The footprint is not the roof. A 6:12 pitch adds about 12% to the area and a 12:12 adds 41%, which is the difference between ordering enough and going back to the merchant. Enter the plan size and the pitch, get the real surface.
Notes
Where the slope factor comes from
It is the hypotenuse over the run: for a rise of R in 12, the factor is sqrt(12*12 + R*R) / 12. A 6:12 works out at 1.118, so every square metre of footprint is 1.118 square metres of roof.
What a square is
A roofing square is 100 square feet, or about 9.29 square metres. Shingles are sold by the bundle, typically three bundles to the square, but check the wrapper because it varies.
Waste allowance
10% is normal on a simple gable. Hips, valleys, dormers and anything cut on the rake push it to 15% or more, because every cut piece leaves an offcut you cannot use.
What this does not do
It does not size structure, and it does not account for the specific coursing of your material. Complex roofs should be measured plane by plane.