Water Pump O-ring Size?

I'm in the process of a timing belt change and in the process I'm changing the water pump - all is going well (2nd time around for me so work is going much easier and more quickly) but I forgot all about the water pump o-ring - Does anyone know the correct size and material? I'm assuming it's just a Buna o-ring but probably a metric size??? If anyone knows so I can pick on up locally I'd appreciate it. BTW - for a 1980 GTSi... if there happens to different sizes or versions through the years. Thanks - Nick