Measuring volume of a gas

Sup /sci/

I have to do an experiment where I measure the volume of a gas. I could do all that eudiometer bullshit, but I'd rather use a manometer instead. If I know atmospheric pressure, temperature, and then the pressure after a reaction in a sealed Erlenmeyer flask attached to the manometer, how can I get the volume of the gas evolved during the process?

What you could get, however, is the numer of moles of gas evolved, which would be of far greater interest.