How to Calculate Roof Square Footage
Published 2026-09-04
Measure the building footprint in square feet, then multiply by the slope factor for your pitch — 1.118 at 6/12. A 40 × 25 footprint at 6/12 is 1,000 × 1.118, or about 1,118 square feet of roof, which is 11.2 roofing squares.
Roof area is never the same as the floor plan underneath it. Getting from one to the other takes one multiplication — and skipping it is the single most common way a shingle order comes up short.
The two-step formula
footprint ft² = length × width (measured on the ground)
roof ft² = footprint ft² × slope factor
roofing squares = roof ft² ÷ 100The slope factor comes from the pitch: √(rise² + 144) ÷ 12 — see how to calculate roof pitch for measuring it, or let the roofing squares calculator do both steps. If you do not know your pitch yet, measure it first — it is a two-minute job with a level.
Measuring the footprint
Walk the perimeter at ground level and measure the outside of the walls, including the eave overhang. Break an L-shaped or T-shaped house into rectangles, measure each, and add them:
| Section | Dimensions | Area |
|---|---|---|
| Main house | 40 × 25 | 1,000 ft² |
| Rear extension | 14 × 12 | 168 ft² |
| Attached garage | 24 × 22 | 528 ft² |
| Total footprint | 1,696 ft² |
An overhang that projects 18 inches on all four sides of a 40 by 25 house adds about 70 ft² — a full square of shingles. Measure to the drip edge, not to the wall.
Change any measurement to update your estimate.
- Shingle bundles
- 37 bundles
- Slope factor
- ×1.118
- Footprint
- 1,000 ft²
Applying the slope factor
That 1,696 ft² footprint at a 6/12 pitch:
1,696 × 1.118 = 1,896 ft²
1,896 ÷ 100 = 19 roofing squaresIf the garage is a lower pitch than the house — very common — calculate the two sections separately with their own factors and add the results. Averaging the pitches does not work.
From squares to bundles
A roofing square is 100 ft² of finished roof — the unit the National Roofing Contractors Association and every supplier quote in — and three bundles cover one square for standard three-tab and most architectural shingles. Heavier designer shingles run four or five bundles per square; the wrapper states it.
| Roof | Squares | Bundles at 3/sq | With 10% waste |
|---|---|---|---|
| 1,000 ft² | 10 | 30 | 33 |
| 1,896 ft² | 19 | 57 | 63 |
| 2,500 ft² | 25 | 75 | 83 |
What the waste percentage covers
Ten percent is the usual allowance, and it is not padding:
- Starter course along every eave, cut from full shingles unless you buy starter strip
- Hip and ridge caps — figure one bundle per 35 linear feet of ridge and hip
- Valley cuts, which waste roughly a third of every shingle crossing them
- Rake trimming at both gable ends
A simple gable roof with no valleys can run closer to 7%. A cut-up roof with dormers, hips and multiple valleys needs 15%, sometimes more.
Do not subtract the openings
Skylights, chimneys and vents are not deducted. The cuts around them waste more material than the opening saves, and you want the offcuts on hand when a shingle splits during install.