Roofing Calculator
Turn a building footprint, overhang and roof pitch into roof area, squares, shingle bundles, underlayment rolls, ridge cap and drip edge for gable or hip roofs.
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About this toolWhat is the Roofing Calculator?
You can measure a roof without climbing onto it. The ground footprint plus the overhang gives the plan area, and the pitch tells you how much longer each sloped surface is than its level shadow. This roofing calculator multiplies the two to get the true roof area, converts it to roofing squares (100 square feet each), and lists the materials that follow from it: shingle bundles, underlayment rolls, ridge cap and drip edge.
Enter the building length along the ridge, the width, the overhang and the pitch as rise in 12 or as an angle in degrees. Choose gable, hip or shed. A hip roof has the same surface area as a gable of the same pitch, but a shorter ridge, four hips that need cap shingles, and eaves on all four sides, so the edge materials change even when the shingle count barely does.
Defaults reflect common products and are all editable: three bundles per square, felt or synthetic underlayment, ridge cap by the linear foot per bundle and drip edge in 10 foot pieces. A table of common pitches with their angle and multiplier is beside the results.
- Gable, hip and shed roofs from footprint, overhang and pitch
- Pitch as rise in 12 or degrees, with a multiplier table
- Squares, bundles, underlayment rolls, ridge cap and drip edge in one result
- Editable bundles per square, roll coverage, cap coverage and piece length
- Imperial or metric inputs, cost per bundle, copy and share
How to use the Roofing Calculator
- Choose feet or metres and the roof shape: gable, hip or shed.
- Enter the building length along the ridge and the width across it, then the overhang measured level from the wall.
- Enter the pitch as rise per 12 inches of run, or switch to degrees if you measured the angle.
- Set waste (10% for a simple gable, 15% or more for hips, valleys and dormers) and check the bundles per square on your chosen shingle.
- Read the area, squares, bundles and edge materials, and enter a price per bundle for the shingle cost.
Worked examples
40 x 30 ft gable at 6/12
Input
Gable, 40 x 30 ft, 1 ft overhang, 6/12, 10% waste, 3 bundles per square
Output
1,503 ft² of roof = 15.03 squares; 50 bundles; 5 rolls of #15 felt; 42 ft of ridge cap; 156 ft of drip edge (16 pieces)
The plan with overhang is 42 x 32 = 1,344 ft². Times 1.118 for the 6/12 pitch gives the roof area.
Same house with a hip roof
Input
Hip, 40 x 30 ft, 1 ft overhang, 6/12, 15% waste
Output
Same 15.03 squares; 52 bundles; ridge 10 ft plus four 24 ft hips = 106 ft of cap (5 bundles); 148 ft of drip edge
The area does not change, but the cap length more than doubles and the extra cutting justifies the higher waste.
Shed roof on a garage
Input
Shed, 24 x 12 ft, 1 ft overhang, 4/12, synthetic underlayment
Output
384 ft² = 3.84 squares; 13 bundles; 1 roll; 82 ft of drip edge
A single plane has no ridge. Drip edge runs along the low eave, both sloped rakes and the high edge.
Background and insight
The pitch multiplier is the length of a rafter per unit of horizontal run: the square root of 1 + (rise / 12)². At 6 in 12 it is 1.118, so a roof covers 11.8% more surface than its footprint; at 12 in 12 (45°) it is 1.414. Because every plane of a simple gable or hip roof has the same pitch, multiplying the total plan area by this one factor gives the whole roof area, however the planes are arranged.
Asphalt shingles are sold so that three bundles cover one square for most three-tab and many architectural products, but heavier laminated shingles can need four or five, which is why the wrapper is the final word. Nominal underlayment coverage is about 400 ft² for a roll of #15 felt, 200 ft² for #30 and around 1,000 ft² for many synthetic rolls, less once laps are taken out.
Hips and valleys need cut shingles along their full length, and dormers add both. A plain gable is often estimated at 10% waste and a cut-up hip roof at 15% or more, which is a judgment call rather than a formula.
Common use cases
Checking a reroof quote
Compare the number of squares on a contractor quote with an independent figure from the footprint and pitch.
Ordering shingles for a shed or garage
Get bundles, underlayment and drip edge for a small DIY roof in one pass.
Estimating before a storm claim
Know the approximate roof area before an adjuster visits, so the numbers in the claim make sense to you.
Comparing pitch options on new builds
See how moving from 4/12 to 8/12 changes material quantities and the edge lengths.
Tips and limitations
- Measure overhang horizontally from the wall to the fascia, not along the slope. The pitch multiplier handles the slope.
- Count layers if you are adding over existing shingles. Many codes allow at most two layers, and tear-off adds disposal cost.
- Starter strip, valley lining, flashing and nails are not included. Order starter by the length of the eaves and rakes.
- Steep roofs above about 8/12 often cost more per square to install because of safety equipment and slower work, even though the materials scale only with area.
- Satellite measurements and drone reports are good cross-checks for complex roofs with several pitches, which this simple model does not handle.
Frequently asked questions
How do I calculate roof area from the footprint?
Add the overhang to each side of the footprint, multiply length by width, then multiply by the pitch factor: the square root of 1 + (rise / 12)². A 42 x 32 ft plan at 6/12 is 1,344 x 1.118 = 1,503 ft².
How many bundles of shingles are in a square?
Three bundles cover one square (100 ft²) for most standard three-tab and many architectural shingles. Some heavy laminated or designer shingles need four or five bundles per square, so check the coverage on the wrapper.
What is a roofing square?
A square is 100 square feet of roof surface, about 9.29 square metres. Roofers price and order by the square, so a 1,503 ft² roof is about 15 squares before waste.
How much waste should I add for a roof?
Around 10% for a simple gable and 15% or more for hip roofs, valleys, dormers and other cut-up shapes. Every hip and valley needs shingles cut along its length.
What is the multiplier for a 6/12 roof pitch?
For 6/12 it is 1.118, which is 26.6 degrees. Other common values: 4/12 is 1.054, 8/12 is 1.202, 10/12 is 1.302 and 12/12 is 1.414.
Does a hip roof need more shingles than a gable roof?
The surface area is the same for the same footprint and pitch, but a hip roof wastes more in cuts and needs much more ridge cap because the hips need capping as well as the ridge. Drip edge runs around all four eaves instead of eaves and rakes.
Is there an in-depth guide to the Roofing Calculator?
Yes. “Roof Pitch, Stair Layout, AC Sizing and Running Costs Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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