Field Guide
How Many Shingles Do I Need? Squares, Bundles & Pitch
Shingles are sold by the bundle and priced by the square (100 sq ft of roof). Measure the roof from the ground, correct for the slope with the pitch factor, add waste, and convert to bundles.
1. How to Calculate Shingles (The Math Explained)
Here's the exact process this calculator uses, with a 30 × 40 ft gable roof at a 6/12 pitch as the example.
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1
Measure the footprint
Measure the length and width of the roof including the overhangs.
Footprint = 40 ft × 30 ft = 1,200 sq ft -
2
Apply the pitch factor
A sloped roof has more surface than its footprint.
Pitch factor = √(6² + 12²) ÷ 12 = 1.118 Roof area = 1,200 × 1.118 = 1,342 sq ft -
3
Add waste and convert to bundles
Standard shingles come 3 bundles to the square.
With 10% waste = 1,342 × 1.10 = 1,476 sq ft Squares = 1,476 ÷ 100 = 14.76 Bundles = 14.76 × 3 = 44.3 → 45 bundles (15 squares)
2. Roof Pitch Factors
Pitch is the rise in inches for every 12 in of horizontal run. You can read it with a level and tape on the roof or on a rafter in the attic, or from your house plans.
| Pitch | Factor | Example: 1,200 sq ft footprint |
|---|---|---|
| 4/12 | 1.054 | 1,265 sq ft of roof |
| 6/12 | 1.118 | 1,342 sq ft |
| 8/12 | 1.202 | 1,442 sq ft |
| 10/12 | 1.302 | 1,562 sq ft |
| 12/12 | 1.414 | 1,697 sq ft |
3. Ridge Cap, Starter & Drip Edge
For gable and hip sections, this calculator works out the edges from the shape. On a gable roof the ridge runs the length of the house and the rakes run up each gable end. On a hip roof, the ridge is the long side minus the short side, and four hips run down to the corners.
Hip & ridge cap: every ridge and hip. Check the bundle label for its linear-foot coverage (often 20 to 33 ft).
Starter strip: along the eaves, and along the rakes if your shingle maker calls for it.
Drip edge: along the eaves and rakes, in 10 ft pieces lapped 2 in.
Pro tip: check the code for ice barrier
In cold climates, codes require ice-and-water shield from the eaves to at least 24 in inside the exterior wall, and it's a good idea in valleys everywhere. Add it to your order separately.
4. Frequently Asked Questions
How many bundles of shingles do I need?
Find the roof area, add waste, divide by 100 for squares and multiply by 3. A 30 × 40 ft footprint at 6/12 pitch is 1,200 × 1.118 = 1,342 sq ft of roof. With 10% waste that's 1,476 sq ft, or 14.76 squares: 45 bundles (15 squares).
What is a square in roofing?
A roofing square is 100 sq ft of roof. Standard 3-tab and architectural shingles come 3 bundles to the square, so each bundle covers about 33⅓ sq ft. Some heavy designer shingles come 4 or 5 bundles to the square; check the wrapper.
How do I calculate the area of a roof from the ground?
Measure the house's footprint including the overhangs, then multiply by the pitch factor for your roof's slope: √(rise² + 144) ÷ 12. A 4/12 roof is 1.054, 6/12 is 1.118, 8/12 is 1.202 and 12/12 is 1.414. This works for gable and hip roofs alike, as long as every plane has the same pitch.
How much waste should I add for shingles?
About 10% for a simple gable roof and 15% for hip roofs or roofs with valleys, dormers and other cut-up areas. The waste covers the cuts at hips, valleys and rakes, plus the shingles used in the starter course on some installs.
How much hip and ridge cap do I need?
Add up the length of every ridge and hip. On a 32 × 48 ft hip roof at 6/12, the ridge is 16 ft and each of the four hips is 24 ft, so 112 ft in all, or 124 ft with 10% waste. Hip and ridge cap bundles cover about 20 to 33 linear ft depending on the product; the coverage is printed on the bundle.
How many rolls of roofing underlayment do I need?
Divide the roof area plus waste by the roll's coverage. Synthetic underlayment usually comes in 10-square rolls, #15 felt in 4-square rolls and #30 felt in 2-square rolls. A 1,476 sq ft roof needs 2 synthetic rolls or 4 rolls of #15 felt. Many codes also require ice-and-water shield along the eaves in cold climates.
What is starter strip and drip edge?
Starter strip is the first course of shingles along the eaves (and often the rakes), with the sealant strip at the edge so the first row can't blow up. Drip edge is the metal flashing on the roof edges that sends water into the gutter; it comes in 10 ft pieces. This calculator gives both from the eave and rake lengths.