What the numbers mean
Roofers measure in squares — one square is 100 square feet of roof surface. Your roof's surface is larger than your home's floor plan, because slope adds area: the steeper the pitch, the more roof sits over the same footprint. When aerial data is available we measure every roof plane and its own slope; when it isn't, you trace the building outline and pick a steepness, and we do the honest math from there.
Why we stop at size
Plenty of tools convert a satellite guess straight into a dollar figure. We don't, on purpose. The real price of a roof depends on things no satellite can see — how many layers come off, what the decking looks like underneath, flashing details, access, and the material you choose. Size is the honest half of the answer; the other half is a free inspection and an exact price in writing before we leave the driveway.