How to use the roofing calculator
- Measure the length and width of the house at ground level and add the roof overhang (eaves) on each side. This is the roof’s “footprint”.
- Enter the pitch — as rise per 12 inches in US mode, or in degrees in UK mode.
- Add 10% waste for a simple gable roof or 15% for hip roofs and roofs with valleys or dormers.
How roof area is calculated
A pitched roof is larger than the footprint it covers. Multiplying the footprint by a slope factor gives the true area — and for roofs where every slope has the same pitch, this works for gable and hip roofs alike.
Pitch angle = arctan(rise ÷ 12) Slope factor = 1 ÷ cos(pitch angle) Roof area = footprint area × slope factor Squares = roof area (ft²) ÷ 100
Worked example (US)
A 40 × 30 ft footprint is 1,200 ft². At a 6/12 pitch the slope factor is 1.118, so the roof is 1,342 ft². With 10% waste that is 14.76 squares — 45 bundles of shingles at 3 per square.
Worked example (UK)
A 10 × 8 m footprint is 80 m². At 35° the slope factor is 1.221, giving 97.7 m² of roof, or 107.4 m² with 10% waste. That is about 1,075 large interlocking tiles or 2,149 slates.
Pitch and slope factors
| Pitch (US) | Degrees | Slope factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 5/12 (≈ 22.5°) | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 (≈ 30°) | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 8.4/12 (35°) | 35.0° | 1.221 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
What this calculator does not include
- Ridge, hip and valley caps or tiles, starter strips and drip edge — measure these by length.
- Underlayment or roofing membrane, battens (UK) and fixings.
- Flat roofs, which are measured by plain area (use the square footage calculator).
Roofing is dangerous work. Always use a qualified roofer for anything beyond simple repairs, and get several quotes — this calculator helps you check them.
Frequently asked questions
What is a roofing square?
A square is the US roofing unit for 100 ft² of roof surface. Shingles, underlayment and labour are usually priced per square. A 2,000 ft² roof is 20 squares.
How many bundles of shingles are in a square?
Most 3-tab and architectural (laminated) shingles come 3 bundles per square. Some heavy designer shingles need 4 or 5 bundles per square — check the wrapper.
How do I find my roof pitch?
In the US, pitch is the rise in inches over 12 inches of horizontal run: hold a level horizontally against a rafter or the roof, measure 12 inches along it, then measure straight up to the roof. In the UK, pitch is usually given in degrees, often listed on house plans. The calculator converts between the two.
How many roof tiles do I need per square metre?
It depends on the tile. Large interlocking concrete tiles need about 10 per m², natural slates (500 × 250 mm) about 20 per m², and traditional plain tiles about 60 per m². The manufacturer’s figure for your gauge and headlap is the one to use.
Does this work for hip roofs?
Yes. If every slope has the same pitch, the roof area is simply the footprint (including overhangs) multiplied by the slope factor — whether the roof is a gable, hip or a combination. Use 15% waste for hip roofs because of the extra cutting.
Is it safe to measure my roof myself?
You can measure the footprint from the ground and get the pitch from the loft or plans, which is all this calculator needs. Leave walking on roofs — especially steep or wet ones — to professionals with the right safety equipment.