Roofing Calculator
Estimate roof area, roofing squares, shingle bundles and material cost from your footprint.
Results update as you type.
About this calculator
A roofing calculator turns the flat footprint of a building into the true sloped roof area and the material needed to cover it. The roof pitch — the rise in inches for every 12 inches of horizontal run — sets a slope multiplier equal to √(rise² + 12²) ÷ 12, which scales the footprint up to the actual surface. Roof area is measured in "squares" of 100 square feet, and asphalt shingles come three bundles to the square. A waste allowance covers cuts, ridges, hips and starter courses.
Worked example: a 40 × 30 ft footprint is 1,200 sq ft. A 4/12 pitch has a multiplier of √(4² + 144) ÷ 12 = √160 ÷ 12 ≈ 1.054, so the roof surface is about 1,265 sq ft, or 12.6 squares. With 10% waste that is about 13.9 squares — roughly 42 bundles of shingles.
This covers the main field area; add extra for ridge caps, valleys, dormers and complex rooflines. Homeowners and roofers use it to estimate a re-roof, compare quotes, and order shingles and underlayment without a big over- or under-buy.
Frequently asked questions
What is a roofing square?
A square is 100 square feet of roof surface — the standard unit roofers and suppliers quote in. A 2,000 sq ft roof surface is 20 squares.
How does roof pitch change the amount of material?
A steeper pitch means more actual surface over the same footprint. A 4/12 roof is about 5% larger than the flat footprint; a 12/12 roof is about 41% larger. The pitch field applies the exact √(rise²+144)/12 multiplier.
How many bundles of shingles are in a square?
Standard three-tab and architectural asphalt shingles are three bundles per square. Heavier designer shingles can be four or five, which you can set under advanced options.
How do I find my roof pitch?
Measure how many inches the roof rises over a level 12-inch run — hold a level out horizontally from the roof and measure down to the surface at the 12-inch mark. A rise of 4 inches is a 4/12 pitch. Most houses fall between 4/12 and 9/12.
How many bundles of shingles will I need for my roof?
Three bundles cover one square (100 sq ft) of standard asphalt shingle, so multiply your roof area in squares — including waste — by three. The example 13.9 squares needs about 42 bundles; heavier designer shingles may be four or five per square.
Is the footprint the same as my house’s floor area?
It is the ground area the roof covers, measured to the outer edges including eaves and overhangs, not the interior floor area. For a simple gable roof that is close to the building outline; measure to the drip edge, then let the pitch multiplier handle the slope.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/roofing/curl
curl "https://calculator.free/api/v1/roofing/?length=40&width=30&pitch=4"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/roofing/?" + new URLSearchParams({
"length": "40",
"width": "30",
"pitch": "4"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.