There are several factors to consider with the hole size needed for a bulkhead. The information below is considered "general" guidelines for the most popular types of bulkheads. (For example they may not apply to schedule 80 bulkheads).

Bulkhead Size Recommended Hole Sizes
3/4 inch 1 1/2 inches 35 to 40 mm
3/4 inch thick wall 1 3/4 inches 45 mm
1 inch 1 3/4 inches 45 mm
1 1/2 inch 2 1/2 inches 60 to 65 mm

Be sure to check with your supplier before ordering, and be sure to have bulkheads in hand before having a tank custom drilled!! 

3/4 inch bulkhead 1.0 inch bulkhead 1 1/2 inch bulkhead

The 3/4 inch (left), 1 inch (middle) and 1.5 inch (right) bulkheads