How the calculator works
You can estimate a roof from the ground: the footprint area times a pitch multiplier gives true surface area, because a sloped plane is longer than its horizontal shadow. The multiplier is √(rise² + 12²) ÷ 12, so at 6/12 pitch every 100 sq ft of footprint is 111.8 sq ft of roof. Roofing is measured in squares (100 sq ft), and architectural shingles ship 3 bundles to the square.
Worked example: a 40 × 30 house (footprint measured to the overhangs) at 6/12 pitch with 10% waste: 1,200 × 1.118 × 1.1 = 1,476 sq ft = 14.8 squares = 45 bundles, plus 4 rolls of underlayment.
What the estimate can't see
- Hips and valleys add cutting waste, so use 15% instead of 10%.
- Ridge caps: figure one bundle of ridge cap shingles per 25–30 linear ft of ridge and hips.
- Starter strips: one bundle per ~100 linear ft of eaves and rakes.
- Ice & water shield: code in cold climates for the first 3–6 ft above eaves and in all valleys, so measure those runs separately.
DIY reality check
A bundle weighs 60–80 lb and a modest roof means moving two tons of shingles up a ladder. Most suppliers offer rooftop delivery for a modest fee. Take it. And this calculator estimates materials, not safety: anything above one story or past 6/12 pitch is harness territory, and many jurisdictions limit total shingle layers to two before a tear-off is required.