Roof Area Calculator
Roof area calculator: enter building size, overhang and pitch for roof square feet, roofing squares, waste and shingle bundles on gable, hip or shed roofs.
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How the Roof Area Calculator Formula Works
A sloping roof has more surface than the footprint under it. If you look straight down on the roof you see the plan area, which is the building size plus the overhang. The surface is larger by a factor that depends only on the pitch: √(1 + (rise/12)2). A 6:12 roof rises 6 inches for every 12 inches of run, so the factor is the square root of 1.25, which is 1.1180.
For the example building of 40 × 30 ft with 1 ft overhangs, the plan area is 1,344 sq ft, so the roof is 1,344 × 1.1180 = 1,502.6 sq ft. One roofing square is 100 sq ft, so that is 15.03 squares. Adding 10% waste gives 16.53 squares, and at 3 bundles per square you buy 50 bundles.
Gable, hip and shed roofs with one pitch have the same surface area for the same plan. The roof type changes the ridge height and the length of ridge and hip lines.
roof area = plan area × √(1 + (rise/12)2); squares = area / 100; bundles = ceil(squares × (1 + waste) × bundles per square)- plan areasq ft or sq mBuilding size plus overhang, seen from above.
- risein per 12 inRoof pitch: how many inches the roof rises over 12 inches of horizontal run.
- waste%10Extra material for cuts, valleys and mistakes.
- bundles per squarecountBundles needed to cover 100 sq ft, from the product label.
- square100 sq ftA roofing square, 100 sq ft of roof surface.
- plan area
- sq ft or sq m
- Building size plus overhang, seen from above.
- rise
- in per 12 in
- Roof pitch: how many inches the roof rises over 12 inches of horizontal run.
- waste
- %
- Extra material for cuts, valleys and mistakes.
- bundles per square
- count
- Bundles needed to cover 100 sq ft, from the product label.
- square
- 100 sq ft
- A roofing square, 100 sq ft of roof surface.
Using the Roof Area Calculator
Enter units
Choose from Imperial (feet), Metric (metres).
Enter roof type
Choose from Gable (two planes), Hip (four planes), Shed (one plane).
Enter building length (ft, or m)
Type the figure you already have, then press Calculate.
Enter building width, the span (ft, or m)
Type the figure you already have, then press Calculate.
Enter eave overhang (ft, or m)
Type the figure you already have, then press Calculate.
Enter rake overhang at gable ends (ft, or m)
Type the figure you already have, then press Calculate.
Enter enter the pitch as
Choose from Rise per 12 of run (such as 6:12), Angle in degrees.
Enter pitch, rise per 12 of run
Any value between 0 and 48.
Enter roof angle (degrees)
Any value between 0 and 79.
Enter extra plan area at the same pitch (sq ft, or sq m)
Type the figure you already have, then press Calculate.
Enter deduct skylights or chimneys, sloped (sq ft, or sq m)
Type the figure you already have, then press Calculate.
Enter waste allowance (%)
Any value between 0 and 100.
Enter bundles per square (check your product)
Choose from 3 bundles per square (common), 4 bundles per square, 5 bundles per square.
Enter price per bundle (placeholder)
Type the figure you already have, then press Calculate.
Read the result
Press Calculate to update the result. Until you do, the last result is dimmed and marked as out of date, so you always know which numbers belong to the values in the boxes.
A Real Worked Example
A gable house is 40 ft long and 30 ft wide, with 1 ft eave and rake overhangs and a 6:12 roof. The plan with overhang is 42 × 32 = 1,344 sq ft. The pitch factor is 1.1180, so the roof is 1,502.64 sq ft, or 139.6 sq m.
That is 15.03 squares. With 10% waste it is 16.53 squares, so at 3 bundles per square you buy 50 bundles, and at $35 each the shingles cost $1,750. The ridge is 42 ft long and sits 7.5 ft above the walls.
A 30 x 40 ft roof with no overhang
Enter 40 × 30 ft, zero overhangs, 6:12 and no waste. A flat roof of that size would be 1,200 sq ft, which is 12 squares. At 6:12 the roof is 1,341.64 sq ft, or 13.42 squares.
