How do you calculate roof area from the footprint?

Multiply the plan footprint area by the slope factor, which is the square root of (rise squared plus run squared) divided by run. A flat roof has a slope factor of 1; a 6-in-12 pitch has a slope factor of about 1.118, so the real surface is roughly 12 percent larger than the footprint.

What is a roofing square?

A roofing square is 100 square feet (about 9.29 square metres) of roof surface. Shingles, underlayment and labour are often priced per square, so the calculator converts your sloped area into squares automatically.

How many shingle bundles will I need?

Standard three-tab and architectural asphalt shingles take about three bundles to cover one roofing square. The calculator multiplies your waste-adjusted squares by three and rounds up, so you always buy whole bundles.

Why does the tool add a waste percentage?

Cuts at hips, valleys, ridges and edges produce offcuts, and you want spare material for repairs. The calculator adds a material-specific waste allowance (7 to 12 percent) on top of the measured area before counting bundles, rolls and cost.

Does roof shape change the area?

For the same footprint and pitch, a gable and a hip roof have almost the same total sloped area, because both equal footprint times slope factor. Shape mainly changes the ridge and hip capping length and the waste, which the tool reflects separately.

Is my data sent anywhere?

No. Every measurement and price stays in your browser and the whole calculation runs client-side. Nothing is uploaded, logged or stored on a server.

Roof Area Calculator

A roof area calculator that converts a simple building footprint and roof pitch into the true sloped surface area of the roof, then estimates the materials and cost to cover it. It is built for homeowners sizing a re-roof, builders pricing a job, and DIYers ordering shingles who need a number that accounts for the slope rather than just the floor plan. Because a pitched roof is always larger than the rectangle it sits on, measuring the ground area alone under-orders material every time — this tool fixes that with the standard slope-factor maths.

How it works

The calculation has three stages. First the tool builds the plan footprint: your building length times width, expanded by the overhang you enter on each side so the eaves and verges are included. Second it applies the slope factor, the ratio of the sloped rafter length to the horizontal run. With a pitch entered as rise over run, the slope factor is:

slope factor = √(rise² + run²) ⁄ run

Multiplying footprint by slope factor gives the actual roof surface area. A flat roof has a factor of 1, a 4-in-12 pitch about 1.054, a 6-in-12 about 1.118, and a steep 12-in-12 roof about 1.414 — so a steeply pitched roof can be over 40 percent larger than its footprint. Third, the tool adds a material-specific waste allowance (asphalt 10 percent, metal 8 percent, tile 12 percent, membrane 7 percent), converts the result into roofing squares (one square = 100 ft²), and turns that into shingle bundles, underlayment rolls and ridge and hip capping length. Finally it multiplies the waste-adjusted area by your material and labour prices for a cost estimate.

Worked example

Take a rectangular house measuring 12 m by 8 m with a 0.4 m overhang all round and a 6-in-12 gable pitch. The expanded footprint is 12.8 × 8.8 = 112.64 m². The slope factor for 6-in-12 is √(36 + 144) / 12 = 1.118, so the actual roof surface is 112.64 × 1.118 ≈ 125.9 m². Adding the 10 percent asphalt waste allowance gives about 138.5 m² to order — roughly 14.9 roofing squares, which is about 45 shingle bundles and 4 underlayment rolls. If material runs £28 per m² and labour £22 per m², the estimate is around 138.5 × 50 ≈ £6,900 before fees. Switch the same roof to a 12-in-12 pitch and the surface jumps to about 159 m² — the same floor plan, far more roof.

Reference and formula note

Slope factor (also called the roof-pitch multiplier) is √(rise² + run²) / run; the pitch angle is arctan(rise / run) in degrees and the percent grade is rise / run × 100. One roofing square is 100 ft² or 9.2903 m². Asphalt shingles average 3 bundles per square; underlayment rolls are sized here at roughly 40 m² (≈430 ft²) of coverage each. These packaging figures are tuned to asphalt — for metal panels, concrete or clay tile, and EPDM or TPO membrane, treat the surface area and squares as exact and confirm bundle, panel or roll counts against the manufacturer pack size. The cost figure is your material price plus labour price per unit area, applied to the waste-adjusted surface, so the displayed currency simply follows whatever your prices are in.