Children as young as 4 or 5 years may be able to use a peak flow meter, if they’re at least 43 inches tall. Peak flow meters provide information that's used with peak flow rate charts or calculators. These charts give estimated information about peak flow zones, so you can determine if your child is in the green, yellow, or red zone for breathing. On peak flow charts, estimated personal best peak flow starts at 43 inches. If your child is shorter than 43 inches, talk with their pediatrician about using a peak flow meter and other options.