Peak Flow Zones Calculator
Peak flow zone from personal best and today reading: green, yellow, or red.
What it does
Peak flow meters turn asthma control into one number: how fast you can blow out, tracked against your own best. Zones then map that percent to action, green for go, yellow for caution, red for emergency.
The tool above takes the personal best with today best of 3. It returns the percent, the zone, and the exact next step.
The default 300 against a 400 best reads 75%, Yellow, so quick relief per plan with a doctor call. A 450 against 500 would read 90% green, while 200 against 500 would read 40% red.
Bands follow American Lung Association and Asthma and Allergy Foundation guidance with AAAAI traffic light wording behind it. All three use the same 80 and 50 edges and the same meter discipline.
How to use
- Enter the personal best in L/min.
- Blow 3 times and enter the highest.
- Read the percent of best first.
- Read the zone color beside it.
- Follow the action line for that zone.
- Bring the meter and plan to visits.
Formula
- personal best
- Highest reading over 2 to 3 calm weeks, L/min.
- today
- Best of 3 blows today on the same meter.
- 80 / 50
- Zone edges: 80 green floor, 50 red ceiling.
Record the highest of 3 blows, never the average. An 80% reading counts green in these sources. Readings above best still read green, and reset the best with a doctor after calm weeks.
Worked example
Example: best 400, today 300
Defaults: personal best 400 L/min, today best of 3 is 300.
- Best of 3 today: 300 L/min against a 400 personal best.
- Divide: 300 / 400 = 0.75.
- Percent: 0.75 x 100 = 75%.
- Zone: 75 sits in 50 to 79, Yellow.
- Act: quick relief per plan, call the doctor.
- Tool shows 75 % of best, Yellow: caution, with the yellow action.
Reference table
| Zone | Range | Meaning |
|---|---|---|
| Green | 80 to 100% | Go, continue control meds |
| Yellow | 50 to 79% | Caution, relief plus doctor call |
| Red | Below 50% | Emergency, act at once |
Limits
- Estimates for information only, not medical advice.
- Personal best needs calm weeks to mean anything.
- Colds and technique shifts move readings.
- Children and pregnancy need doctor-set plans.
Common mistakes
- Averaging 3 blows instead of taking the best.
- Switching meters mid month.
- Testing right after quick relief.
- Skipping the written action plan.
- Resetting personal best alone after one high blow.
Try it yourself
Quick checks. Answers sit tucked under each question.
Best 500, today 450?
450 / 500 = 90%: Green, stay on control medicine.
Best 500, today 200?
200 / 500 = 40%: Red, quick relief now and emergency help if it holds.
Best 400, today 320?
320 / 400 = 80%: Green edge, keep tracking twice daily.
Questions
How do I find my personal best?
The top reading over 2 to 3 calm weeks, best of 3 blows twice daily on the same meter.
Is yellow serious with no wheeze yet?
Yes. A 20 to 30% drop can open an episode, so treat yellow rules as binding.
What do I do in the red zone?
Use quick relief at once and get care now. Go to emergency help if readings stay red.
Can I compare meters directly?
No. Meters differ, so each meter needs its own personal best.
Does personal best ever move up?
Yes, after a calm stretch. Set the new best with your doctor, not alone.
What should travel with me?
Carry the written plan and the meter. Early yellow action beats late emergency care.