A peak flow meter is an inexpensive, portable, handheld device for people with asthma that is used to measure how well air moves out of your lungs. Measuring your peak flow using this meter is an important part of managing your asthma symptoms and preventing an asthma attack and should form part of your personal asthma action plan Peak flow is measured using a small handheld device called a peak flow meter. The scale is in units of liters per minute (lpm). When you blow into the mouthpiece the small marker will move depending on how fast you are blowing. Where the marker ends up, is your peak flow reading. The highest reading from three attempts should be recorded as your personal best peak flow reading.