Roof details
Type feet and inches like 12'6" or just 12.5. Add one roof plane for each flat part of the roof. Everything is saved on this device.
About the roofing calculator
This free roofing calculator works out the real area of your roof and the shingles you need. Measure the roof from above (the size of the house plus the overhang), enter the pitch, and the calculator adds the slope. You get squares, bundles of shingles, and the ridge cap, starter strip, drip edge, underlayment and nails.
A roof is always bigger than the floor under it, because it slopes. A 6/12 roof is 12% bigger than its footprint. This tool does that step for you.
Features
- Footprint or slope — type the size from above, or measure on the roof itself
- Pitch — pick 2/12 to 12/12 and see the angle, or type your own
- Several planes — add a plane for each flat part, and subtract a chimney or skylight
- Squares and bundles — one square is 100 sq ft, and there are 3 bundles to a square
- Waste — 5%, 10%, 15% or 20%
- Extras — ridge cap, starter strip, drip edge, underlayment rolls and nails
- Cost — add a price per bundle
- Metric — m² and metres, with the same bundles
How to use it
- Choose Feet & inches or Metres & cm
- Enter the length and width of the first roof plane. Use Add another roof plane for each more
- Choose whether you measured from above or on the roof, and pick the pitch
- Pick the waste for your roof
- Optional: enter the eaves, rakes and ridge lengths
- Read the squares and bundles, then press Copy the list
Roof pitch and slope factor
Pitch is the rise in inches for each 12 inches of run, like 6/12. The slope factor is how much to multiply the flat area by to get the real roof area. It is the length of the slope divided by the flat length.
| Pitch | Angle | Slope factor |
|---|---|---|
| 2/12 | 9.5° | 1.014 |
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Not sure of your pitch? Hold a 12 inch level against the rafter, make it level, and measure how far down the roof is at the 12 inch mark.
Worked examples
These numbers come from the calculator:
| Roof | Pitch | Result |
|---|---|---|
| 40 × 25 ft footprint, 10% waste | 6/12 | 1,118 sq ft, 12.3 squares, 37 bundles |
| 50 × 30 ft footprint, 10% waste | 6/12 | 1,677 sq ft, 18.45 squares, 56 bundles |
| 60 × 40 ft footprint, 10% waste | 6/12 | 2,683 sq ft, 29.52 squares, 89 bundles |
| Two planes of 40 × 14 ft, 15% waste | 8/12 | 1,346 sq ft, 15.48 squares, 47 bundles |
| 25 × 20 ft on the slope minus a 3 × 3 ft chimney, 10% | none | 491 sq ft, 5.4 squares, 17 bundles |
Take the 40 × 25 ft roof: the footprint is 1,000 sq ft. Times 1.118 is 1,118 sq ft of roof. With 10% waste that is 1,230 sq ft, or 12.3 squares. Three bundles a square gives 36.9, so you buy 37 bundles.
Ridge cap, starter, drip edge and underlayment
Fill in the optional edges to get these lines. They are rough guides, so check the packs you buy:
- Ridge cap: one bundle covers about 25 linear ft of ridge or hip
- Starter strip: one bundle covers about 100 linear ft of eaves and rakes
- Drip edge: metal strips of 10 ft (3 m) along eaves and rakes
- Underlayment: a roll of synthetic covers 10 squares and a roll of felt about 4 squares
- Nails: about 2½ lb of roofing nails for each square
For the two-plane 8/12 roof with 80 ft of eaves, 56 ft of rakes and 40 ft of ridge, you need 2 bundles of ridge cap, 2 bundles of starter and 14 pieces of drip edge.
Frequently asked questions
How many bundles of shingles per square?
Three bundles cover one square (100 sq ft). Some premium shingles come in four bundles a square, so check the label.
How do I find my roof area from the footprint?
Multiply the flat area by the slope factor for your pitch. A 6/12 roof uses 1.118.
How much waste should I add?
10% for a simple gable roof, 15% for hips and valleys, and 20% for a roof with many cuts or dormers.
Does the calculator work for metal roofing?
It gives the roof area, so you can divide by the coverage of a panel. Bundles and ridge cap are for asphalt shingles.
What is the lowest pitch for shingles?
Shingles are normally for 4/12 and up. From 2/12 to 4/12 they need special underlayment. Under 2/12 use another roof type. Ask the maker.
Should I measure from the ground?
You can. Measure the house from above, add the overhang on each side, and use the footprint option with the pitch.
Good to know
- Safety first: do not climb a wet or steep roof. Measure from the ground or a plan when you can.
- Add the overhang to the house size.
- All numbers are estimates. Ask your supplier to confirm.
- Everything stays on your device.
Related tools
- Square Footage Calculator — areas of any shape
- Online Protractor — measure a roof angle
- Drywall Calculator — sheets for the inside walls
- Fence Calculator — posts and boards for the yard
- Unit Converter — feet, metres and squares