At 3 bundles per square that rounds up to 41 bundles, about 40.2 before rounding.
A hip roof, 36 x 28 ft at 8:12
Choose hip with 1.5 ft overhangs all round, 8:12 and 12% waste. The plan is 39 × 31 = 1,209 sq ft and the factor is 1.2019, so the roof is 1,453.04 sq ft, 14.53 squares.
With waste that is 16.27 squares, or 49 bundles at 3 per square. The ridge and four hips add up to 104.9 ft of cap, and the ridge stands 9.3 ft above the walls.
A shed roof, 20 x 12 ft at 3:12
Choose shed with 0.5 ft overhangs, a 3:12 pitch and 5% waste. The plan is 21 × 13 = 273 sq ft, the factor 1.0308, and the roof 281.40 sq ft, which is 2.81 squares.
With waste it is 2.95 squares and 9 bundles at 3 per square. The high side stands 3.0 ft above the low side.
A metric gable roof, 12 x 8 m at 30 degrees
Choose metric, enter 12 × 8 m with 0.4 m overhangs and pick degrees with an angle of 30. The factor is 1.1547, which equals 1 divided by cos 30 degrees, and the plan is 112.6 sq m.
The roof is 130.07 sq m, or 1,400 sq ft, 14.00 squares. With 10% waste you buy 47 bundles at 3 per square.
A 10 x 10 ft roof plus a porch
Enter 10 × 10 ft with no overhang, a 4:12 pitch, 120 sq ft of extra plan area for a porch and a 6 sq ft skylight deduction. The plan area is 220 sq ft and the factor is 1.0541.
The roof is 231.90 − 6 = 225.90 sq ft, 2.26 squares, and with 10% waste 8 bundles.
What Else to Know
How to Calculate Roof Area and Roof Square Footage
To calculate the roof area, work from the plan. Measure the building length and width, add the overhang on each side, and multiply for the plan area. Then multiply by the pitch factor to allow for the slope. A 40 × 30 ft building with 1 ft overhangs and a 6:12 roof is 1,344 × 1.1180 = 1,502.6 sq ft.
The pitch factor is √(1 + (rise/12)2). It comes from the Pythagorean theorem, since the roof surface is the hypotenuse of a right triangle with a base of 12 and a height of the rise. Roof pitch is described by Wikipedia as rise over run, such as 6:12.
The calculator then gives the roofing squares, waste, bundles and, with a price, the shingle cost. This one method answers the roof sq ft, how to calculate roof square footage and roof area with pitch searches.
Roof Pitch Factor Table and the Pitch Calculator
The table lists the pitch factor from 0:12 to 12:12. A 4:12 roof is 1.0541, 6:12 is 1.1180, 8:12 is 1.2019 and 12:12 is 1.4142. These values come straight from the formula, so they match the Pythagorean relationship exactly.
If you know the angle instead, rise per 12 is 12 x tan(angle) and the factor is 1 divided by cos(angle). A 30 degree roof is 6.93:12 with a factor of 1.1547. Pick degrees in the calculator and enter the angle; it shows the pitch and angle for the roof you entered.
To find the pitch of an existing roof, measure rise over a 12 in run with a level and a ruler from inside the attic, or work from the plans. Do not walk a roof to measure it.
Roofing Squares, Bundles and Waste
A roofing square is 100 sq ft of roof. Fine Homebuilding defines it as the quantity of shingles needed to cover 100 sq ft, and says three bundles to a square is most common, with heavier shingles needing four or five. Treat 3 as typical and read the coverage on your product's label.
Divide the roof area by 100 for squares, add a waste allowance, then multiply by bundles per square and round up. For 15.03 squares with 10% waste, that is 16.53 squares and 50 bundles.
There is no single waste number. Fine Homebuilding's author plans 1% on a simple roof and 5% or more on one with open valleys, while many estimators use 10% to 15%. The 10% default is a placeholder; change it to your installer's advice.
