AHI Calculator (Apnea-Hypopnea Index)

What is the apnea-hypopnea index?

The apnea-hypopnea index, or AHI, is the number of scored apneas and hypopneas divided by hours of measured sleep.

An adult apnea requires at least a 90% drop in the appropriate airflow signal for at least 10 seconds. Under the AASM recommended adult rule, a hypopnea requires at least a 30% airflow reduction for at least 10 seconds plus either at least 3% oxygen desaturation or an arousal. Reports may also show a 4% desaturation-based value for coverage or historical reasons. AHI describes event frequency, not the total physiologic burden. It does not show how long events lasted, how low oxygen fell, whether events clustered in REM sleep or the supine position, or how sleepy the patient feels. Two people with an AHI of 20 can have different oxygen exposure, arousal intensity, symptoms, and cardiovascular risk. Read the index with the rest of the sleep report.

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How this AHI calculator works

Use values from the same scored study. Add obstructive, central, and mixed apneas to the hypopnea count only when the calculator asks for total respiratory events. Then divide by total sleep time in hours. Convert minutes to decimal hours by dividing by 60. For example, 330 minutes equals 5.5 hours. Do not divide by time in bed when a polysomnogram provides measured total sleep time.

Check whether the report already lists AHI, obstructive AHI, central apnea index, REM AHI, supine AHI, or respiratory disturbance index. These are not interchangeable. RDI may include respiratory effort-related arousals. A home sleep apnea test often reports REI based on recording or monitoring time because sleep stages are not measured. Recalculating REI as AHI does not create missing sleep-time data.

Enter counts only if the denominator and scoring rule are known. Event totals from one night divided by sleep time from another night have no clinical meaning. If a report lists separate 3% and 4% hypopnea-rule indices, keep both labels and ask which one guides the diagnosis, treatment, or coverage decision.

To convert a clock-style sleep duration, convert the minutes before dividing. Four hours 45 minutes is 4.75 hours, not 4.45.

If 57 respiratory events occurred, the calculation is 57 ÷ 4.75 = 12 events per hour. Keep one decimal place when useful, but do not imply that 12.0 is biologically more certain than 12. The scorer, sensors, sleep-stage distribution, and night-to-night variation all limit precision.

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Calculation examples

Example one: 20 apneas plus 50 hypopneas over 5 hours of sleep gives 70 ÷ 5 = 14 events per hour. That falls in the commonly used mild adult range, close to the moderate boundary.

Example two: the same 70 events over 4 hours gives 17.5 per hour, which falls in the moderate range. Accurate total sleep time changes the classification. Example three: 12 central apneas during 6 hours of sleep gives a central apnea index of 2 per hour. Do not label all events central unless the report classifies them that way. Example four: a home test records 60 events over 7.5 hours of monitoring, producing REI 8 per hour. If actual sleep was shorter, true events per hour of sleep could be higher, but this calculator cannot recover the missing denominator.

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Adult AHI severity thresholds

Common adult categories are below 5 events per hour, 5 to 14.9 mild, 15 to 29.9 moderate, and 30 or more severe.

The boundaries describe frequency and should not be read as sharp biological transitions. Diagnosis also depends on the event type, symptoms, and clinical context. Pediatric scoring and diagnostic thresholds differ, so adult categories should not be applied to children. An AHI below 5 does not make every report normal. Respiratory effort-related arousals, REM-limited obstruction, upper-airway resistance, hypoventilation, or substantial snoring may still matter. An AHI of 6 with marked sleepiness, atrial fibrillation, or oxygen loss may carry more clinical weight than an AHI of 12 without symptoms or desaturation. A clinician interprets the complete pattern rather than treating the category label alone.

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Read the rest of the sleep report

Review oxygen nadir, time below 90% saturation, oxygen desaturation index, arousal index, sleep efficiency, REM and non-REM AHI, supine and nonsupine AHI, event duration, and heart rhythm notes. A low oxygen nadir can reflect respiratory disease or artifact as well as apnea, so signal quality and the tracing matter. REM-predominant or positional disease can be hidden by a whole-night average.

Check the central apnea index and whether Cheyne-Stokes breathing, hypoventilation, or mixed events were reported. Central events can occur with heart failure, stroke, high altitude, opioids, or treatment-emergent physiology and need a different assessment from straightforward upper-airway obstruction. Limb movements and parasomnias can fragment sleep without raising AHI.

