How to calculate roof pitch
Roof pitch is how many inches the roof rises for every 12 inches it runs horizontally. Put a level against the rafter or roof deck, mark 12 inches along it, and measure straight up to the roof. If that measurement is 6 inches, you have a 6/12 pitch.
To turn pitch into an angle, take the arctangent of rise / 12. A 6/12 roof is about 26.6 degrees, a 4/12 is about 18.4 degrees and a 12/12 is exactly 45 degrees.
How to calculate roof square footage
Measure the outside of the house, add the overhang on every side, and multiply length x width to get the footprint. Then multiply by the pitch multiplier, which is the square root of 1 + (rise / 12) squared. A 6/12 multiplier is 1.118, so a 2,000 sq ft footprint has about 2,236 sq ft of roof.
This works for simple gable and hip roofs. Dormers, valleys and multi-level roofs should be measured section by section, or pulled from a satellite measurement report, and added together.
Squares, bundles and waste
One roofing square covers 100 sq ft. Standard architectural shingles come three bundles to a square, so 25 squares is 75 bundles. Always round bundles up.
Add waste before you order. Around 10% is typical for a simple gable roof, 15% for hips and valleys, and more for cut-up roofs with lots of penetrations or steep pitches.
From measurement to signed roof
Once you have squares, the price comes together fast. QuotesForge lets you build the roof estimate by square, add tear-off, underlayment, flashing and disposal as line items, then send a branded proposal the homeowner can sign and put a deposit on from their phone.