Gable, Hip, Shed and Metal Roofs
For one pitch, gable, hip and shed roofs have the same surface for the same plan area, because every plane has the same slope. What changes is the plan size with overhang, the ridge height and the length of ridge and hip lines. On a 40 × 30 ft building at 6:12 with 1 ft overhangs, a gable roof is 1,502.6 sq ft and a hip roof is 1,502.6 sq ft, because the hip uses the eave overhang all round.
A shed roof is one plane that rises across the whole width, so it is higher at the ridge side. The area formula is unchanged. The calculator reports the high side rise and leaves ridge length blank for a shed.
Metal and steel roofs use the same area. Panels are sold by length and by cover width, so ask your supplier for the coverage of the panel and divide the roof area by it.
Overhangs, Valleys, Hips, Dormers and Skylights
The overhang counts, because the roof covers it. For a 40 × 30 ft gable at 6:12, going from no overhang to 2 ft all round raises the roof from 1,342 to 1,673 sq ft. The table of overhangs shows each step.
Valleys and hips do not change the area you measure, but they raise waste and need linear feet of flashing or cap shingles. Fine Homebuilding suggests two extra shingles per linear foot of valley, and says a bundle of standard shingles caps about 35 linear feet of ridge or hip. The calculator gives ridge and hip length for gable and hip roofs.
Add dormers, porch roofs and other planes as extra plan area at the same pitch, and subtract skylights or chimneys if you want the net. If a roof has two different pitches, run the calculator once for each and add the results.
Common Sizes: 30x40, 10x10 and a Typical House Roof
A 30 × 40 ft roof has a plan of 1,200 sq ft. At 6:12 with no overhang it is 1,341.6 sq ft, 13.42 squares. A 10 × 10 ft roof is 100 sq ft in plan and 111.8 sq ft at 6:12. There is no normal roof size, since it follows the footprint, pitch and overhang. A 2,000 sq ft footprint at 6:12 gives about 2,236 sq ft.
The tables show roof area for plan areas of 1,000 to 3,000 sq ft and for common building sizes, in square feet, squares and square metres.
To work in metres, choose metric. A roof of 1,500 sq ft is 139.4 sq m, and a roofing square is 9.29 sq m.
By Address and Satellite, Limits and Related Calculators
Satellite and by-address tools estimate roof size from images. Their results depend on image date, tree cover and the pitch they assume, so use them as a cross-check. This calculator works from your measurements instead.
It assumes every plane has one pitch and a simple rectangular outline. It does not handle curved roofs, cut-up roofs with many dormers beyond the extra area box, or roofs with different pitches in one run. It gives an estimate, not a bid. Follow the shingle maker's instructions and local code, and have a roofer check the quantities before you order. Roofs are dangerous, so measure from the ground, the plans or the attic.
For related work, the rafter length calculator finds rafter lengths for the same pitch, the percentage calculator works out waste on any quantity, and the board foot calculator helps with lumber.
