Apnea-Hypopnea Index (AHI) Calculator: Events per Hour and Sleep Apnea Severity
Calculate your apnea-hypopnea index (AHI or home-test REI) from apneas, hypopneas and sleep time, with adult or child severity, RDI, ODI, central apnea index and change from a past AHI.
How it works
AHI = (apneas + hypopneas) ÷ hours of sleep · REI (home test) = (apneas + hypopneas) ÷ hours of recording · RDI = (apneas + hypopneas + RERAs) ÷ hours of sleep · ODI = oxygen desaturations ÷ hours · Adults: under 5 normal, 5 to under 15 mild, 15 to under 30 moderate, 30 or more severe · Children: under 1 normal, 1 to under 5 mild, 5 to under 10 moderate, 10 or more severe
The apnea-hypopnea index (AHI) is the main number on a sleep study report. It counts every apnea (breathing stops for 10 seconds or more) and every hypopnea (breathing becomes shallow for 10 seconds or more, with an oxygen drop or an arousal) and divides by the hours you were asleep. Adults are graded by the American Academy of Sleep Medicine scale: under 5 events per hour is normal, 5 to under 15 is mild, 15 to under 30 is moderate and 30 or more is severe sleep apnea. Children breathe faster and tolerate fewer events, so 1 or more per hour is abnormal and 10 or more is severe. Enter the counts from your report and the total sleep time. For a home sleep test, choose “Home test”: these devices usually cannot tell when you are awake, so they divide by the recording time and report a respiratory event index (REI), which can underestimate the AHI. Under More options you can add central apneas for a central apnea index, RERAs for the respiratory disturbance index (RDI), oxygen desaturations for the ODI, and a previous AHI to see the change after treatment or weight loss. A table shows how many events would put this night into each severity band. The STOP-BANG and Epworth calculators estimate the risk of sleep apnea from symptoms before a test; this one reads the result of the test. It does not replace a sleep specialist’s interpretation, which also weighs oxygen levels, sleep position and symptoms.
Worked example
A sleep study records 42 apneas and 60 hypopneas over 6 hours 30 minutes of sleep. Total events = 42 + 60 = 102 and sleep time = 6.5 hours, so AHI = 102 ÷ 6.5 = 15.7 events per hour: moderate sleep apnea on the adult scale (15 to under 30). The apnea index is 42 ÷ 6.5 = 6.5 per hour and the hypopnea index is 60 ÷ 6.5 = 9.2 per hour. For this sleep time, 32 or fewer events would have been normal, 33–97 mild, 98–194 moderate and 195 or more severe.
Sources: StatPearls (NCBI Bookshelf): Obstructive sleep apnea (AHI, REI, severity, hypopnea rules) · Cleveland Clinic: Apnea-hypopnea index (adult and pediatric severity) · American Family Physician: Obstructive sleep apnea in children · Omni Calculator: AHI calculator
Frequently asked questions
How is the AHI calculated?
Add the apneas and hypopneas recorded during the night and divide by the hours of sleep. For example, 102 events in 6 hours 30 minutes give 102 ÷ 6.5 = 15.7 events per hour.
What is a normal AHI?
For adults, fewer than 5 events per hour is normal. 5 to under 15 is mild, 15 to under 30 moderate and 30 or more severe sleep apnea. For children, fewer than 1 event per hour is normal.
What is the difference between AHI, RDI and REI?
The AHI counts apneas and hypopneas per hour of sleep. The RDI also counts respiratory effort-related arousals (RERAs), so it is equal to or higher than the AHI. The REI is the home-test version, divided by recording time instead of sleep time, so it tends to read lower.
What AHI should I see on my CPAP machine?
CPAP machines report an estimated residual AHI each night. A reading of 5 or more is often a sign that the therapy is not controlling the apnea well, for example because of mask leaks or pressure settings, and is worth raising with your sleep team.
Can my AHI change from night to night?
Yes. Sleeping on your back, alcohol, nasal congestion, weight change and the amount of REM sleep all move the AHI, and different scoring rules for hypopneas (3% or 4% oxygen drop) change it too. That is why one borderline night is usually interpreted alongside symptoms.
Related calculators
This calculator explains sleep study numbers; it does not diagnose sleep apnea or replace a sleep specialist. Seek care promptly for breathing pauses with gasping, severe daytime sleepiness or drowsy driving.