Breath shows when you breathe in to when you breathe out. This goes in conjunction with tidal volume, which reflects how much air exchanges during a breath. In order to see this, you need a chart, usually from a respirometer. The respirometer tracks your breathing and graphs points that you breathe in and breathe out. The breath is from one peak to the next peak. The tidal volume is from the peak to a trough.
Start at one peak on the respirometer chart and follow that down. The bottom is the trough. Go back up to another peak; from one peak to the next peak is the breath.
Find the measurement at the top of a peak.
Find the measurement at the corresponding trough after the peak.
Subtract the trough measurement from the peak measurement to calculate tidal volume.