Reference Tables
| Pitch (rise:12) | Angle (degrees) | Pitch factor | Extra area vs plan |
|---|---|---|---|
| 0:12 | 0.0 | 1.0000 | 0.0% |
| 2:12 | 9.5 | 1.0138 | 1.4% |
| 3:12 | 14.0 | 1.0308 | 3.1% |
| 4:12 | 18.4 | 1.0541 | 5.4% |
| 5:12 | 22.6 | 1.0833 | 8.3% |
| 6:12 | 26.6 | 1.1180 | 11.8% |
| 7:12 | 30.3 | 1.1577 | 15.8% |
| 8:12 | 33.7 | 1.2019 | 20.2% |
| 9:12 | 36.9 | 1.2500 | 25.0% |
| 10:12 | 39.8 | 1.3017 | 30.2% |
| 11:12 | 42.5 | 1.3566 | 35.7% |
| 12:12 | 45.0 | 1.4142 | 41.4% |
| Plan area (sq ft) | 4:12 | 6:12 | 8:12 | 10:12 | 12:12 |
|---|---|---|---|---|---|
| 1,000 | 1,054 | 1,118 | 1,202 | 1,302 | 1,414 |
| 1,500 | 1,581 | 1,677 | 1,803 | 1,953 | 2,121 |
| 2,000 | 2,108 | 2,236 | 2,404 | 2,603 | 2,828 |
| 2,500 | 2,635 | 2,795 | 3,005 | 3,254 | 3,536 |
| 3,000 | 3,162 | 3,354 | 3,606 | 3,905 | 4,243 |
| Building (ft) | Plan (sq ft) | Roof at 6:12 (sq ft) | Squares at 6:12 | Squares, flat roof |
|---|---|---|---|---|
| 10 × 10 | 100 | 111.8 | 1.12 | 1.00 |
| 20 × 20 | 400 | 447.2 | 4.47 | 4.00 |
| 24 × 24 | 576 | 644.0 | 6.44 | 5.76 |
| 30 × 40 | 1,200 | 1,341.6 | 13.42 | 12.00 |
| 40 × 50 | 2,000 | 2,236.1 | 22.36 | 20.00 |
| 50 × 60 | 3,000 | 3,354.1 | 33.54 | 30.00 |
| Roof area (sq ft) | Squares | Squares with 10% waste | Bundles at 3 per square | Bundles at 4 per square |
|---|---|---|---|---|
| 500 | 5.00 | 5.50 | 17 | 22 |
| 1,000 | 10.00 | 11.00 | 33 | 44 |
| 1,500 | 15.00 | 16.50 | 50 | 66 |
| 2,000 | 20.00 | 22.00 | 66 | 88 |
| 2,500 | 25.00 | 27.50 | 83 | 110 |
| 3,000 | 30.00 | 33.00 | 99 | 132 |
| 4,000 | 40.00 | 44.00 | 132 | 176 |
| Angle (degrees) | Rise per 12 | Pitch factor |
|---|---|---|
| 10 | 2.12 | 1.0154 |
| 15 | 3.22 | 1.0353 |
| 20 | 4.37 | 1.0642 |
| 25 | 5.60 | 1.1034 |
| 30 | 6.93 | 1.1547 |
| 35 | 8.40 | 1.2208 |
| 40 | 10.07 | 1.3054 |
| 45 | 12.00 | 1.4142 |
| Waste | Squares to buy | Bundles at 3 per square |
|---|---|---|
| 0% | 15.03 | 46 |
| 5% | 15.78 | 48 |
| 10% | 16.53 | 50 |
| 12% | 16.83 | 51 |
| 15% | 17.28 | 52 |
| 20% | 18.03 | 55 |
| Overhang (ft) | Plan area (sq ft) | Roof area (sq ft) | Squares |
|---|---|---|---|
| 0.0 | 1,200 | 1,341.6 | 13.42 |
| 0.5 | 1,271 | 1,421.0 | 14.21 |
| 1.0 | 1,344 | 1,502.6 | 15.03 |
| 1.5 | 1,419 | 1,586.5 | 15.86 |
| 2.0 | 1,496 | 1,672.6 | 16.73 |
| Roof type | Roof area (sq ft) | Ridge height above walls (ft) | Ridge and hip length (ft) |
|---|---|---|---|
| Gable | 1,502.6 | 7.50 | 42.0 |
| Hip | 1,502.6 | 7.50 | 106.0 |
| Shed (rise across 30 ft) | 1,502.6 | 15.00 | none |
| Ridge and hip length (ft) | Bundles of cap shingles |
|---|---|
| 20 | 1 |
| 40 | 2 |
| 70 | 2 |
| 105 | 3 |
| 140 | 4 |
| 200 | 6 |
| Plan area (sq ft) | Plan (sq m) | Roof at 6:12 (sq m) | Roof at 6:12 (sq ft) | Squares |
|---|---|---|---|---|
| 1,000 | 92.9 | 103.9 | 1,118 | 11.18 |
| 1,500 | 139.4 | 155.8 | 1,677 | 16.77 |
| 2,000 | 185.8 | 207.7 | 2,236 | 22.36 |
| 2,500 | 232.3 | 259.7 | 2,795 | 27.95 |
| 3,000 | 278.7 | 311.6 | 3,354 | 33.54 |
| Mistake | Effect | Fix |
|---|---|---|
| Using the plan area as the roof area | Roof 1 to 41% too small at pitches from 2:12 to 12:12 | Multiply by the pitch factor |
| Leaving out the overhang | Short by the eave and rake strips | Add the overhang to the plan size |
| Mixing up degrees and rise:12 | Wrong factor | Choose the pitch type first |
| Counting waste twice | Buying too much | Use one waste percentage |
| Forgetting dormers and porches | Area too low | Add them as extra plan area |
| Assuming 3 bundles per square for every shingle | Bundle count off | Check the label for coverage |
Frequently Asked Questions About the Roof Area Calculator
How do you calculate the area of a roof?
