You will need a 1.75" hole for a 1" bulkhead. That's using the standard schedule-40 bulkhead. Schedule 80 bulkheads exist, we don't really need to use them on aquariums, but if you got your hands on one of those then it would need a bigger hole due to the thicker wall thickness.
A 1.5" bulkhead will need a hole, I would guess, at least 2.25", maybe even 2.5". I have some 1.5" bulkheads at home I can double check the size of hole they need.
I have a 1.75" hole saw for glass so I can help you with a 1.75" hole for the 1" bulkheads. Unfortunately if you need anything larger I can't help, but you should be able to go into any glass shop or aquarium shop and have them do it for you.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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