Roofing Calculator
Work out how much roofing material you need: enter the footprint of the house and the roof pitch to get the sloped roof area, the number of roofing squares, bundles of shingles and rolls of underlayment, with a waste allowance built in.
Bundles are the field shingles only — starter strip, ridge cap and drip edge are ordered separately. The waste allowance is already included and every count rounds up.
Formula and calculation
Worked example
A 40 ft by 30 ft house with a 6/12 roof has a 1,200 sq ft footprint. Multiply by the 1.118 pitch factor and the roof surface is 1,341.64 sq ft, which is 13.42 roofing squares. Adding 10% waste gives 14.76 squares to order: 45 bundles of shingles, 4 rolls of underlayment, and about $1,919 in shingles at $130 per square.
How to measure the footprint
Stand back and sketch the plan view of the house, then measure each rectangle at ground level. The roof is bigger than the walls: a typical 12 in overhang adds 2 ft to both the length and the width, which on a 40 by 30 house is over 100 extra square feet of footprint. Steeper roofs need more material for the same footprint, which is why the pitch factor matters more than the size of the house. If the roof has more than one level, measure each footprint separately and add the squares together.
What the estimate leaves out
The bundles here are the field shingles only. Starter strip along the eaves, ridge cap over the peaks and hips, and drip edge along the rakes are ordered separately, and they are a real line on the invoice. Roofing also needs nails, underlayment, ice and water shield in cold climates, and flashing where the roof meets a wall or chimney. Tear-off of the old roof, decking repair and labor are not included either. Treat the number here as the covering material, and add 10–15% to the whole invoice for the rest.
What the results mean
- Roof area
- The sloped surface area of the roof in square feet — the number you actually cover with shingles. It is larger than the footprint by the pitch factor.
- Roofing squares
- The roof area in roofing squares. One square is 100 square feet, and shingles, underlayment and labor are all priced by the square in the US.
- Pitch factor
- How much longer the sloped roof is than its footprint for this pitch: multiply the footprint by it to get the roof area. The standard table value: 6/12 is 1.118, 8/12 is 1.202, 12/12 is 1.414.
- Order this much
- The number of squares to order: the roof area plus the waste allowance, in squares.
- Bundles of shingles
- How many bundles, panels or tiles to buy, depending on the covering you picked. It rounds up to whole units and includes the waste allowance. Asphalt shingles are the common case: 3 bundles cover one square.
- Underlayment
- Rolls of number 15 felt underlayment to buy, 400 square feet (4 squares) per roll, waste included. Synthetic underlayment covers about 10 squares per roll, so you would need fewer rolls of it.
- Material cost
- The covering material at the price per square you entered. It excludes underlayment, starter strip, ridge cap, drip edge, fasteners and labor.
Frequently asked questions
How many bundles of shingles do I need for a 1,200 sq ft roof?
Divide the roof area by 100 to get squares, then multiply by 3 bundles per square. A 1,200 sq ft roof is 12 squares, or 36 bundles before waste; with 10% waste it is 40 bundles. Enter your footprint and pitch above and the bundle count comes out already rounded up.
Do I use the house footprint or the roof area?
You measure the footprint, then the pitch factor converts it to roof area. A 1,200 sq ft footprint at 6/12 pitch is about 1,342 sq ft of roof surface, because the slope makes the roof longer than its shadow. Shingles are bought on the roof area, never the footprint.
How much waste should I allow for a roof?
Ten percent is the usual allowance for a simple gable roof. Hip roofs, valleys, dormers and complicated cut-up rooflines waste more, so 15% is safer there. The last course along the ridge and the starter course are cut from full shingles, which is where most of the waste comes from.
How many roofing squares is a 2,000 square foot house?
A 2,000 sq ft footprint is 20 squares before the pitch; at 6/12 pitch the sloped area is about 2,236 sq ft, or 22.4 squares, and with 10% waste about 24.6 squares. Fetch the actual footprint of the roof, not the finished floor area, since an overhang and a second storey change it.
What pitch is too low for shingles?
Asphalt shingles are rated for pitches of 2/12 and steeper, and below 4/12 most manufacturers require a double underlayment layer. Under 2/12 the roof is treated as flat and needs a membrane or a standing-seam system instead. Check the shingle warranty and your local code before ordering for a low-slope roof.