Find the plan area from above, including the overhang, then multiply by the pitch factor. A 40 × 30 ft building with 1 ft overhangs has a plan of 1,344 sq ft. At 6:12 the factor is 1.1180, so the roof is 1,502.6 sq ft. Add extra areas and deduct openings if needed.
What is the formula for calculating roof area?
Roof area = plan area x √(1 + (rise/12)2). The square root is the pitch factor, from the Pythagorean theorem. At 6:12 it is 1.1180; at 12:12 it is 1.4142. For a 1,500 sq ft plan at 6:12 the roof is 1,677 sq ft.
How many squares is a 30x40 roof?
A 30 × 40 ft plan is 1,200 sq ft, or 12 squares if the roof were flat. At 6:12 the roof is 1,341.6 sq ft, which is 13.42 squares. Add overhang and waste, and the squares to buy increase. Enter your pitch for your own number.
How much area is 1 square of roofing?
One roofing square is 100 sq ft of roof surface, which is 9.29 square metres. Fine Homebuilding defines it as the quantity of shingles needed to cover 100 sq ft of roof. A roof of 1,503 sq ft is 15.03 squares.
How do I calculate the square footage of a roof?
Measure the building's length and width, add the overhangs, and multiply for the plan area. Then multiply by the pitch factor. For 42 × 32 ft with a 6:12 pitch that is 1,344 × 1.1180 = 1,502.6 sq ft. Use the calculator to add waste and bundles.
How many square feet is a 30x40 roof?
The plan area is 1,200 sq ft. With a 6:12 pitch and no overhang the roof is 1,341.6 sq ft. With 4:12 it is 1,264.9, and with 8:12 it is 1,442.2. A 1 ft overhang all round raises the 6:12 figure to 1,502.6.
How many sq ft is a normal roof?
There is no normal size. It follows the building footprint, pitch and overhang. A 1,500 sq ft footprint at 6:12 is about 1,677 sq ft of roof, and a 2,000 sq ft footprint is about 2,236 sq ft. The table of plan areas and pitches gives more.
How much is 100 square feet of roof?
100 sq ft is one roofing square. At 3 bundles per square it takes 3 bundles of common shingles, or 4 with 10% waste. The cost is bundles times your price per bundle: at an example $35 per bundle, 4 bundles is $140. Prices and coverage vary by product.
How do I calculate my roof square footage?
Start with the footprint and overhang, pick your pitch, and apply the pitch factor. Our example roof is 1,503 sq ft, which is 15.0 squares. Then add a waste allowance, and if you will use shingles, divide by the coverage per bundle. The page does these steps.
How many squares is a 10x10 roof?
A 10 × 10 ft plan is 100 sq ft, or 1 square if flat. At 4:12 it is 105.4 sq ft, 1.05 squares. At 6:12 it is 111.8 sq ft. Round up when you buy and add waste, since a partial bundle cannot be bought. Enter your own pitch and overhang in the calculator for a roof that matches your plans.
How do I calculate roof area with pitch?
Convert the pitch to a factor: factor = √(1 + (rise/12)2). A 4:12 roof is 1.0541, 6:12 is 1.1180 and 9:12 is 1.2500. Multiply the plan area by it. A 2,000 sq ft plan at 9:12 is 2,500 sq ft.
