Enter your house footprint, roof type, pitch and overhang, and this roof area calculator gives you the actual roof surface area, with the flat footprint shown alongside so you can see exactly what the overhang and the pitch added. Results update as you type. No signup.
Outside wall to outside wall along the long side of the house.
Outside wall to outside wall along the short side.
Two sloped planes meeting at a ridge along the long side. Overhang on all four sides.
Not sure? Use the roof pitch calculator.
How far the roof edge extends past the wall. 12 in is typical; enter 0 for none.
length × width
(length + 2 × overhang) × (width + 2 × overhang)
footprint with overhang × pitch multiplier
Gable: 2 planes at 6:12, multiplier 1.118roof area − house footprint
gable: ridge = long side with overhang
Take the roof surface area to the roofing square calculator and use its “I know the roof area” mode to get squares, waste and shingle bundles.
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How the roof area calculator works
The area of a roof is almost never the area of the house under it. Two things make it bigger: the roof extends past the walls at the eaves and rakes, and it slopes, so the shingled surface is longer than the horizontal distance it covers. This calculator applies both corrections in order, and shows each step separately so you can see how much the overhang added and how much the pitch added.
The variables
- Footprint length and width are the outside dimensions of the house walls. Measure at ground level, outside face to outside face. Do not use the interior square footage from a listing; it excludes wall thickness and, on a two-story house, counts space under the same roof twice.
- Roof type sets how the roof is shaped over that rectangle: gable, hip, shed or flat. What each one assumes is stated in the next section, including where two types produce the same answer.
- Roof pitch is rise in inches per 12 inches of run. Each option carries the multiplier that converts horizontal area to sloped area. Measure yours with the roof pitch calculator if you do not know it.
- Overhang is how far the roof edge projects past the wall, in inches. Most American homes have 12 to 24 inches at the eaves. The calculator applies the same overhang to all four sides.
Formula 1: house footprint
footprint = length × width
A 40 by 30 foot house has a 1,200 square foot footprint. This is the starting point and is shown in the first result so you can compare it with the finished number.
Formula 2: footprint with overhang
extended footprint = (length + 2 × overhang) × (width + 2 × overhang)
Overhang is added to both ends of both dimensions before anything else happens, because the roof plane starts at the edge of the overhang, not at the wall. With a 12 inch overhang the 40 by 30 house becomes a 42 by 32 roof outline, which is 1,344 square feet: 144 square feet more than the footprint, and that is before the pitch is applied. Overhang is entered in inches and converted to feet inside the formula.
Formula 3: actual roof surface area
roof area = extended footprint × pitch multiplier
The pitch multiplier is √(rise² + run²) ÷ run, which is the ratio of sloped length to horizontal length. At 6:12 it is 1.118, so the 1,344 square foot outline becomes 1,503 square feet of roof surface. For a flat roof the multiplier is 1.000 and the roof area equals the extended footprint.
Formula 4: ridge and hip length
This is the one output that changes between gable, hip and shed, and it is useful for pricing ridge cap and hip cap. Using the extended dimensions, with the long side as L and the short side as W:
- Gable: ridge = L. No hips.
- Hip: ridge = L − W, plus four hip rafters each (W ÷ 2) × √(2 + (rise ÷ 12)²) long. The hips run diagonally from each corner up to the ends of the shortened ridge.
- Shed and flat: zero. A shed roof has a high eave, not a ridge.
What each roof type assumes
This is the part most roof calculators get wrong or hide, so it is worth being explicit.
- Gable: two rectangular planes at the same pitch meeting at a ridge along the long dimension. Every point of the extended footprint is under a plane at that pitch, so surface area = extended footprint × multiplier.
- Hip: four planes at the same pitch, two trapezoids and two triangles. It looks different, but every point of the extended footprint is still under a plane at the same pitch, so the surface area is identical to a gable over the same rectangle. The calculator does not invent a difference. What does differ is the ridge and hip length (much more hip cap) and the waste factor, which the roofing square calculator handles.
- Shed: one plane at the stated pitch across the full width. By the same logic the surface area again equals extended footprint × multiplier, the same as a gable at the same pitch. There is no ridge, so the ridge length is zero. In practice shed roofs are usually built at lower pitches, which is where the real difference in area comes from.
- Flat: multiplier fixed at 1.000 and the pitch dropdown is disabled. Real flat roofs have a small drainage slope of about 1/4 inch per foot, which adds under 0.1 percent to the area and is ignored.
In short: the roof type changes the multiplier only for flat roofs, changes the ridge and hip figure for all four, and is stated on the result card so you always know which assumption produced the number.
What changes the number
- Overhang is bigger than people expect. On a 40 by 30 house, going from no overhang to 12 inches adds 144 square feet of outline and 161 square feet of 6:12 roof surface. At 24 inches it adds 296 and 331 respectively. Measure the actual eave projection rather than guessing.
