How to use the roofing calculator
- Enter the house length and width at the walls, and how far the roof overhangs on each side. If you already know the level area the roof covers, choose Area and type it.
- Choose the roof pitch. If you do not know it, the roof pitch calculator explains how to measure it from the attic.
- Pick gable or hip. This does not change the roof area, only the ridge, hip and edge lengths.
- Check the waste and bundles per square. 10% and 3 are filled in. Change them to match your roof and the bundle label.
Optional extras: the coverage from an underlayment roll label gives a roll count, and your own price per square gives a rough material cost. Choose Meters to work in meters and square meters. Nothing is saved.
How roof area and squares are worked out
Here is a 40 ft by 30 ft house with a 12 inch overhang all round and a 6/12 gable roof. Press Try an example to see it above.
| Step | Working | Result |
|---|---|---|
| Outline with overhangs | (40 + 2) x (30 + 2) | 1,344 sq ft |
| Pitch multiplier for 6/12 | square root of (1 + (6 / 12)²) | 1.118 |
| Roof area | 1,344 x 1.118034 | 1,502.64 sq ft |
| Roofing squares | 1,502.64 / 100 | 15.03 |
| With 10% waste | 15.03 x 1.10 | 16.53 squares |
| Shingle bundles | 16.53 x 3, rounded up | 50 bundles |
In metric, a 12 m by 9 m house with a 30 cm overhang has a 12.6 m by 9.6 m outline, 120.96 m². At 6/12 that is 135.24 m² of roof, or 14.56 squares (16.01 with 10% waste, 49 bundles).
How many squares is my roof?
Shingles are measured and ordered by the roofing square. GAF, the shingle maker, puts it simply: "A roofing square is equal to 100 square feet of roof area". So divide the roof area in square feet by 100.
The roof is always bigger than the level area under it, and the steeper the pitch, the bigger it gets. For 1,500 sq ft of level area, overhangs included:
| Pitch | Multiplier | Roof sq ft | Squares | With 10% waste | Bundles |
|---|---|---|---|---|---|
| 3/12 | 1.031 | 1,546 | 15.46 | 17.01 | 52 |
| 4/12 | 1.054 | 1,581 | 15.81 | 17.39 | 53 |
| 5/12 | 1.083 | 1,625 | 16.25 | 17.88 | 54 |
| 6/12 | 1.118 | 1,677 | 16.77 | 18.45 | 56 |
| 7/12 | 1.158 | 1,737 | 17.37 | 19.10 | 58 |
| 8/12 | 1.202 | 1,803 | 18.03 | 19.83 | 60 |
| 9/12 | 1.250 | 1,875 | 18.75 | 20.63 | 62 |
| 10/12 | 1.302 | 1,953 | 19.53 | 21.48 | 65 |
| 12/12 | 1.414 | 2,121 | 21.21 | 23.33 | 71 |
Use the footprint, not the floor area of the house. A two-story house with 2,000 sq ft of floor space may have only about half that under its roof.
How many bundles of shingles per square?
GAF says: "For most standard asphalt shingles (3-tab and architectural), there are three bundles per square." So each bundle covers about a third of a square, roughly 33 sq ft.
Not every shingle follows that rule. GAF adds that "Some specialty shingles may require up to five bundles per square due to their added weight", and gives its own Woodland line as four bundles per square. The bundle label states the coverage, so change Bundles per square to match. The calculator always rounds bundles up to whole ones.
Gable or hip: why the roof area is the same
Every part of a roof at one pitch is larger than the patch of ground under it by the same multiplier. A gable roof and a hip roof over the same outline at the same pitch cover the same level area, so they have the same total roof area. The hip roof simply has four sloping sides where the gable has two sides and two upright gable walls.
What changes is the edges. For the 42 ft by 32 ft outline at 6/12:
| Edge | Gable | Hip |
|---|---|---|
| Ridge | 42 ft | 10 ft |
| Hips | none | 4 x 24 ft = 96 ft |
| Eaves | 84 ft | 148 ft |
| Rakes (sloping gable edges) | 4 x 17 ft 10 11/16 in | none |
Ridge and hip lengths tell you how much ridge cap you need, and eaves and rakes tell you how much drip edge and starter you need. A hip roof also needs more cut shingles along its hips, which is one reason a higher waste figure suits it.
How much extra for waste?
Shingles are cut at the edges, hips and valleys, so you buy more than the roof area. GAF's guide says "You'll want to add 10-15% to the material total for trim allowance and waste." The calculator fills in 10%; raise it for a roof with valleys, dormers or many hips. The waste is added before the bundles are rounded up, so a few bundles to spare is normal.
Underlayment rolls
Underlayment rolls differ a lot in size, so the calculator does not assume one. Type the coverage printed on the roll's label under Underlayment rolls, and check whether that figure allows for the overlaps. The roll count is the roof area divided by that coverage, rounded up.
For asphalt shingles, the 2024 International Residential Code, section R905.2.2, says that from 2/12 "up to 4 units vertical in 12 units horizontal (33-percent slope), double underlayment application is required". In that range the calculator counts two layers. Below 2/12 the code does not allow asphalt shingles at all.
What this roofing estimate leaves out
- Complicated roofs. Dormers, valleys, porches at a different pitch and roofs at more than one pitch need each plane measured and added up.
- Starter strips, ridge caps and flashing. These are often sold separately. Use the ridge, hip, eave and rake lengths above to estimate them from the product labels.
- Prices. We do not quote roofing costs. The cost line only multiplies the price you type by the squares with waste.
A roofer measures the roof itself before ordering, and the shingle maker's instructions decide how the shingles go on.
Questions and answers
How many squares is a 2,000 square foot roof?
Divide by 100: 2,000 sq ft of roof area is 20 squares. If 2,000 sq ft is the level area under a 6/12 roof, the roof itself is 2,236.07 sq ft, or 22.36 squares before waste.
How many bundles of shingles are in a square?
Three for most standard 3-tab and architectural asphalt shingles, according to GAF. Some heavier specialty shingles need four or five, so check the bundle label.
How many square feet does a bundle of shingles cover?
With three bundles per square, one bundle covers about 33 sq ft, a third of 100 sq ft. The exact coverage is printed on the bundle.
Does a hip roof need more shingles than a gable roof?
The roof area is the same at the same pitch over the same outline. A hip roof does need more ridge cap and more cut shingles along the hips, so allow a little more waste.
Can I use my house square footage for the roof?
Only for a one-story house, and only as a start. The roof covers the footprint plus the overhangs, and you must multiply by the pitch multiplier to get roof area.
How much waste should I add for shingles?
GAF suggests adding 10 to 15% for trim allowance and waste. Use the higher end for roofs with valleys, dormers or hips.
Sources
Checked on 8 October 2026. If a rule has changed, please tell us.