Green · 80% or more
Doing well
Peak flow is at least 80% of personal best. Continue the clinician-authored green-zone plan and routine controller treatment as prescribed.
Free personal-best asthma calculator
Asthma Peak Flow Zone Calculator
Calculate personalized green, yellow, and red asthma peak-flow zones from an established personal best—and see where a current reading falls.
Exact L/min ranges Symptom safety override 2026 source-linked guidance
Educational decision support only. Use these calculated ranges only with an individualized asthma action plan. Severe symptoms or poor response to prescribed rescue treatment require urgent evaluation regardless of peak flow.
Personal-best method
Calculate personalized asthma peak-flow zones
Enter the personal-best peak expiratory flow established when asthma was stable. Add today's reading to identify its percentage and zone.
Built by a Registered Respiratory Therapist
Calculated locally in your browser
Asthma action-plan zones
What green, yellow, and red peak flow mean
Peak-flow zones compare the current reading with the patient's established personal best. They are designed to support a written action plan—not to replace symptoms, clinical judgment, or prescribed instructions.
Yellow · 50–79%
Caution
Asthma may be worsening. Follow the individualized yellow-zone instructions and reassess according to the written plan.
Red · below 50%
Medical alert
Follow the prescribed red-zone instructions and contact the appropriate clinician or emergency service. Severe danger signs warrant emergency help.
Why personal best matters: peak-flow meters and individual baseline values vary. Action-plan zones should be anchored to the patient's own stable best using the same meter whenever possible, not generated from age, sex, or height alone.
Peak-flow technique
How to measure peak expiratory flow
A peak-flow reading is effort-dependent. Consistent position, meter technique, timing, and a forceful initial blast all influence the result.
Follow the manufacturer's instructions and the technique demonstrated by the clinical team. Clean and maintain the device as directed, and use the same meter for comparison whenever possible.
- 1
Stand or sit upright
Move the marker to the bottom of the scale and keep fingers away from the indicator and air outlet.
- 2
Take a full breath
Fill the lungs completely, place the mouthpiece between the teeth, and seal the lips around it.
- 3
Blast out hard and fast
Peak flow measures the fastest expiratory flow, so the effort should be a short, forceful blast rather than a slow exhalation.
- 4
Repeat and record
Repeat the maneuver three times and record the highest acceptable reading—not an average of the attempts.
Finding a reliable personal best
- Establish it with the clinician while asthma is stable and symptoms are well controlled.
- Measure at the recommended times over approximately two to three weeks.
- Use correct technique and the same peak-flow meter whenever possible.
- Review the value periodically because the personal best can change with growth, treatment, or health status.
When the number can mislead
- Symptoms can begin before a meaningful drop in peak flow.
- Some patients perceive airflow limitation poorly and may need more structured monitoring.
- Cough, submaximal effort, a poor seal, or device differences can alter the measurement.
- A single reading does not diagnose asthma or establish treatment response by itself.
Common questions
Asthma peak-flow calculator FAQ
How are asthma peak-flow zones calculated?
Peak-flow zones are calculated as percentages of an individual's established personal best. Green is 80% or more, yellow is 50% through 79%, and red is below 50%. The calculator converts those percentages into non-overlapping whole-number L/min ranges.
Should I use personal-best or predicted peak flow?
For an individualized peak-flow action plan, use a personal-best value established when asthma is stable, with good technique and the same meter. Predicted values do not account for an individual's usual measurement or meter variation.
How do I determine my personal-best peak flow?
A clinician may ask the patient to record peak flow regularly for roughly two to three weeks while asthma is well controlled. The personal best should be reviewed with the clinician before it is used to set action-plan zones.
Can severe asthma symptoms override a green-zone peak flow?
Yes. Symptoms, response to prescribed rescue treatment, and clinical appearance matter. Severe breathlessness, difficulty walking or speaking, blue or gray lips, confusion, drowsiness, or poor response to rescue treatment require urgent assessment regardless of the meter reading.
Why should I record the highest of three peak-flow attempts?
Peak expiratory flow is effort-dependent. Repeating the maneuver and recording the highest acceptable result helps reduce the effect of an incomplete effort. Follow the instructions supplied with the meter and the technique taught by the clinical team.
Does this calculator replace an asthma action plan?
No. It calculates zone boundaries and provides educational context only. Medication instructions, timing, clinician contacts, and emergency steps must come from the patient's individualized written asthma action plan.
Clinical references and calculation method
Evidence and source links were checked August 15, 2026. Zone thresholds are calculated from the entered personal best: green begins at 80%, yellow spans 50% through 79%, and red is below 50%. Whole-number boundaries are rounded to avoid overlapping L/min ranges.