Here is a couple chart's that should help you all at one time or another.
http://internetwoodworking.com/w5/equivs.html
http://www.newmantools.com/decinch.htm
MM conversion chart too. Simple formula is the mm is multiplied by .03937, which is constant. Thus 8mm x .03937 = .31496, pretty close to 5/16.
I use a letter "I " drill for the .273 grommets and I use an 11/32 for the larger grommets. They fit snug. I sometimes have to take an exacto knife to clean out the paint and sealer as the hole once and a while will be too snug but not a real problem and easy to fix. The only problem is the they are not forstner bits which most like to use, but my drilling set up is probably different than most. Might not be what people are looking for, but the charts are worth knowing.
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