- Pitch scales everything. The same 42 by 32 outline is 1,417 square feet at 4:12 and 1,900 square feet at 12:12. A wrong pitch is the largest single error in a do-it-yourself estimate.
- L-shaped and T-shaped houses. Split the footprint into rectangles at the inside corner, run each one separately, and add the results. The overlap where the two roofs meet is small and the valley there is covered by the waste factor later.
- Garages, porches and bump-outs have their own footprint, often their own pitch and sometimes their own roof type. Run them as separate rectangles.
- Dormers, chimneys and skylights are not subtracted or added here. Dormers add a little area and a lot of cutting; both are normally covered by raising the waste factor rather than by measuring them.
From area to squares and cost
Roof area on its own does not order materials. Divide it by 100 to get roofing squares, add waste, and convert to bundles with the roofing square calculator, which has an “I know the roof area” mode built for exactly this handoff. Then take the square count to the roof replacement cost guide to see what it should cost to replace.
What overhang adds
A 40 by 30 foot footprint at a 6:12 pitch, gable or hip, with the overhang applied to all four sides.
| Overhang | Roof outline | Extended footprint | Roof surface | vs no overhang |
|---|---|---|---|---|
| 0 in | 40.0 × 30.0 ft | 1,200 sq ft | 1,342 sq ft | +0 sq ft |
| 12 in | 42.0 × 32.0 ft | 1,344 sq ft | 1,503 sq ft | +161 sq ft |
| 18 in | 43.0 × 33.0 ft | 1,419 sq ft | 1,586 sq ft | +245 sq ft |
| 24 in | 44.0 × 34.0 ft | 1,496 sq ft | 1,673 sq ft | +331 sq ft |
Roof type comparison
The same 42 by 32 foot roof outline (40 by 30 house with 12 inch overhang) at 6:12.
| Roof type | Multiplier used | Roof surface | Ridge + hip length |
|---|---|---|---|
| Gable | 1.118 | 1,503 sq ft | 42 ft |
| Hip | 1.118 | 1,503 sq ft | 106 ft |
| Shed | 1.118 | 1,503 sq ft | 0 ft |
| Flat | 1.000 | 1,344 sq ft | 0 ft |
What pitch adds
The same 42 by 32 foot outline, 1,344 square feet flat, at common pitches.
| Pitch | Multiplier | Roof surface | Added by pitch |
|---|---|---|---|
| 4:12 | 1.054 | 1,417 sq ft | +73 sq ft |
| 6:12 | 1.118 | 1,503 sq ft | +159 sq ft |
| 8:12 | 1.202 | 1,615 sq ft | +271 sq ft |
| 10:12 | 1.302 | 1,750 sq ft | +406 sq ft |
| 12:12 | 1.414 | 1,900 sq ft | +556 sq ft |
Related calculators and guides
- Roof Pitch Calculator: find your pitch and its multiplier from rise and run or an angle.
- Roofing Square Calculator: convert roof area into squares, waste and shingle bundles.
- Roof Replacement Cost: what a new roof costs per square and what pushes the price up.
Roof area FAQ
How do I calculate the area of my roof?
Measure the outside length and width of the house, add the overhang to both ends of each, multiply them together, then multiply by the pitch multiplier for your roof slope. A 40 by 30 house with 12 inch overhangs at 6:12 is 42 × 32 × 1.118, about 1,503 square feet.
Does a hip roof have more area than a gable roof?
No. Over the same rectangle at the same pitch, a hip roof and a gable roof have exactly the same surface area. The hip roof has far more hip and ridge line, needs more cap shingles and generates more waste, but the field area is identical.
How much roof area does overhang add?
A 12 inch overhang on all sides of a 40 by 30 house adds 144 square feet to the roof outline and about 160 square feet of sloped roof at 6:12, roughly 12 percent. A 24 inch overhang adds about 25 percent.
Is roof area the same as the square footage of the house?
No. Roof area is larger because of overhang and pitch, and on a two-story house it is much smaller than the total living area because both floors sit under one roof. Always measure the footprint, not the living area.
How do I measure an L-shaped roof?
Divide the footprint into two rectangles at the inside corner, run each one through the calculator with its own pitch and overhang, and add the two roof areas together.
What pitch should I use if I do not know it?
Measure it. The roof pitch calculator explains how to do this from inside the attic with a level and a tape. If you must guess, 6:12 is the most common pitch on American homes, but a wrong guess can put the area off by 10 percent or more.
How many squares is my roof?
Divide the roof surface area by 100. A 1,503 square foot roof is about 15 squares before waste. Use the roofing square calculator to add waste and convert to shingle bundles.