Roofing Calculator
Size a re-roof from the ground up. Enter your house length and width (or a known footprint) and the roof pitch, and the tool returns true sloped area, roofing squares, shingle bundles, underlayment rolls, nails, ridge cap and drip edge, with an optional material cost. Switch between imperial and metric, and add an eaves overhang for accuracy.
Formula
Worked example
A 50 ft by 30 ft house is a 1,500 ft² footprint. With a 6:12 pitch the slope factor is √(6² + 12²) ÷ 12 ≈ 1.118, so true roof area = 1,500 × 1.118 ≈ 1,677 ft². Adding 10% waste gives 1,845 ft², about 18.4 squares. At 3 bundles per square that is 56 bundles of architectural shingles.
How the roofing estimate works
Roofers measure material in squares, where one square equals 100 square feet of finished roof surface. The catch is that a sloped roof has more surface than the flat footprint it sits on, so you cannot simply divide the building outline by 100. This calculator first works out your covered footprint, either from the house length and width (plus any eaves overhang) or from a footprint area you enter directly, then multiplies it by a slope factor that depends on the pitch. The steeper the roof, the larger the factor. It then adds a waste percentage for trimming and laps, and finally divides by 100 to give the number of squares to order. The slope factor itself is the hypotenuse of a right triangle with a rise and a 12-inch run, divided by that run, which is why a 12:12 roof carries a factor of about 1.414.
Material breakdown: bundles, underlayment, nails and trim
Beyond the bare square count, the tool turns your order into a shopping list. Asphalt shingles ship three bundles to a square, so it multiplies squares by three and rounds up. Underlayment, whether felt or synthetic, typically covers about two squares per roll, so it divides squares by two. Roofing nails run around 320 per square at four nails per shingle, and steep or high-wind roofs that call for six nails per shingle need proportionally more. Ridge and hip cap covers the peaks, and drip edge runs the full perimeter of the eaves and rakes, so both are reported as linear measurements. Metal, tile and membrane systems are bought by area rather than in bundles, so for those the list shows the field area to order instead of a bundle count.
Choosing pitch, waste and material
Pitch is expressed as rise over a 12-inch run, so a 6:12 roof climbs six inches for every foot of horizontal travel. If you do not know your pitch, a level and tape measure on the roof or in the attic will give you the rise over 12 inches. Waste allowance is the next judgment call: a plain gable roof with two simple planes wastes little, so 10% is generous, while a roof broken up by hips, valleys, dormers and skylights can waste 15% to 20%. The material you pick sets a default price per square so the optional cost estimate has a sensible starting point, but local quotes vary widely, so override the price with a real number from your supplier. Buying slightly long is cheaper than a second delivery, and keeping a few spare bundles makes future repairs match the original color lot.
What this estimate does and does not include
The result covers the field shingles or panels and the common accessories: underlayment, nails, ridge cap and drip edge. It does not size flashing around chimneys, walls and penetrations, valley metal, vents, sealant or fasteners for accessories, which you add on top. The cost figure is for materials only and excludes labor, tear-off, disposal, permits and any deck repairs, which often make up half or more of a re-roof bill. For load-bearing or structural decisions, and for steep or multi-story work, consult a licensed roofer. This tool is a material-quantity and budgeting estimate, not an engineering or safety specification.
Roof pitch slope factors
| Pitch (rise:12) | Approx. angle | Slope factor |
|---|---|---|
| Flat | 0° | 1 |
| 4:12 | 18.4° | 1.0541 |
| 6:12 | 26.6° | 1.118 |
| 8:12 | 33.7° | 1.2019 |
| 10:12 | 39.8° | 1.3017 |
| 12:12 | 45° | 1.4142 |
Multiply the flat footprint by the slope factor to get true roof area.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of finished roof surface. Shingles, underlayment and labor are commonly quoted per square, so an 1,800 ft² roof is 18 squares.
Why is my roof area bigger than my house footprint?
Because the roof is sloped. A pitched plane has more surface than the flat ground it covers. The slope factor, for example about 1.118 at 6:12, scales the footprint up to the true sloped area.
How many bundles of shingles are in a square?
Standard asphalt shingles, both 3-tab and architectural, come three bundles to a square. So a roof of 20 squares needs about 60 bundles, before rounding up for waste. Heavier designer shingles can run four or more bundles per square.
How much waste should I add?
Plan on about 10% for a simple gable roof, and 15% to 20% for complex roofs with many hips, valleys, dormers or skylights, where each cut produces an offcut you cannot reuse.
Does the cost estimate include labor?
No. The optional cost figure prices the field material only, using your price per square. Labor, tear-off, disposal, permits, flashing and deck repairs are separate and often add up to half or more of a full re-roof.