Peak Flow Zone Calculator

Calculate green, yellow, and red asthma zones from a known personal-best peak flow. Add a current reading to see where it falls. This educational tool supports—but does not create or replace—a personalized asthma action plan.

Personal rangesConvert one personal-best value into clear, non-overlapping whole-number zones.

Current zone checkOptionally compare today's highest correctly measured reading with personal best.

Private by designValues remain in this browser tab and are not uploaded or stored by the tool.

Calculate your peak flow zones

Use a personal best established when asthma was well controlled—not a guessed value or a population reference number. If your written plan lists exact L/min cutoffs, those instructions take priority.

Prepared by: Starlight Tools editorial team. Last reviewed: July 30, 2026.

Default method: green at 80% or more of personal best, yellow from 50% to below 80%, and red below 50%.

Privacy: Inputs and results stay in this tab. This calculator does not transmit, save, or track them.

1. Enter peak flow values

Personal best (L/min) Enter a whole number from 20 to 1,200 L/min.

Current reading (L/min) optional Use the highest of three good attempts.

2. Choose zone boundaries

Boundary method Some clinicians use different thresholds. Use custom mode only to reproduce a written action plan.

Calculation: boundary = personal best × percentage ÷ 100. The exact boundary is applied first; displayed whole-number lower limits are rounded upward.

Your peak flow zones

Enter a personal best and select Calculate zones.

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Current reading zone

Personal best—

Current readingNot entered

Percent of personal best—

Current reading compared with personal best

Action-plan reminder Your calculated ranges and current-zone context will appear here. Symptoms can require action even when a number is in the green zone.

Calculated zone table

Calculated peak flow zones in litres per minute
Zone	Percent of personal best	Calculated range	General meaning
Green	At least 80%	—	Doing-well range in the common traffic-light system. Continue to follow the green-zone instructions in the written plan.
Yellow	50% to below 80%	—	Caution or worsening range. Follow the written plan's yellow-zone actions and watch symptoms.
Red	Below 50%	—	Medical-alert range. Follow the red-zone instructions promptly and get emergency help for danger signs.

Formula, boundaries, and rounding

Standard zone formulas

green boundary = personal best × 0.80 yellow boundary = personal best × 0.50

Green includes readings at or above its boundary. Yellow includes readings at or above its lower boundary but below green. Red is below the yellow boundary.

Whole-number display rule

Peak flow is normally recorded as whole litres per minute. The calculator uses exact percentages for classification, then rounds each displayed lower boundary upward to avoid overlaps.

Example: with a personal best of 451 L/min, 80% is 360.8 and 50% is 225.5. The displayed zones are green ≥361, yellow 226–360, and red ≤225 L/min.

Your written plan wins. The 80% and 50% boundaries are common general guidelines, not universal treatment thresholds. If a clinician has written different percentages or exact L/min values in your action plan, use those instructions.

Personal best and accurate measurement

Personal best is the highest peak expiratory flow you can achieve when asthma is well controlled. The American Lung Association describes establishing it over a two- to three-week period; a clinician can help decide when and how to measure and when to update it.

Use the same meter where possible. Different meters and scales can produce readings that are not directly interchangeable.
Reset the marker. Set it to zero or the lowest mark, then stand or sit in the same position each time.
Fill the lungs, seal, and blow. Take a deep breath, seal your lips around the mouthpiece, and blow out once as hard and fast as possible—not as long as possible.
Repeat three times. Record the highest of three correctly performed attempts, not an average. Coughing, tongue obstruction, or a poor seal can invalidate an attempt.
Record context and symptoms. Time of day, medicines, symptoms, illness, and technique can affect readings. Use the monitoring schedule in the written plan.

Safety and medical limits

Get emergency help now for severe trouble breathing, inability to walk or speak normally because of breathlessness, blue or grey lips or skin, sudden confusion or drowsiness, or when prescribed quick-relief medicine is not helping. Follow the emergency instructions in the asthma action plan and do not delay help to finish a measurement.

A peak flow number is only one part of asthma monitoring. Symptoms can worsen before the reading falls, and serious breathing difficulty can occur with a reading that looks acceptable. Never use a green-zone result to dismiss severe or rapidly worsening symptoms.

This page is an educational calculation aid, not a medical device, diagnosis, treatment plan, or substitute for clinical care. It does not tell you which medicine or dose to take. Ask a qualified health professional to create and review a written asthma action plan.

Sources and methodology

CDC: Living with Asthma — personalized action plans and the green-zone personal-best concept.
CDC: Asthma Action Plan — green, yellow, and red peak flow fields and symptom-based danger signs.
American Lung Association: Measuring Your Peak Flow Rate — personal best, standard 80/50 traffic-light zones, measurement technique, and the need to follow clinician-set zones.
National Heart, Lung, and Blood Institute: Managing Asthma — personal-best monitoring, the 80–100% green range, and accurate peak-flow technique.

Implementation note: zone classification uses the unrounded current-reading percentage. Whole-number range labels use ceiling-rounded lower boundaries so no integer L/min value appears in two zones. Source and content review completed July 30, 2026.

Peak flow zone FAQ

How are peak flow zones calculated?

The common traffic-light method compares a reading with personal best. Green is at least 80%, yellow is at least 50% but below 80%, and red is below 50%. A clinician may set different boundaries in a personalized action plan.

What is personal-best peak flow?

Personal best is the highest peak flow achieved when asthma is well controlled, usually established over time with good technique. It is more useful for personal monitoring than a population predicted value.

Should I use predicted peak flow or personal best?

This calculator is designed for a known personal best. Predicted values are population estimates based on characteristics such as age, sex, and height and are not interchangeable with a personal best unless a clinician has told you to use one in your plan.

What if my current reading is higher than my personal best?

A higher reading may reflect normal variation, technique, or a new best. Record it and ask the clinician who manages the action plan whether the personal best and zone thresholds should be updated.

Why does the calculator round zone limits?

Peak flow is normally recorded as whole litres per minute. The calculator applies the exact percentage first, then rounds each lower boundary upward so the displayed whole-number ranges do not overlap.

Does a green-zone reading mean there is no asthma problem?

No. Symptoms and breathing difficulty still matter, and a green reading does not rule out an asthma attack or another problem. Follow the symptom instructions in the written action plan.

What should I do with a yellow- or red-zone reading?

Follow the matching instructions in the personalized asthma action plan. A red-zone reading needs prompt action. Seek emergency help for severe breathing difficulty, inability to speak or walk normally, blue or grey lips or skin, confusion, or when reliever medicine is not helping.

Does this calculator store my peak flow readings?

No. Inputs, calculations, copying, and download preparation happen locally in the browser. This tool does not upload or save the values.