Free Roofing Calculator

How the roofing calculator works out roof area, squares and pitch

Every roofing estimate runs in the same order, and the calculator above follows it. Get the first three numbers right — footprint, pitch, roof area — and everything downstream (bundles, rolls, trim, cost) follows from them.

1. Footprint. Measure the house on the ground and add the overhang on every side: a 12 in overhang adds 2 ft to both the length and the width. Length x width is the flat area the roof covers.
2. Pitch multiplier. Pitch is the rise in inches over 12 in of run. The multiplier is the square root of 1 + (rise / 12)², so a 6/12 roof is 1.1180 and a 12/12 roof is 1.4142. The roof pitch chart lists every value.
3. Roof area. Footprint x multiplier is the real sloped surface. It is the same for a gable or a hip roof of equal pitch, because every plane rises at the same rate.
4. Squares. Roof area / 100. Roofing is priced and ordered by the square.
5. Waste. Add about 10% for a simple gable, 15% for a hip and up to 20% for a cut-up roof with valleys and dormers. The waste factor calculator goes through the cases.
6. Materials. Standard architectural shingles come three bundles to the square (33.3 sq ft a bundle), synthetic underlayment about 10 squares a roll, drip edge in 10 ft lengths along the eaves and rakes, and hip and ridge cap by the linear foot of ridge and hips.

Worked example: a 40 x 30 ft house with a 6/12 roof

The same house with a hip roof has the same 1,503 sq ft of surface but more cutting, so it takes 15% waste (52 bundles), drip edge on all four eaves (148 ft, no rakes) and 106 ft of hip and ridge cap instead of 42. See the hip roof calculator for the hip rafters.

Roof size to squares and bundles

Footprint measured outside the eaves, 10% waste, three bundles per square. For a hip roof add a further 5% or so; for heavier designer shingles that come four or five bundles to the square, use the shingle calculator and change the coverage.

How to measure a roof for the calculator (without climbing on it)

1. Measure the house walls on the ground. Run a tape or measuring wheel along the length and the width of the outside walls. Garages and porches under the same roof get measured as their own rectangles.
2. Measure the overhang. Stand under an eave and measure from the wall to the fascia — 12 in is typical, 16 to 24 in on many newer houses. The calculator adds it to both sides of the length and width.
3. Find the pitch from the ground or the attic. In the attic, hold a level against a rafter, mark 12 in along it and measure straight down to the rafter: that number is the rise. From outside, count siding courses on the gable end, or use the roof pitch calculator.
4. Split complex roofs. Add an L-shaped house as two rectangles; work any section with a different pitch on its own. Dormers and valleys go on top as extra waste.
5. Check it against an aerial view. Satellite measurements of the footprint should land within a few percent of your tape — if they differ by more, measure again. The step-by-step measuring guide has diagrams.

Roof cost by material for the same 40 x 30 ft house

The calculator's material picker uses these installed bands (materials and labour, before tear-off). On the 1,503 sq ft roof from the worked example:

Slate and cedar bands are This Old House's 2026 cost guides; asphalt is this site's 2026 cost-per-square guide. Price one material in detail with the asphalt shingle, slate or cedar shake cost calculator. For metal panels use the metal roofing cost calculator, and for tile the tile roof calculator.

Resale: the Cost vs. Value 2025 report (Zonda / JLC) puts a 30-square asphalt shingle tear-off and replacement at $31,871 nationally, returning $21,501 at sale (67.5% recouped); the metal roof version costs $51,865 and returns $25,972 (50%). Those are full-service contractor jobs, so they run above the planning bands here.

What a roof calculator cannot see

• Dormers, valleys and chimneys. Add them as extra planes or raise the waste; valleys and chimney flashing have their own calculators.
• Old layers. A tear-off adds labour and disposal — the tear-off cost calculator prices it.
• Bad decking. Rotten sheathing only shows once the old roof is off; the plywood calculator sizes a re-deck.
• Local prices. Labour varies more by region than materials do. Compare roof cost by state before trusting any single figure.
• Ventilation. A new roof is the time to fix it — the roof vent calculator balances intake and exhaust.

Every calculator on this site runs in your browser, with no sign-in and no email. Each one shows the formula it used, and the figures in the guides come from the same functions, so the two cannot disagree.