How do I measure the volume of air in a breath?

I'm not sure, but thinking a little, I think that perhaps blowing in some kind of balloon, or better, directly into a gas syringe.. You can try this, but I'm not a specialist in that.

How do I measure the volume of air in a breath?
If you have a flowmeter, you cann blow into it and measure the time and calculate the volume.

I better explain more, a digital flowmeter shows the flow rate, which is volume/time, so if you measure how long you have inhaled in a flowmeter and multiply it by the flow rate that you can read on the flowmeter you can find the volume. However, to be accurate you have to inhale with the same speed. It is tricky. Does it help?

A Giant Syringe could work, as you blow into the end the plunger would push out, then you measure the length of travel and diameter of syringe.
Maybe 4" diameter by 2 feet? You may have to custom manufacturer.
Some additional info:
Breath measurements and units in the metric system (http://www.800mainstreet.com/e2/e22.html)

The lowest tech (and dubiously accurate) way I can think of:

Take a plastic bag or balloon or something and breathe into it. Seal it off.
Put a large container (a small bucket) inside a larger container (a larger bucket) and fill the small bucket to the very top with water.
Submerge the balloon/bag entirely in the water, while avoiding putting your hands in.

Now measure the volume of water that spilled into the larger bucket. This will be equal to the volume of air in the bag (or very close, at least). You can use a measuring cylinder/jug to do this.

(This method was famously used by Archimedes to find if a golden crown was made with pure gold!)

I was even thinking filling a drywall bucket to the top with water, then blow through a tube in the bucket of water and measure how much flows over the top.