Roof Area Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Roof area equals the building footprint multiplied by the pitch multiplier. A 40 ft × 30 ft house with a 6:12 roof has a 1,200 sq ft footprint and a 1.1180 multiplier, giving 1,341.6 sq ft of actual roof surface — about 13.4 roofing squares.
Your measurements
Enter the values you know — results update live.
The longest side of the building footprint, measured on the ground.
The shorter side of the footprint. Include the eave overhang if you want the covered area.
Enter 6 for a 6:12 roof. Use 0 for a genuinely flat roof.
Analysis
Live results from your measurements.
Show the working
- Footprint with overhang:
(40 + 2×1) × (30 + 2×1)= 1,344 sq ft - Pitch multiplier:
√(6² + 144) ÷ 12= 1.118 - Sloped area:
1,344 × 1.118= 1,502.6 sq ft - Roofing squares:
1,502.6 ÷ 100= 15.03 - With waste:
1,502.6 × 1.1= 1,652.9 sq ft
What this calculator does
Roof area is the true surface of the roof planes, which is always larger than the footprint of the building underneath. The extra comes from the slope: the steeper the roof, the longer the rafter line has to be to cover the same horizontal distance. Roofers convert that surface into squares — one square is 100 square feet — because shingles, underlayment and labour are all priced per square. Getting the area right is the single biggest driver of an accurate material order; a 10% error on a 30-square roof is three squares of shingles either wasted or missing halfway through the job.
How to use it
- Measure the length and width of the building footprint at ground level, including any bump-outs.
- Add the eave overhang to each side if your measurement stopped at the wall.
- Multiply length by width to get the footprint area.
- Find the roof pitch and look up or calculate the pitch multiplier.
- Multiply footprint area by the multiplier to get the true sloped area.
- Add a waste allowance — 10% on a simple gable, 15% or more with hips and valleys.
The formula
Sloped roof area
roof area = footprint length × footprint width × pitch multiplier
Works for any roof where all planes share the same pitch, including gables and hips.
Pitch multiplier
multiplier = sqrt(rise^2 + 144) / 12
The ratio of rafter length to horizontal run for an X:12 pitch.
Roofing squares
squares = roof area in square feet / 100
The trade unit for ordering and pricing roofing material and labour.
Worked example
A 48 ft × 28 ft house with an 8:12 roof and a 1.5 ft overhang all round.
- Footprint with overhang: 51 × 31 = 1,581 sq ft
- Multiplier for 8:12: √(64 + 144) ÷ 12 = 1.2019
- Sloped area: 1,581 × 1.2019 = 1,900.2 sq ft
- Squares: 1,900.2 ÷ 100 = 19.0
- With 12% waste: 1,900.2 × 1.12 = 2,128.2 sq ft
Order for roughly 21.3 squares — 22 squares once you round up to full bundles.
Reference chart
| Pitch | Multiplier | Area increase | Angle |
|---|---|---|---|
| 2:12 | 1.0138 | 1.4% | 9.46° |
| 3:12 | 1.0308 | 3.1% | 14.04° |
| 4:12 | 1.0541 | 5.4% | 18.43° |
| 5:12 | 1.0833 | 8.3% | 22.62° |
| 6:12 | 1.118 | 11.8% | 26.57° |
| 7:12 | 1.1577 | 15.8% | 30.26° |
| 8:12 | 1.2019 | 20.2% | 33.69° |
| 9:12 | 1.25 | 25% | 36.87° |
| 10:12 | 1.3017 | 30.2% | 39.81° |
| 12:12 | 1.4142 | 41.4% | 45° |
| 14:12 | 1.5366 | 53.7% | 49.4° |
| 16:12 | 1.6667 | 66.7% | 53.13° |
| 18:12 | 1.8028 | 80.3% | 56.31° |
Roof area by roof type for the same 1,200 sq ft footprint at 6:12
| Roof type | Approximate area | Waste allowance | Notes |
|---|---|---|---|
| Simple gable | 1,342 sq ft | 10% | Two planes, straight cuts at the rakes. |
| Hip | 1,342 sq ft | 15% | Same surface, far more cutting at the hips. |
| Cross gable | 1,380 sq ft | 15% | Valleys add area and waste. |
| Gambrel | 1,520 sq ft | 12% | Two pitches per side; measure each separately. |
| Mansard | 1,690 sq ft | 18% | Near-vertical lower slope adds significant area. |
Common mistakes
- Ordering material against the footprint instead of the sloped area — that under-orders by 5% to 40% depending on pitch.
- Forgetting the eave and rake overhangs, which on a typical house add 5% to 8% of area.
- Applying one pitch to a roof that changes pitch, such as a gambrel or a porch tie-in.
- Using a flat 10% waste on a hip roof with multiple valleys. Cut-heavy roofs run 15% to 20%.
- Rounding squares down. Always round up to the next full bundle.
- Ignoring dormers, which add both area and a lot of flashing detail.
Assumptions and limits
- The footprint is rectangular and every roof plane shares the same pitch.
- Overhang is applied equally on all four sides.
- Area excludes chimneys, skylights and other penetrations — these rarely reduce material need.
- One roofing square equals 100 square feet of finished roof surface.
Frequently asked questions
How many squares is a 2,000 square foot roof?
A 2,000 square foot roof is 20 roofing squares, because one square covers 100 square feet. Add waste before ordering: at 10% you would buy 22 squares of shingles.
Is roof area the same as house square footage?
No. House square footage is floor area inside the walls. Roof area is the sloped surface above, which includes overhangs and is enlarged by the pitch multiplier — usually 20% to 40% larger.
How do I calculate roof area without climbing up?
Measure the building footprint at ground level, add the overhangs, and multiply by the pitch multiplier for your roof pitch. You only need the pitch, which you can read from inside the attic.
How much waste should I add to a roof order?
Use 10% for a simple gable, 15% for a hip roof, and up to 20% where there are multiple valleys, dormers or a complex ridge line. Waste covers starter courses, cuts and damaged pieces.
Does roof area include the overhang?
Yes. Shingles cover the overhang, so it must be in the area you order against. A 1.5 ft overhang on a 40 × 30 house adds about 220 square feet of footprint before the pitch multiplier.
How do I measure area on a roof with several pitches?
Break the roof into planes that each have a single pitch, calculate the area of each plane separately using its own multiplier, then add the results together.
Related guides
- Common Roof Pitches Explained: 3:12 Through 12:12A pitch-by-pitch breakdown of every slope you will meet on a house — what each one costs, what it can be covered with, and which climates it suits.
- Roof Pitch Multiplier: Chart, Formula and How to Use ItThe single number that turns a floor plan into a material order — where it comes from, when it fails, and the hip/valley factor that goes with it.
- Roofing Squares and Shingle Bundles: How Many You NeedThe estimating unit every roofer quotes in — plus the starter strip, hip and ridge cap, underlayment and nail quantities that never appear in the headline square count.
- Shed and Lean-To Roof Pitch: How to Choose and Calculate ItSingle-slope roofs use the whole building depth as run, which changes every calculation. Here is the geometry, the wall-height math and the pitch to pick for a shed, lean-to or carport.
- Metal Roof Pitch: Minimums, Panel Lengths and Cost by SlopeMetal covers a wider slope range than any other roofing, but only if you match the profile to the pitch. Minimums, panel take-off, weight and the cost curve across slopes.
- Roof Replacement Cost: How Roofing Quotes Are Actually BuiltRoofers price per square, not per square foot of house. Here is how material, labour, tear-off, steepness and access combine into the number on your quote — and where the extras hide.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.