Roofing Calculator
Squares, shingle bundles & underlayment for a roof
A 40 by 30 ft footprint under a 6/12 pitch is 14.8 roofing squares with 10% waste, which is 45 bundles of shingles at three to the square. Enter your own footprint and pitch for squares, bundles and rolls of underlayment. Replacing gutters at the same time? The gutter calculator covers the roofline.
What a roofing square is
This doubles as a shingle calculator and a roof area calculator, since both come from the same footprint and pitch.
Roofers price everything in squares, and one square is 100 ft² of finished roof surface. Shingles, underlayment, labour and disposal are all quoted per square, so it is the unit to think in from the start.
The catch is that roof area is not the footprint of the house. A sloped roof covers more surface than the ground beneath it, and the steeper the roof the bigger the gap. That is what the pitch multiplier corrects for.
Roof pitch multiplier table
Pitch is the rise in inches per 12 inches of horizontal run. The multiplier is the hypotenuse: √(rise² + 144) ÷ 12.
| Pitch | Multiplier | Angle | Notes |
|---|---|---|---|
| Flat / 1⁄12 | 1.003 | 4.8° | Needs membrane, not shingles |
| 2⁄12 | 1.014 | 9.5° | Minimum for shingles, with double underlayment |
| 3⁄12 | 1.031 | 14.0° | Low slope |
| 4⁄12 | 1.054 | 18.4° | Walkable, very common |
| 5⁄12 | 1.083 | 22.6° | |
| 6⁄12 | 1.118 | 26.6° | The most common residential pitch |
| 8⁄12 | 1.202 | 33.7° | No longer walkable |
| 10⁄12 | 1.302 | 39.8° | Steep, needs staging |
| 12⁄12 | 1.414 | 45.0° | Very steep |
The practical consequence: the same 1,200 ft² footprint is 13.9 squares at 4⁄12 and 18.7 squares at 12⁄12 — 35% more material for the identical house. Steep roofs also cost far more in labour, because above about 6⁄12 a roofer cannot simply walk on it.
Shingles and squares by house size
| Footprint | Area | Roofing squares | Shingle bundles |
|---|---|---|---|
| 20 × 20 ft | 400 ft² | 4.9 | 15 |
| 30 × 25 ft | 750 ft² | 9.2 | 28 |
| 40 × 30 ft | 1,200 ft² | 14.8 | 45 |
| 50 × 30 ft | 1,500 ft² | 18.4 | 56 |
| 60 × 40 ft | 2,400 ft² | 29.5 | 89 |
Standard three-tab and architectural shingles come 3 bundles to a square, each bundle covering about 33⅓ ft². Heavier designer shingles sometimes run 4 or 5 bundles per square, so check the wrapper rather than assuming.
How much waste to add
| Roof | Waste | Why |
|---|---|---|
| Simple gable, no penetrations | 10% | Starter course and ridge cuts only |
| Hip roof | 12 to 15% | Every hip is an angled cut |
| Multiple valleys or dormers | 15% | Heavy cutting on both sides |
| Complex, many planes | 15 to 20% | Offcuts rarely reusable |
Everything sold separately from the shingles
Field shingles are usually about half the material bill. The rest is easy to forget when comparing quotes:
- Underlayment: one roll per ~400 ft² for #15 felt. Synthetic rolls cover far more, often 1,000 ft².
- Ice and water shield along eaves and in every valley. Required by code in cold climates, typically 3 ft up from the eave or past the interior wall line.
- Starter strip along every eave and rake. Cutting up field shingles for this is a common shortcut that voids some warranties.
- Ridge cap, sold by the linear foot; measure ridges and hips.
- Drip edge on eaves and rakes, in 10 ft lengths.
- Roofing nails: about 2½ lb per square with 4 nails per shingle, or 320 nails per square. High-wind areas need 6.
- Flashing for every chimney, wall junction, vent and skylight.
- Ridge vent and disposal, which is a real cost — a 20-square tear-off fills a large skip.
Measuring a roof from the ground
You do not have to climb up. Measure the building footprint at ground level including the eave overhang on each side, then apply the pitch multiplier. To find the pitch without going up, hold a level horizontally against a gable end, mark 12 inches along it, and measure straight down to the roof line — that drop in inches is your rise.
Two things the footprint method will not catch: dormers add their own roof area, and a roof over an L-shaped or multi-wing house needs each rectangle measured separately and added. For anything with more than two planes, measure each section and sum them rather than using an overall footprint.
Roofing work carries real fall risk. Measure from the ground where you can, and treat this as a material estimate rather than a substitute for a professional survey.
What the roofing estimate assumes
Roof area is footprint × pitch multiplier × (1 + waste). The multiplier is the true hypotenuse ratio, √(rise² + 12²) ÷ 12, so a 6⁄12 pitch gives √180 ÷ 12 = 1.118.
- Squares are roof area ÷ 100. Bundles use 33⅓ ft² each, the standard three-bundles-per-square coverage for three-tab and most architectural shingles.
- Underlayment assumes ~400 ft² per roll, which matches #15 felt. Synthetic underlayment rolls commonly cover 1,000 ft² and would need roughly 40% of the count shown.
- Footprint means the area the roof covers, including eave overhangs, not the interior floor area.
- One roof plane geometry. Dormers, L-shaped wings and multi-level roofs need each section measured and added separately.
- Not included: ice and water shield, starter strip, ridge cap, drip edge, nails, flashing, vents and disposal. These commonly total as much again as the field shingles.
Reviewed September 2026.
Both unit systems. Metric mode measures the footprint in metres and reports the roof in m². Shingles are still counted in squares and bundles, because that is how they are sold; one square is 9.29 m².
Sources, checked
- How many bundles of shingles are in a square?, GAF. Three bundles to a 100 ft² square for most shingles.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Roofing Calculator: frequently asked questions
How many bundles of shingles do I need?
Three bundles per roofing square, since each covers about 33⅓ ft². A 40 × 30 ft house with a 6/12 roof is 14.8 squares including waste, which is 45 bundles. Designer shingles can run 4 or 5 bundles per square.
What is a roofing square?
100 square feet of finished roof surface. Shingles, underlayment, labour and disposal are all estimated and priced by the square, so it is the unit to work in.
How do I calculate roof area from the footprint?
Multiply the footprint by the pitch multiplier. A 6/12 roof is 1.118× its footprint, a 4/12 is 1.054× and a 12/12 is 1.414×. The multiplier is √(rise² + 144) ÷ 12.
What is the pitch multiplier for a 6/12 roof?
1.118. A 1,200 ft² footprint under a 6/12 roof is 1,342 ft² of actual roof surface, or 1,476 ft² once 10% waste is added.
How much extra should I add for waste?
10% for a simple gable roof, 12 to 15% for a hip roof, and 15% or more where there are valleys and dormers. Angled cuts along hips and valleys produce offcuts that cannot be reused.
How many nails per square of shingles?
About 320, which is four per shingle, or roughly 2½ lb of roofing nails per square. High-wind zones require six nails per shingle, so budget 480 and about 4 lb.
What is the minimum roof pitch for shingles?
2/12 with doubled underlayment, and 4/12 for standard installation. Anything shallower needs a membrane roof rather than shingles, because wind-driven rain can push back up under the courses.
How do I measure roof pitch from the ground?
Hold a level horizontally against the gable end, mark 12 inches along it, and measure straight down to the roof line. That drop in inches is the rise, so a 6 inch drop is a 6/12 pitch.