What is the pitch factor for a 6/12 roof?
It is 1.1180, the square root of 1.25. That means the surface is about 11.8% larger than the plan area. A 6:12 roof has an angle of 26.6 degrees. The table lists the factor for every pitch from 0:12 to 12:12, with the angle and the extra area it adds, so you can check any roof quickly.
How do I calculate roof area in square meters?
Measure in metres and use the same method. A 12 × 8 m building with 0.4 m overhangs and a 30 degree roof has a plan of 112.6 sq m, factor 1.1547 and a roof of 130.1 sq m. Choose metric and degrees in the calculator.
Can I find roof square footage by address or satellite?
Some map tools estimate it from images, and results vary with image age, tree cover and the pitch they assume. Treat any such figure as a starting point. This calculator uses your own measurements and pitch, so you can check an estimate against the plan area. Do not climb a roof to measure.
How do I calculate a shed roof's square footage?
A shed roof has one sloping plane. Multiply the plan area, including overhang, by the pitch factor. A 20 × 12 ft shed with 0.5 ft overhangs at 3:12 has a plan of 273 sq ft and a roof of 281.4 sq ft, about 2.81 squares. Choose the shed type.
How do I calculate hip roof area?
A hip roof with one pitch has the same factor on all four planes, so use the plan area with overhang times the factor. A 36 × 28 ft hip roof with 1.5 ft overhangs at 8:12 is 1,209 × 1.2019 = 1,453.0 sq ft. The calculator also gives ridge and hip length.
How do I calculate a metal or steel roof's square footage?
The area is the same as for shingles: plan area with overhang times the pitch factor. Metal panels are sold by length and coverage width, so ask for the panel coverage and divide the area. For 1,503 sq ft with a 3 ft cover, that is about 501 panel-feet.
How many bundles of shingles per square?
Three bundles per square is the most common for standard shingles, according to Fine Homebuilding, and heavier shingles may need four or five. Check the label on your product, because bundles show the coverage. Enter your number in the calculator and it rounds up whole bundles.
How much waste should I add to roof area?
There is no single rule. Fine Homebuilding's author plans about 1% on a simple roof and 5% or more with open valleys, while many estimators use 10% to 15%. The default is 10% as a placeholder. On 15.03 squares, 10% waste adds 1.50 squares.
How do valleys and hips change roof area?
They do not change the surface area, which comes from the plan and the pitch. They change how much material you cut and waste, and they need linear feet of flashing or cap shingles. Fine Homebuilding suggests two extra shingles per foot of open valley. Raise the waste allowance for roofs with many valleys.
Does overhang count in roof area?
Yes, the roof covers the overhang, so add it to the building size. A 40 × 30 ft building with 1 ft overhangs has a roof of 1,502.6 sq ft; with no overhang it is 1,341.6. The calculator has separate eave and rake overhangs.
How do I convert roof angle in degrees to pitch?
Rise per 12 = 12 x tan(angle). A 30 degree roof is 6.93:12, and 45 degrees is 12:12. The pitch factor is also 1 divided by cos(angle): 1.1547 at 30 degrees. Choose degrees in the calculator to enter the angle.
How do I deduct skylights and chimneys?
Enter their sloped area in the deduction box. A 6 sq ft deduction on 231.9 sq ft of roof leaves 225.9. Many roofers do not deduct small openings, because cutting around them wastes material. Use your judgment, and ask your roofer.
References
- [1] Wikipedia contributors. “Roof pitch.” Wikipedia. Accessed 2026-10-07.
- [2] Wikipedia contributors. “Pythagorean theorem.” Wikipedia. Accessed 2026-10-07.
- [3] Fine Homebuilding. “Don't Fall Short on Shingles: How to Estimate Materials for Roofing Projects.” Fine Homebuilding. Accessed 2026-10-07.
- [4] Wikipedia contributors. “Hip roof.” Wikipedia. Accessed 2026-10-07.
- [5] Wikipedia contributors. “Square foot.” Wikipedia. Accessed 2026-10-07.
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