Measure from the ground and the attic.Walking on a roof is a fall risk and can damage shingles. Nothing in this guide needs you on the roof.
1. Measure the footprint
The footprint is the area the roof covers seen from directly above: the outside walls plus the overhangs. It isn't your home's living area. A two-story house has roughly half as much roof footprint as floor area, and an attached garage or porch adds footprint even though it isn't living space.
For a simple rectangular house:
- Measure the outside walls along the ground: length and width, in feet.
- Measure the overhangs. The eave overhang is how far the roof edge sticks out past the wall along the low edges; the rake overhang is the same at the sloped gable ends. They are often different, so measure both from the ground where you can see the roof edge.
- Add the overhangs to both sides. Footprint = (wall length + both rake overhangs) × (wall width + both eave overhangs).
If the overhang is the same all round, add twice the overhang to each wall dimension. Measure separate roof sections, such as a garage or porch roof, on their own and add them up later.
2. Find the pitch
Pitch is how many inches the roof rises for every 12 inches of horizontal run, written like 6/12. From the attic, hold a level horizontally against the underside of a rafter, mark 12 inches along the level from where it touches, and measure straight up from the mark to the rafter. That measurement is the rise.
The roof pitch calculator converts the rise to an angle and percent slope, and works the other way if you only know the angle.
3. Convert to roof area
A sloped roof has more surface than its footprint. The roof-area multiplier (also called the pitch factor) converts one to the other:
roof-area multiplier = √(rise² + 12²) ÷ 12
roof area = footprint × roof-area multiplier
| Pitch | 4/12 | 5/12 | 6/12 | 7/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|---|---|
| Multiplier | 1.054 | 1.083 | 1.118 | 1.158 | 1.202 | 1.302 | 1.414 |
This works for any roof whose planes all share one pitch, gable and hip roofs included, because each plane's area is its footprint times the same factor.
4. Convert to squares
Divide the roof area by 100. One roofing square covers 100 square feet of roof (GAF). Labor, tear-off and underlayment are usually priced on this number, without waste.
Standard asphalt shingles usually come three bundles to the square, but some products need more, so go by the coverage printed on the bundle wrapper (GAF).
5. Add waste for shingles
Shingles are cut to fit at edges, hips, valleys and around chimneys, so you order more than the roof area. How much more depends on the roof: long, simple planes waste little; many hips, valleys and dormers waste more. GAF, a shingle manufacturer, says contractors usually include a 10–15% waste factor (GAF). Use the allowance your roofer or the shingle's installation instructions give.
Waste applies to the shingle order only. It isn't part of the roof's area, so don't add it before calculating labor or tear-off.
Worked example
A simple gable house
- Outside walls
- 50 ft × 30 ft
- Overhangs
- 1.5 ft all round (assumed for the example)
- Footprint
- (50 + 3) × (30 + 3) = 53 × 33 = 1,749 sq ft
- Pitch
- 6/12, one pitch throughout
- Roof-area multiplier
- √(36 + 144) ÷ 12 = 1.118034
- Roof area
- 1,749 × 1.118034 = 1,955.44 sq ft
- Squares
- 1,955.44 ÷ 100 = 19.55 squares
- Waste allowance
- 10% (an example; use your roofer's figure)
- Shingles to order
- 19.55 × 1.10 = 21.51 squares
- Bundles at 3 per square
- 21.51 × 3 = 64.5, so 65 bundles
Labor and tear-off would be priced on the 19.55 squares; only the shingle order uses 21.51.
Several pitches and complex roofs
- Different pitches: measure each section's footprint and pitch separately, convert each to roof area, then add the areas. A gambrel or mansard roof has two pitches on each side, so each slope is its own section.
- Dormers, porches and additions: treat each as a section. Don't count the same footprint twice where one roof sits over another.
- Hip roofs: if all planes share one pitch, the whole footprint × one multiplier is still correct. Hip and valley lines need more ridge cap and valley material, which you count in linear feet, not squares.
GableOne pitch: one multiplier. 
GambrelTwo pitches per side: two sections. 
MansardSteep sides and a low top: measure each.
When this isn't enough
Ground measurements are an estimate. Overhangs are hard to measure exactly from below, hidden roof sections are easy to miss, and small errors grow when multiplied across a large roof. Rely on a professional measurement when:
- the roof has several pitches, dormers, turrets or curves;
- you can't see or reach the edges to measure the overhangs;
- you are ordering materials yourself and a shortfall would be costly.
A roofer's on-site measurement or a measurement report is the better basis for ordering. If your own figure is well below a quote's shingle quantity, ask the contractor how they measured; either number could be the one that's off. Asphalt shingles also need a minimum slope: 2/12 under many US residential codes, with two layers of underlayment from 2/12 up to 4/12, for example as summarized by the City of Grand Rapids and the City of Minneapolis. Your local code decides.
Questions
Is roof area the same as the square footage of my house?
No. Living area counts floors; roof area is the sloped surface over the footprint, including overhangs. A two-story house can have a roof area smaller than its living area.
Should I include the overhangs?
Yes. The roofing covers the overhangs too, so add the eave and rake overhangs to the wall dimensions before converting to roof area.
My roof has two different pitches. What do I do?
Treat each pitch as its own section: measure that section's footprint, multiply by its own multiplier, and add the results.
How much waste should I add?
It depends on the roof's shape and the shingle. GAF says contractors usually allow 10–15%; your roofer or the shingle's instructions will give the figure for your roof.
How many bundles are in a square?
Usually three for standard asphalt shingles, but some products need more. Check the coverage printed on the bundle.
When do I need a professional measurement?
When the roof is complex, when you can't measure the overhangs, or when you are ordering materials yourself. A contractor's own measurement is what their quote will be based on.
Sources
- How Many Bundles of Shingles Are in a Square?, GAF (shingle manufacturer). Read October 26, 2026. Square definition, bundles per square, typical waste factor.
- Roof Assemblies (Chapter 9, Michigan Residential Code), City of Grand Rapids, Michigan. Read October 21, 2026. Minimum slope and underlayment on low slopes (R905.2.2, R905.1.1).
- Roofing and re-roofing with asphalt shingles (handout, last updated May 2016), City of Minneapolis, Minnesota — Construction Code Services. Read October 26, 2026. Minimum slope and double underlayment from 2:12 to 4:12.
The pitch, multiplier and square conversions are geometry and unit arithmetic; the formulas are on the methodology page.