WaterBear,

In the formulas I've posted, the round to square is the

accurate one since it was derived from the ASHRAE formula.

Since you're the one in need of a shorcut, you can verify

the accuracy of the formula for all cases. I repeat, I can

vouch for the accuracy on round to square cases only. The

formulas for the rectangular duct was derived using an equal

area method using the area of the square duct equivalent of

the round duct. I've not tested the conversion formulas and

the accuracy might be questionable. Sorry for the confusion,

as I was hoping that you'll be able to see through the formulas.