Roofing Calculator
Squares, shingle bundles, underlayment and nails — from the footprint you can actually measure, with the pitch multiplier done for you.
The footprint of the building, including eave overhang — not the roof surface. The pitch below converts one to the other.
Rise in inches per 12 inches of horizontal run. Measure it with a level and a tape in the attic if you can't get on the roof.
The actual sloped surface, all planes added together — not the footprint.
If a roofer quoted you in squares, enter that number. One square = 100 sq ft.
Check the wrapper before you order. Assuming 3 when the shingle packs 4 is the most common way a roofing order comes up a pallet short.
Add every ridge and hip. Cap bundles cover about 20 linear ft each.
For starter strip along the bottom edge — about 105 ft per bundle.
You need
Enter dimensions
| Also on the order | Amount |
|---|---|
| Pitch multiplier | — |
| Underlayment | — |
| Roofing nails (1¼″) | — |
| Hip & ridge cap | — |
| Starter strip | — |
Architectural bundles commonly run $33–$45.
How to Calculate Roofing Materials
A roof is bigger than the house under it, and the pitch is what decides by how much. Every estimate is the same four steps:
- Footprint area: length × width, including the eave overhang.
- Pitch multiplier: √(rise² + 144) ÷ 12. This is the slope length of a rise-in-12 triangle — a 6/12 roof is 1.118× its footprint.
- Squares: roof area ÷ 100, plus 10–20% waste.
- Bundles: squares × bundles per square, rounded up.
Roofing formulas
Roof area = footprint × √(rise² + 144) ÷ 12
Bundles = (roof area × 1.10 ÷ 100) × bundles per square, rounded up
Worked example — 40 × 30 ft house, 6/12 pitch: footprint 1,200 sq ft × 1.118 = 1,342 sq ft of roof. Add 10% waste → 1,476 sq ft = 14.8 squares. At 3 bundles per square that's 45 bundles, 2 rolls of synthetic underlayment and about 34 lb of nails.
Pitch Multipliers
Multiply your footprint by this to get roof surface:
| Pitch | Multiplier | Angle | 1,200 sq ft footprint becomes |
|---|---|---|---|
| 3/12 | 1.031 | 14° | 1,237 sq ft |
| 4/12 | 1.054 | 18° | 1,265 sq ft |
| 6/12 | 1.118 | 27° | 1,342 sq ft |
| 8/12 | 1.202 | 34° | 1,442 sq ft |
| 10/12 | 1.302 | 40° | 1,562 sq ft |
| 12/12 | 1.414 | 45° | 1,697 sq ft |
The Bundles-Per-Square Trap
Almost every guide online tells you three bundles make a square and stops there. That's true for three-tab and for most standard architectural shingles — and wrong often enough to cost you a second trip.
- 3 bundles/square — three-tab and standard architectural. Roughly 26–29 shingles per bundle for three-tab, 22–25 for architectural.
- 4 bundles/square — heavyweight architectural and several designer lines.
- 5 bundles/square — premium designer, slate-look and shake-look profiles.
- Read the wrapper. Coverage per bundle is printed on it. That number, not a rule of thumb, is what your order should be built on.
Before You Order
- Measure the footprint, not the roof. Getting on a steep roof with a tape is the dangerous way to get a number you can calculate from the ground.
- Valleys and hips eat shingles. Every one means angled cuts and discarded offcuts — that's what the 15% waste tier is for.
- Ice-and-water shield is usually code at the eaves in freeze-thaw climates, and it's separate from your field underlayment. Check locally.
- High-wind zones want six nails per shingle, not four — that's 480 per square instead of 320, and it's often a warranty condition.
- Order the ridge cap as cap, not by cutting up field shingles, if the manufacturer's warranty calls for matching cap.
Frequently Asked Questions
How many bundles are in a square?
Three for three-tab and most architectural — but 4 or 5 for heavyweight and designer lines. Check the wrapper; assuming 3 is the most common way an order comes up short.
How do I get roof area from my footprint?
Multiply by the pitch multiplier, √(rise² + 144) ÷ 12. A 6/12 roof is 1.118× the footprint; 12/12 is 1.414×.
What is a roofing square?
100 sq ft of roof surface. Roofing is quoted and sold by the square, which is why every estimate converts to it.
How much waste should I add?
10% on a simple gable, 15% with valleys, hips or dormers, 20% on a cut-up roof or where the pattern must align.
How many nails per square?
About 320 at four per shingle; 480 in high-wind zones at six per shingle. Roughly 140 nails to the pound.
How much underlayment?
Synthetic rolls cover 10 squares, #15 felt 4 squares, #30 felt 2. Ice-and-water shield at the eaves is extra and often required by code.
References
- Asphalt Roofing Manufacturers Association — residential asphalt shingle guidance: asphaltroofing.org
- NRCA — The NRCA Roofing Manual (steep-slope assemblies): nrca.net
- International Residential Code, Chapter 9 — roof assemblies, underlayment and fastening: codes.iccsafe.org
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