# Q- Surface area reported from Models: outward facing polys only?

I have a quick question on the surface area number reported by the Models module. I did a quick exercise where I made a 5cm sphere (well, as close as I could get by hand using the ruler) and compared reported values to hand-computed values - 4pir^2, etc.

The value I get for surface area is what I would expect, but I was a little confused from earlier posts (Using surface cut of segment editor to measure area) regarding counting front-facing and back-facing polygons in the area calculation, and needing to divide by 2.

For a â€śclosed surface representationâ€ť, such as a segment converted to a model, it seems that the computed surface area is the outward-facing polygons only, and one does not need to divide the reported quantity by 2. Is this correct?

Thanks!!

-Hollister

I think what was meant for surface cut is that on a 2D image if you draw say a circle then your 3D surface is like a circle disc shaped object. Therefore the surface area includes both the front side and the back side so you would divide by 2 to get the area of the circle. I guess technically there is an edge to the disc which has surface area, but dividing by 2 will get you close.

You only need to divide the surface area by 2 if you want to measure area of an open surface by creating a very flat closed surface around it (as the closed surface wraps around both sides of the open surface).

For open surface measurements I would no longer recommend creating a closed surface and divide by 2, but instead you have better options, such as:

A. Use closed curve markups:

B. Use cross section area measurement module:

1 Like