Roofing Calculator
Enter the footprint of the building and the pitch of the roof. You get the actual roof surface, the number of roofing squares, and the bundles of shingles to order, with the pitch multiplier shown so you can check it.
How many squares of shingles do I need?
A roofing square is 100 square feet of roof surface, and shingles come 3 bundles to the square. Take the footprint, multiply by the pitch multiplier to get the real sloped surface, add a waste allowance, then divide by 100. A 40 ft x 30 ft house at a 6/12 pitch has 1,342 sq ft of roof, which with 10 % waste is 14.76 squares, or 45 bundles.
Shingles you need
1,341.64 sq ft of roof plus 10 % waste is 14.76 squares, or 45 bundles.
- Roof surface
- 1,341.64 sq ft
- Squares
- 14.76100 sq ft per square
- Bundles
- 453 bundles per square
- Underlayment
- 4 rolls400 sq ft per roll
The numbers above are a worked example for a 40 ft x 30 ft building at a 6/12 pitch. Allow this page to run scripts in your browser to use your own dimensions, or follow the formula further down the page and do it by hand.
How the roofing calculator works, step by step
The trap in roofing is that the roof is bigger than the house. You can measure the footprint from the ground, but shingles cover the sloped surface, and the steeper the roof the larger that surface is. Get the multiplier wrong and you are short by several squares on delivery day, on a job where running out halfway means an exposed deck.
Measure to the edge of the overhang, not to the wall. The eaves and rakes are roof too.
This is Pythagoras. A roof that rises 6 in over 12 in of run has a sloped length of the square root of (122 + 62) = 13.42 in for every 12 in of run, which is a multiplier of 1.1180.
bundles = squares x 3
Worked example: a 40 ft x 30 ft house at 6/12
- Footprint: 40 x 30 = 1,200 sq ft
- Multiplier at 6/12: 1.1180
- Roof surface: 1,200 x 1.1180 = 1,341.6 sq ft
- With 10 % waste: 1,475.8 sq ft
- Squares: 1,475.8 / 100 = 14.76 squares
- Bundles: 14.76 x 3 = 45 bundles
The pitch multiplier table
If you would rather not do the arithmetic, read the multiplier off this table and multiply your footprint by it. Every figure here is the square root of (1 + (rise / 12) squared), rounded to four decimal places.
| Pitch | Multiplier | Roof over a 1,200 sq ft footprint |
|---|---|---|
| 3/12 | 1.0308 | 1,237 sq ft |
| 4/12 | 1.0541 | 1,265 sq ft |
| 5/12 | 1.0833 | 1,300 sq ft |
| 6/12 | 1.1180 | 1,342 sq ft |
| 7/12 | 1.1577 | 1,389 sq ft |
| 8/12 | 1.2019 | 1,442 sq ft |
| 9/12 | 1.2500 | 1,500 sq ft |
| 10/12 | 1.3017 | 1,562 sq ft |
| 12/12 | 1.4142 | 1,697 sq ft |
To find your pitch without climbing on the roof, hold a level horizontally against a rafter or the gable end, measure 12 in along it, and measure straight down from that point to the roof line. That drop in inches is the rise.
What a square is, and what a bundle covers
Roofing is the one trade that still sells by the square: 100 square feet of finished roof. Architectural shingles are packed 3 bundles to the square. As a concrete reference, Owens Corning lists its TruDefinition Duration shingles at 3 bundles per square, 98.4 sq ft of coverage per square and 64 shingles per square, at an exposure of 5 5/8 in.
Note that 98.4 figure: the actual coverage is slightly under the nominal 100 sq ft, which is one more reason the waste allowance is not optional. Heavier designer shingles can run four or five bundles to the square, so check the wrapper before you multiply.
Waste, and the parts people forget
| Roof | Waste | Why |
|---|---|---|
| Simple gable, no penetrations | 10 % | Rake and ridge cuts only |
| Hips and valleys | 15 % | Every course is cut at an angle |
| Dormers, turrets, many penetrations | 15 to 20 % | Short courses and offcuts everywhere |
| Steep roof needing staging | 15 % | More breakage handling material on the slope |
The calculator covers field shingles, underlayment and ridge cap. It does not cover drip edge, valley flashing, step flashing, pipe boots, ice and water shield at the eaves, or the nails, and every one of those is needed. Ice and water shield in particular is code in cold regions for the first few feet above the eaves, and it is not what the underlayment figure here represents.
Frequently asked questions
How many bundles of shingles in a square?
Three, for standard three-tab and architectural shingles. Owens Corning lists 3 bundles per square for its Duration line. Heavy designer and premium shingles can be four or five bundles per square, so read the wrapper rather than assuming.
How many squares is a 2000 square foot roof?
Twenty squares of roof surface, because a square is 100 sq ft. But be careful which 2,000 you mean: if that is the floor area of the house, the roof over it is larger. At a 6/12 pitch a 2,000 sq ft footprint carries 2,236 sq ft of roof, which is 22.4 squares before waste.
How do I find my roof pitch from the ground?
Hold a 2 ft level horizontally against the gable end so it touches the roof line at one end. Measure 12 in along the level from that point, then measure straight down to the roof. That measurement in inches is the rise, and the pitch is written as rise/12.
Do I include the overhang?
Yes. Measure the footprint to the outer edge of the eaves and rakes, not to the walls. On a house with 18 in overhangs all round, leaving them out understates a 40 ft x 30 ft roof by more than 200 sq ft, which is two squares.
How much underlayment do I need?
One roll of synthetic underlayment typically covers 400 sq ft, so divide the roof surface including waste by 400. Check your roll: felt is commonly sold in 400 sq ft rolls for 15 lb and 200 sq ft for 30 lb.
How much ridge cap do I need?
Measure every ridge and every hip and add them up. A bundle of ridge cap covers roughly 20 to 25 linear feet depending on the product. You can cut cap from three-tab shingles, but you cannot cut it from architectural shingles, which is a common and expensive surprise.
Can I put new shingles over old ones?
Most codes allow a second layer but not a third, and it is rarely a good idea: you cannot inspect the deck, the new shingles telegraph the old ones, and the weight matters. Tearing off also lets you see the rot before it becomes a structural problem rather than after.
Source
Bundles and coverage per square come from Owens Corning, TruDefinition Duration Shingles product page (bundles and coverage per square). The pitch multiplier is not a product figure at all: it is Pythagoras, so it is exact for any roof. Shingle packaging varies by line and by manufacturer, and where your wrapper disagrees with this page, the wrapper wins.