Roof Size Calculator
Enter the building's length and width, the overhang at the eaves and the roof pitch, and choose the roof style. The calculator gives the sloped roof area, the number of roofing squares and the slope length.
- From the footprint
- Any pitch
- Gable, hip or shed
Pitch is the rise in inches for every 12 inches across. Use a phone level app if you do not know it.
A roof is always bigger than the floor under it. It overhangs the walls and slopes up. The pitch factor tells you how much larger: a 6/12 roof is about 12% bigger than its footprint, and a 12/12 roof about 41% bigger.
Roof pitch chart
Pitch factors. Search for a pitch.
| Pitch | Angle | Factor | Area on a 1,000 sq ft footprint |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 1014 |
| 4/12 | 18.4° | 1.054 | 1054 |
| 6/12 | 26.6° | 1.118 | 1118 |
| 8/12 | 33.7° | 1.202 | 1202 |
| 10/12 | 39.8° | 1.302 | 1302 |
| 12/12 | 45° | 1.414 | 1414 |
Multiply the footprint with overhangs by the factor to get the roof area.
Pitch and roof area
A 40 × 30 ft house. Slide the pitch.
The end view shows the slope. The small triangle is 12 across and the pitch up.
How to work out roof area
Footprint times factor. Step through it.
Footprint
Length × width.
Hip roofs have the same area as gable roofs of the same pitch, because every face slopes at the same angle.
Gable, hip or shed roof
Roof styles. Compare them.
Two slopes meeting at a ridge.
Slopes on all four sides.
One single slope.
Low pitches under about 2/12 need special membranes instead of shingles.
Before measuring
Roof checks. Tick each item.
Tick each item as you go.
Measure from the ground where you can. Climbing on a roof is dangerous, especially when it is wet.
What you get
Roof area. Each tile explains one result.
Sloped surface.
Area ÷ 100.
In degrees.
Slope multiplier.
Eave to ridge.
Add dormers, porches and garages separately and sum them.
Frequently Asked Questions
No question matches that search.
How do I calculate roof area?
Multiply the footprint including overhangs by the pitch factor for your roof's slope.
What is a roof pitch factor?
How much longer a sloped roof is than its flat footprint. It equals the square root of one plus the pitch over twelve squared.
How many squares is my roof?
Divide the roof area in square feet by 100.
Is a hip roof bigger than a gable roof?
Not in area if the pitch is the same, but it has more edges and more waste.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches across, or use a level app on the rafter.
How much bigger is a roof than the house?
About 10% to 40% for common pitches, plus the overhangs.
Should I include the overhang?
Yes. The overhang is covered in roofing too.
Know your roof before you buy
Start with the footprint, add the overhang and multiply by the pitch factor. The roof area and squares are what roofers and suppliers ask for.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.