Unless you really want to learn formulas, why not use an online calculator?

No, that's not correct at all.

3.5" is equivalent to 3.5/12 ft = 0.292 ft. There are 27 cubic feet in a yard, so 27/0.292 = 92.5 square feet per yard at a depth of 3.5 inches. However, that seems like an awkward way to figure things out. To figure out how many yards you need, you should take your depth in feet (in this case, 0.292 ft) and multiply it by your square footage. The result would give you cubic feet; to get cubic yards, you would divide that number by 27.

For example, if you have a 10' X 20' slab, you would have 200 square feet; multiplying that by 0.292 ft gives a result of 58.4 cubic feet. Dividing this by 27 gives you a result of 2.17 cubic yards.

If you want to do this for a different thickness of slab, say 5", you would need to figure out what fraction of a foot 5" is: 5/12 = 0.42 ft. For that same 10' X 20' slab, the cubic feet would be the product of 0.42 times 200 = 84 cubic feet; dividing that by 27 gives you 3.1 cubic yards.

And whatever result you get, order a little more concrete so you don't end up a wheelbarrow short at the end of your carefully calculated pour.

or simply put

(length in feet) x (width in feet) x (thickness in feet) = volume in cubic feet

(volume in cubic feet) / 27 = volume in cubic yards.

if you need to convert from feet to inches then divide by 12.

i.e. (thickness in inches) / 12 = (thickness in feet)

It is divide by 80.

3.6 is the number by calculating it out and 4.2 is the number when divided by 80. Close enough for a good starting reference point.

Including extra I would have added at 3.6

But we thought you wanted confirmation of the right formula. Oh well...

I know how to calculate volume. This is a reference formula some contractors use to estimate cubic yards.

Ok...but when you seemed to indicate in your first post that you were going to multiply square footage by 80 it wasn't clear you knew how to calculate volume properly. And when you asked for confirmation of a formula it wasn't clear you were asking for a rough estimation technique. Anyway, glad you got it resolved.

so the bottom line is that your contractor figures 15% more as a contingency.

Direct would be divide by (1/(3.5/12/27) = 92.6)

92.6/80 = 1.158

You can easily calculate a formula for any thickness slab now or you can just add a 15%waste figure to the actual number.

I was still trying to guess what 'at 3.5 deep' meant, since there were no units. Units are important. :-D