how to measure umbrella size?

May be it sounds stupid, but how umbrellas are measured? What does it mean -

45"? The reason I'm asking is - I bought two 45" umbrellas (Westcott and

Photogenic) from Adorama.com. Both have 42" in diameter (I measured the max

diameter, between two opposite rods). Is it ok, or I got the wrong items?

You have it right but is 3" worth worrying about in this case?

To measure the umbrella to the manufactors measurements measure the other side the curved one. You will find that the measurement matches.

If the rain is coming in at 45degrees ...and you don't get wet standing upright...it's a big one. :~)

It would make sense to measure the ribs along the curve, or the length of the rib when folded. Personally, I consider the specification to be nominal - only the results count.

The umbrella dimension is basically the rib length, measured on the curve over the top.

 

The straight across diameter will be less. How much less depends on the curvature or depth... whether it is a shallow more flat umbrella or a deep curved umbrella. Many 45s are around 40 inches (flatter). Alienbees are about 37 inches (deep). I like the flatter ones.

 

I have one 60 inch Photogenic Eclipse, which measures 48 inches across.