Roofing Calculator
Enter your roof footprint and pitch to estimate roofing squares, shingle bundles and rolls of underlayment for a typical asphalt shingle roof.
How to estimate shingles for your roof
- Measure the length and width of the building footprint in feet — you are measuring the horizontal area the roof covers, not the sloped surface. Multiply length × width, then multiply by the pitch multiplier for your roof slope to get the true roof area: a 6:12 pitch adds about 12% and a 12:12 pitch about 41% more surface than the footprint.
- Roofing is sold by the "square" — one square covers exactly 100 sq ft of roof. Divide the roof area by 100 to get squares. Standard 3-tab and architectural asphalt shingles come 3 bundles to the square.
- Add 10% waste for a simple gable roof or 15% for hip roofs and roofs with valleys and dormers, then multiply the squares by 3 to get bundles and round up. For underlayment, divide the roof area by the coverage of one roll — about 400 sq ft for a standard roll of felt — and round up.
Roof area (sq ft) = Footprint length × width × pitch
multiplier. Squares = Roof area ÷ 100. Bundles = Squares × 3 ×
(1 + waste %), rounded up. Underlayment rolls = Roof area ÷
400, rounded up.
Roof pitch multipliers
| Roof pitch | Multiplier | Notes |
|---|---|---|
| Flat / low slope | 1.00 | Up to about 3:12 — membrane or roll roofing may be required |
| 4:12 | 1.054 | Minimum pitch for standard shingles with double underlayment |
| 6:12 | 1.118 | Very common residential pitch |
| 8:12 | 1.202 | Steep — walkable with care |
| 10:12 | 1.302 | Steep — roof jacks recommended |
| 12:12 | 1.414 | 45° slope — not walkable, extra labor |
Multiply the building footprint area by the factor to get the true sloped roof area.
Frequently asked questions
1,500 sq ft is 15 squares. At 3 bundles per square that is 45 bundles — order about 50 with a 10% waste allowance.
One square equals exactly 100 sq ft of roof surface. It is the standard ordering unit: most asphalt shingles are packaged 3 bundles to the square, so each bundle covers about 33.3 sq ft.
About 33.3 sq ft for standard 3-tab and architectural shingles (3 bundles = 1 square = 100 sq ft). Some heavyweight designer shingles come 4 bundles per square — check the wrapper.
Hold a 12 in level horizontally against the roof surface (or a rafter in the attic) and measure straight down from its end to the roof. That drop in inches is the rise: 6 in of drop over a 12 in run is a 6:12 pitch.
Roughly 200–250 lb for architectural asphalt shingles (65–80 lb per bundle). Spread stacks out if you load bundles on the roof, and expect the tear-off from an average home to fill a 10–20 yard dumpster.
Most building codes allow a maximum of two layers, but a tear-off is the better choice if the old shingles are curling, the deck may be damaged, or you want the full manufacturer warranty. Overlays can also void wind warranties.
This calculator assumes about 400 sq ft per roll — typical for a roll of #15 felt. #30 felt covers about 200 sq ft per roll, while many synthetics come in 1,000 sq ft rolls. In cold climates add ice-and-water shield at eaves and valleys; one 65 ft roll covers about 200 sq ft.
Figure 4 nails per shingle — about 320 nails per square, or 480 in high-wind zones that require 6 per shingle. A 5 lb box of 1-1/4 in roofing nails holds roughly 1,000 nails, enough for about 2–3 squares.
3-tab shingles typically last 15–20 years, architectural (dimensional) shingles 25–30 years and premium designer shingles up to 40. Ventilation, climate and installation quality matter as much as the shingle itself.
Yes. Starter strips run along all eaves and rakes, and hip/ridge cap shingles cover the peaks — figure one bundle of cap shingles per 20–35 linear ft of ridge. Most pros also add 1–2 bundles of field shingles to cut starters from, or buy factory-made starter.