Isn't the Whole Story) — why the headline number often misses important patterns.
Understanding Flow Limitation: What Your PAP Machine Doesn't Tell You — the metric that explains residual symptoms even when AHI looks fine.
What Is Residual AHI on CPAP, and What Counts as Good? — how leak can make a night's AHI less trustworthy.
How AirwayLab scores your data — the methodology behind the metrics on this page.
Check your own nights
Upload your SD card to AirwayLab and see leak charted next to events, night by night, in your browser.
Sources
- [M1] ResMed AirSense 11 user guide, Mask Fit section: leak under 24 L/min reads as a good seal, over 24 L/min as "Adjust the mask". Grade: manufacturer user guide. This is a mask fit check, not a therapy-data threshold.
- [M2] ResMed AirFit F20 full face mask user guide, venting, troubleshooting and pressure-flow table, cited in the FAQ. Grade: manufacturer specification.
- [M3] ResMed AirSense 11 user guide, Mask Fit, troubleshooting and device messages, cited in the FAQ. Grade: manufacturer user guide.
- [M4] OSCAR (open-source PAP analysis), Philips Respironics PRS1 loader source, large-leak event type. Grade: open-source code reference, not a manufacturer document.