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?

>>1,4
I think both of you niggers need to quit abusing the word "evolved".