Find the true sloped area of a roof. This free roof area calculator scales the building footprint by the pitch (degrees or rise:12) to give roof surface area, roofing squares and material bundles.
Enter the building’s plan footprint and the roof pitch (as an angle or a rise:12 ratio). The calculator scales the footprint by the slope to give the true roof surface area.
Sloped roof area = plan footprint × slope factor, where slope factor = 1 ÷ cos(pitch). A steeper roof has more surface than its footprint. “Roofing squares” are 100 ft² units used to order materials; add overhangs and waste separately.
How Roof Area Calculator Works
A sloped roof always covers more surface area than the flat footprint of the building beneath it, because the slope stretches that footprint over a longer diagonal surface. This calculator takes your building’s plan dimensions and roof pitch and scales the footprint up to the true sloped roof area, then converts that into roofing squares and material bundles.
Formula: Slope factor = 1 ÷ cos(pitch angle). Roof area (sloped) = footprint area × slope factor. Roofing squares = roof area in ft² ÷ 100. Bundles needed = roofing squares × bundles per 100 ft², rounded up. If pitch is given as a rise:12 ratio, pitch angle = atan(rise ÷ 12).
- Building length & width — the flat plan dimensions of the building footprint, in metres or feet (or enter the footprint area directly)
- Roof pitch — the steepness of the roof, entered either as an angle in degrees or as a rise:12 ratio (e.g. 6:12)
- Slope factor — 1 ÷ cos(pitch), the multiplier applied to the footprint to get the true sloped roof area
- Bundles per 100 ft² — an optional shingle coverage rate used to estimate how many material bundles you’ll need
Example Scenarios
| Footprint | Pitch | Roof Area (Sloped) |
|---|---|---|
| 96 m² (12m × 8m) | 30° | 110.9 m² |
| 96 m² | 45° | 135.8 m² |
| 96 m² | 6:12 (26.6°) | 107.3 m² |
| 96 m² | 9:12 (36.9°) | 120.0 m² |
| 200 ft² | 30° | 230.9 ft² |
| 1,500 ft² | 40° | 1,958.1 ft² |
Tips for Using the Roof Area Calculator Before You Order
Measure the footprint at the eaves, including overhangs, not along the house walls. The roof area calculator can only scale what you give it, and overhangs often add a noticeable strip all round.
If you can’t safely climb up, measure the pitch from inside the loft with a level and tape, or from the gable end. Entering rise:12 or degrees correctly matters, because the roof area calculator applies a multiplier that grows quickly on steep roofs.
- Split roofs with wings, dormers or mixed pitches into sections and run the roof area calculator for each one.
- Add extra material for hips, valleys, ridges and starter courses, which the roof area calculator surface figure doesn’t include.
- Check bundles per square on the shingle wrapper, since coverage varies by product.
Treat the roof area calculator result as a planning figure, and ask your roofer or supplier to confirm quantities before buying. The Wikipedia article on roof pitch explains how pitch is measured and expressed.
Roof Area Calculator FAQ
Why is roof area always bigger than the building’s footprint?
The footprint is a flat, top-down measurement, but the roof itself is tilted at the pitch angle. Tilting a surface stretches it over a longer diagonal, so the true roof surface (footprint ÷ cos of the pitch) is always larger than the flat area below it, except on a flat roof.How do I convert a roof pitch like “6/12” into degrees?
A rise:12 pitch means the roof rises that many inches for every 12 inches of horizontal run. Convert it to degrees with the arctangent of rise ÷ 12 — for example, 6:12 works out to about 26.6°. Just select the Rise:12 option and enter the rise value directly.What is a “roofing square” and why does it matter?
A roofing square is a standard unit equal to 100 ft² of roof surface, used throughout the roofing trade to price labor and order materials like shingles, underlayment, and flashing. The calculator converts your sloped roof area into squares automatically.How many shingle bundles do I need for my roof?
Enter your shingle product’s coverage rate in the “bundles per 100 ft²” field (commonly around 3 bundles per square for standard three-tab shingles, but this varies by product) and the calculator multiplies that rate by your roofing squares and rounds up to a whole number of bundles.Does this calculator include overhangs or material waste?
No. It calculates the sloped area of the building’s plan footprint only. Real roofs typically extend past the wall line with overhangs, and installers add extra material for waste, valleys, and cuts, so budget additional area on top of this raw result.What roof pitch is too steep for this formula to make sense?
The formula works for any pitch from 0° up to just under 90°, since cos(90°) is 0 and would make the slope factor infinite. In practice, pitches near vertical aren’t really “roofs” anymore, so most residential roofs fall well under 45°.Related Calculators
Need to work out the footprint itself first? The general area calculator handles multiple shapes, while the rectangle area calculator is a quick way to get length-times-width for a simple rectangular building. If your roof design involves angled sections or gables, the triangle calculator can help you work out those angles and side lengths.