Formula
Pitch multiplier = √(1 + (rise ÷ 12)²)Roof area = footprint × multiplierSquares = roof area × (1 + waste) ÷ 100 ft²Bundles = squares × 3 (typical 3-tab or architectural shingles) How it works
A sloped roof has more surface than the ground it covers. Measure the house footprint including the eaves overhang, then multiply by a pitch factor: a 6/12 roof rises 6 inches for every 12 inches of run, so its surface is √(1 + 0.5²) ≈ 1.118 times the footprint.
Roofing is ordered in “squares” of 100 ft². Most shingles come three bundles to the square, though some heavy designer shingles need four or five — check the wrapper. Add waste for starter strips, ridge caps and cut-offs at hips and valleys.
This method assumes the whole roof has one pitch. For roofs with mixed slopes, dormers or large porches, work out each section separately and add them together, or ask a roofer to measure from aerial imagery.
Frequently asked questions
How do I find my roof pitch?
Hold a level horizontally against the rafter or roof surface, measure 12 inches along it, and measure straight up to the roof. That rise in inches is the pitch (x/12).
Does the gable shape matter?
Not for area. Gable and hip roofs with the same footprint and the same pitch on every plane have the same surface area; hips just create more waste.