Roofing Calculator
Calculate roof area, roofing squares and shingle bundles from your house footprint and roof pitch.
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How to Calculate
Roof area = footprint (including overhangs) × pitch multiplier, where multiplier = √(12² + rise²) ÷ 12.
One roofing square = 100 ft². Standard 3-tab and architectural shingles come 3 bundles per square.
Add 10% waste for simple gable roofs and 15% for hips, valleys and dormers.
See our methodology for how every calculator on this site is built.
Worked Example: A Simple Gable Roof
The house footprint including overhangs is 44 ft by 32 ft. The roof is a 6/12 pitch, and it's a plain gable with no valleys, so 10% waste is reasonable.
- Footprint: 44 × 32 = 1,408 ft²
- Pitch multiplier: √(12² + 6²) ÷ 12 = √180 ÷ 12 = 13.42 ÷ 12 = 1.118
- Roof area: 1,408 × 1.118 = 1,574 ft²
- With waste: 1,574 × 1.10 = 1,732 ft²
- Squares: 1,732 ÷ 100 = 17.3
- Bundles: 17.3 × 3 = 51.9, so 52 bundles
Also plan starter strip along the eaves and rakes, and hip and ridge cap along the 44 ft ridge. Those are sold separately and are not included in the field shingle count.
Pitch Multipliers
| Pitch | Angle (approx.) | Multiplier | Walkable? |
|---|---|---|---|
| 3/12 | 14° | 1.031 | Yes; check shingle minimum slope |
| 4/12 | 18.4° | 1.054 | Yes |
| 5/12 | 22.6° | 1.083 | Yes |
| 6/12 | 26.6° | 1.118 | Yes, with care |
| 7/12 | 30.3° | 1.158 | Use roof jacks |
| 8/12 | 33.7° | 1.202 | Use roof jacks and harness |
| 9/12 | 36.9° | 1.250 | Steep; harness required |
| 10/12 | 39.8° | 1.302 | Steep |
| 12/12 | 45° | 1.414 | Steep |
Most asphalt shingle manufacturers set 2/12 as an absolute minimum and require extra underlayment below 4/12. Roofs flatter than that usually call for rolled roofing or a membrane.
Measuring Without Getting on the Roof
You can get the footprint from the ground with a tape and add the overhang measured at the eaves and rakes. For pitch, hold a level against a rafter tail or the underside of the gable trim, mark 12 inches along the level, and measure straight up to the roof line. Satellite measurement reports and aerial imaging services are also available and are what many contractors use for bids.
Things the Basic Formula Misses
Hips and valleys
Every valley needs extra shingles for the cut edges, and hip roofs waste more along every hip line. A cut-up roof with dormers can easily need 15% or more extra. If the roof has multiple pitches, calculate each section separately with its own multiplier.
Starter, ridge cap and accessories
Measure the eaves and rakes in linear feet for starter strip, and the ridge and hips for cap shingles. A bundle of ridge cap commonly covers about 20 to 35 linear feet; check the wrapper. Also count drip edge, ice and water shield for eaves and valleys in cold climates, and pipe boots.
Tear-off weight
One layer of old asphalt shingles weighs roughly 2 to 3 lb per ft². On a 17 square roof, that's well over two tons of debris. Size the dumpster for it.
Rough Cost
Architectural shingles typically run around $30 to $50 per bundle, or roughly $100 to $150 per square in material. A full professional replacement with tear-off, underlayment and flashing often lands in the $400 to $700 per square range, and more on steep or complex roofs.
Sources
Common questions
Frequently Asked Questions
What is a roofing square?
100 square feet of roof surface.
How many bundles of shingles per square?
Usually 3; some heavy designer shingles need 4 or 5.
How do I find my roof pitch?
Measure the vertical rise over 12 inches of horizontal run, e.g. 6 inches rise = 6/12.
Can I put new shingles over old ones?
Most codes allow one layer over an existing layer if the deck is sound. Two existing layers means a full tear-off.
How much underlayment do I need?
The same area as the roof. Synthetic rolls commonly cover about 10 squares, felt rolls 2–4 squares, depending on weight.