Roofing Calculator
Find the area of your roof from the house size and pitch, how many squares, shingle bundles, metal sheets or tiles to buy, and what it will cost.
Measure the outside walls. Overhangs are added below.
The roof covers the floor area ÷ storeys.
1 for a square house; 1.5 is a typical oblong.
Share of the half-width (seen from above) under the steep part.
Often 12–24 in (30–60 cm). 0 if none.
Porch, garage or dormer roofs, as sloped area.
Big skylights or chimneys. Small ones are not worth it.
From the tile maker’s data sheet.
The width after the side overlap.
Typical: 3 for asphalt shingles. Check the pack.
Leave blank for the typical figure shown.
Complex means valleys, dormers and lots of angles.
Coverage after the laps. Typical figures: check the roll.
Per layer, in the same unit as labour.
How your roof is worked out
Flat area × pitch multiplier = roof area, then ÷ 100 for squares, plus waste, then × bundles per square.
- 1
Pitch multiplier
6/12 = 26.57°√(1 + 0.5²)= 1.118
- 2
Roof area
42 × 32 ft= 1,344 × 1.118= 1,502.64 sq ft
- 3
Squares + 10% waste
1,502.64 ÷ 100= 15.03 × 1.1= 16.53 squares
- 4
Count the bundles
16.53 × 3= 49.59, round up= 50 bundles
Read more: the formulas for every roof shape, a worked example and typical coverage figures
A roof is a set of sloped faces over the house. Seen from above, they cover the footprint plus the overhangs. Because they slope, their real area is bigger than that flat area, by the pitch multiplier. Roofers then count the area in squares of 100 sq ft, add some extra for cutting and waste, and turn squares into bundles, sheets or tiles.
The formula
pitch multiplier = √(1 + (rise ÷ run)²) · degrees = arctan(rise ÷ run) · x/12 = 12 × rise ÷ run
gable or shed = (length + 2 × gable overhang) × (width + 2 × eaves overhang) × multiplier
hip = (length + 2 × overhang) × (width + 2 × overhang) × multiplier
gambrel = 2 × roof length × (lower run × lower multiplier + upper run × upper multiplier)
mansard = steep band (flat area × lower multiplier) + top (flat area × upper multiplier)
squares = roof area in sq ft ÷ 100 (or m² ÷ 9.29) · to buy = squares × (1 + waste % ÷ 100)
bundles = squares to buy × bundles per square, rounded up
sheets = area to buy ÷ (sheet length × cover width) · tiles = m² to buy × tiles per m²
hip length = half the width × √(2 + (rise ÷ run)²) · hip-roof ridge = length − width
Worked example
A gable roof on a house 40 ft long and 30 ft wide, with a 6/12 pitch and 1 ft overhangs all round. Seen from above the roof covers (40 + 2) × (30 + 2) = 42 × 32 = 1,344 sq ft. A 6/12 pitch rises 6 for every 12 across, so rise ÷ run = 0.5 and the multiplier is √(1 + 0.5²) = √1.25 = 1.118. The roof area is 1,344 × 1.118034 = 1,502.64 sq ft (139.6 m²), which is 1,502.64 ÷ 100 = 15.03 squares. Adding 10% waste gives 15.026 × 1.1 = 16.53 squares to buy. At 3 bundles a square that is 16.53 × 3 = 49.59, so 50 bundles of shingles. The ridge is 42 ft long, so at about 25 ft per bundle of ridge caps you need 2 bundles. Each slope is 16 × 1.118 = 17.89 ft from eave to ridge, so the four rakes total 71.55 ft; with 84 ft of eaves that is 155.55 ft of drip edge, or 16 pieces of 10 ft. The ridge sits 15 × 0.5 = 7.5 ft above the walls.
Pitch, angle and multiplier
| Pitch | Angle | Percent | Multiplier |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50% | 1.118 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 12/12 | 45° | 100% | 1.414 |
Typical coverage figures used
- Asphalt shingles: typically 3 bundles per square (some heavy designer shingles are 4 or 5). Wood shakes: typically 4 bundles per square.
- Ridge and hip caps: typically about 25 ft (7.6 m) per bundle for shingles, 20 ft for shakes, 10 ft pieces for metal, and ridge tiles about 450 mm long.
- Starter strip: typically about 105 ft per bundle, along the eaves and rakes.
- Underlayment: synthetic rolls typically cover 10 squares, #15 felt 4 squares and #30 felt 2 squares after laps.
- Nails: about 80 shingles per square, 4 nails each (6 in high-wind areas), so about 320 nails a square. Metal screws: typically about 80 per square.
- Tiles per m²: typically about 10 for concrete interlocking tiles, 16 for clay pantiles, 60 for small plain tiles and 19 for 500 × 250 mm slates. The real count depends on the tile size and the lap, so use the maker’s figure.
- Waste: about 10% for gable and shed roofs, 12% for gambrel, 15% for hip and mansard, 20% for complex roofs with valleys and dormers.
These are rule-of-thumb figures; brands and products differ, so check the pack. Checked 11 October 2026.
Low-slope roofs
In the United States, the International Residential Code sets the lowest pitch for each material: asphalt shingles 2/12 (with two layers of underlayment below 4/12), clay and concrete tiles 2½/12, wood shakes and slate 4/12, and lapped metal panels with sealant ½/12. Other countries have their own rules, and makers often ask for more. Below about 2/12, use a flat-roof membrane.
Gambrel and mansard roofs
Both have a steep lower slope and a shallower upper slope. A gambrel (barn roof) does this on two sides, with gable ends. A mansard does it on all four sides, often with a nearly flat top. Enter the upper pitch as the main pitch, the steep pitch below it, and how much of the half-width the steep part covers when seen from above.
Common questions
How do I work out the area of my roof?
Measure the flat footprint of the roof, including the overhangs, then multiply by the pitch multiplier. A house 40 ft by 30 ft with 1 ft overhangs covers 42 × 32 = 1,344 sq ft. At a 6/12 pitch the multiplier is √(1 + 0.5²) = 1.118, so the roof is about 1,502.64 sq ft.
What is a roofing square?
A roofing square is 100 square feet of roof, about 9.29 m². Shingles, underlayment and labour are often priced per square. A roof of 1,502.64 sq ft is 15.03 squares. With 10% extra for waste you buy 16.53 squares.
How many bundles of shingles do I need?
Most asphalt shingles come 3 bundles to a square. Multiply the squares you are buying, with waste, by 3 and round up. For 16.53 squares that is 16.53 × 3 = 49.59, so 50 bundles. Check the pack, as some heavy shingles come 4 or 5 bundles to a square.
How much waste should I add for a roof?
Start at about 10% for a simple gable or shed roof and about 15% for a hip roof, as more shingles are cut at the hips. Complex roofs with valleys, dormers and many angles often need about 20%. The calculator picks a figure from the roof shape, and you can change it.
How do I convert roof pitch to degrees?
Pitch x/12 means the roof rises x for every 12 across. Degrees = arctan(x ÷ 12). So 6/12 is arctan(0.5) = 26.57°, and 12/12 is 45°. As a percent, 6/12 is 50%. The pitch multiplier is √(1 + (x ÷ 12)²), which is 1.118 for 6/12.
How do I measure a roof without climbing on it?
Measure the outside walls of the house on the ground, then add the overhang at the eaves and the gable ends. Find the pitch from plans, from a pitch gauge app held against a gable end, or by measuring the rise over 12 inches of run on a rafter in the attic.
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