Also compare total recording time, total sleep time, and sleep efficiency. A short or unusually restless laboratory night may contain limited REM sleep or little time in the person's usual position. Note whether supplemental oxygen, positive airway pressure, oral appliances, or sedating medication were used during the recording. These conditions can make the result inappropriate as an untreated baseline.

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Why the hypopnea rule changes AHI

AASM's recommended adult hypopnea rule accepts a qualifying arousal or at least 3% desaturation. An alternative rule historically used a 4% desaturation requirement. Counting arousal-linked or 3% desaturation events usually produces a higher AHI than requiring 4%. The difference can move a result across a category boundary without any change in the person's breathing that night.

Compare serial studies only after checking the scoring rule, sensors, denominator, and test type. A home test cannot score EEG arousals when it does not measure EEG. Laboratory software and scorer judgment also affect borderline events. Do not average two indices produced by different definitions. Keep the label attached to each value and ask the interpreting clinician which standard applies.

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Age, sex, body position, and comorbidity

AHI tends to increase with age, and obstructive apnea patterns differ by sex and hormonal stage. Women may have more REM-predominant events, shorter events, or symptoms such as insomnia and fatigue. Men often develop disease earlier, but the gap narrows after menopause. Body weight and neck anatomy affect risk, yet lean people can have clinically important obstruction.

Heart failure, atrial fibrillation, resistant hypertension, stroke, chronic lung disease, neuromuscular weakness, pregnancy, and opioid use change interpretation and testing choices. Supine sleep and REM sleep often worsen obstruction. A study containing little REM or little supine sleep may underrepresent the person's typical maximum severity. Significant cardiorespiratory or neuromuscular disease usually favors in-laboratory polysomnography over a basic home test.

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Diagnosis and treatment pathway

AASM recommends polysomnography as the standard diagnostic test when a comprehensive evaluation raises concern for obstructive sleep apnea. A technically adequate home test is an option for selected uncomplicated adults with increased risk of moderate to severe disease. A negative or inconclusive home result should be followed by polysomnography when suspicion remains. Questionnaires and an independently calculated AHI cannot substitute for scored physiologic data.

Treatment may include positive airway pressure, a fitted oral appliance, weight-management treatment, positional therapy, selected surgery, or combinations. Choice depends on symptoms, severity, anatomy, oxygen burden, comorbidities, preferences, and response. Device-reported residual AHI uses proprietary detection during airflow therapy and is not identical to diagnostic polysomnographic AHI. Persistent symptoms or elevated device values warrant review of mask leak, use time, pressure, central events, and sleep duration.

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Limitations, next steps, and urgent symptoms

This calculator performs division.

It cannot verify whether events were scored correctly, whether hours represent sleep or monitoring, which hypopnea definition was used, or whether the study was technically adequate. Night-to-night AHI varies with position, REM amount, alcohol, congestion, medicines, altitude, and random variation. A single number also omits hypoxic burden, arousal intensity, symptoms, and treatment preference. Discuss a report with the ordering clinician, especially when symptoms and AHI disagree, central events are present, or a negative home test conflicts with loud snoring and witnessed pauses. Stop driving if sleepy. Seek urgent care for worsening shortness of breath, chest pain, new neurologic symptoms, or repeated fainting. Call emergency services for blue or gray color, severe breathing difficulty while awake, prolonged unresponsiveness, or failure to recover normally after an observed breathing event.

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How it works

AHI = total events / hours slept. Normal <5; mild 5 to 14; moderate 15 to 29; severe ≥30.

Frequently asked questions

• What AHI needs CPAP?AHI ≥15 (moderate) or ≥5 with symptoms typically warrants CPAP per AASM guidelines.

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References

1. Berry et al., AASM. Rules for Scoring Respiratory Events in Sleep
2. NIH NHLBI. Obstructive Sleep Apnea
3. AASM. Clinical Use of a Home Sleep Apnea Test
4. Kapur et al., AASM. Diagnostic Testing for Adult Obstructive Sleep Apnea
5. Malhotra et al.. Metrics of Sleep Apnea Severity Beyond AHI
6. Soori et al.. Pitfalls of AHI Severity Grading in OSA
7. Ruehland et al.. Impact of AASM Hypopnea Criteria on AHI
8. Duce et al.. AASM Respiratory Event Criteria Increase Hypopnea Incidence
9. Patil et al., AASM. Positive Airway Pressure Treatment of Adult OSA
10. Ramar et al., AASM. Oral Appliance Therapy for Obstructive Sleep Apnea