If you want to try your hand at hydraulic engineering then here's the equation for an orifice flow, which is exactly what a submerged bulkhead is:
Q = CxAx(2gh)^(1/2)
where:
Q = flow rate in cubic feet per second
C = discharge coefficient. Using 0.75 for this value would be conservative. If you have a screen over your bulkhead I'd lower this value to around 0.6 or so.
A = cross sectional area of the bulkhead in feet squared. See Broomer5 explanations above.
g = acceleration due to gravity (32.2 feet/sec^2)
h = distance from the centerline of the bulkhead to the water surface. Also known as the hydraulic head. This value is in feet.
As Broomer5 mentioned, by using equations to calculate flow rates you're not going to come up with an exact answer that matches exactly what's happening in real life. The reason for this in this equation is the coefficient of discharge. No one knows exactly what this number should be and if you ask 10 different hydraulic engineers you're going to get 10 different answers (anywhere from 0.6 to 1.0). This value also varies given the real world conditions of the problem at hand. But, with this equation, you can closely ESTIMATE a flow rate that's pretty close to what's happening in real life.
You have to be careful about units. You'll notice in this equation everything is in feet and seconds and the flow rate is in cubic feet per second whereas everything in the aquarium industry uses gallons per hour.
An example:
Let's say you have an open bulkhead 1" in diameter. It's located in your aquarium/sump/refugium, whatever, so as the distance from the centerline of the bulkhead to the water surface is approximately 5 inches. Now to calculate the flow rate.
C = 0.75
A = 0.005454 feet squared. See Broomer5 post for equation for the area of a circle
g = 32.2 feet/sec^2 (This is a constant)
h = 0.4167 feet (5 inches divided by 12)
Q = 0.75x0.005454x(2x32.2x0.4167)^(1/2)
Q = 0.02119 cubic feet per second
Now we need to convert this flow rate back to something we're more familiar with.
1 cubic foot = 7.4805 gallons
1 hour = 3600 seconds
so multiply your answer by 26929.8 to get gal/hour
Q = 571 gallons per hour
This flow rate is going to change depending on how much head you have above the centerline of the bulkhead.
I hope this helps a little but I'm afraid I'm drastically confusing people. If this is the case then ignore this post and go with the averages that Brommer5 posted. That's much easier!
But if you're an aspiring engineer then go for it!!