Enter your roof footprint and pitch — or a roof area you've already measured —
and get the roof area in squares, the bundles of architectural shingles to buy, and the
underlayment rolls to cover it. The numbers use the standard pitch multiplier table,
3 bundles per square, and a 10% waste factor so you don't run short on the roof.
Squares, bundles & rolls·Footprint + pitch, or direct area·Standard pitch multipliers
Read this first
This is a planning estimate for a simple gable roof, not a substitute for a
measured takeoff. It treats your footprint as one clean gable and applies the standard
pitch multiplier. Real roofs with hips, valleys, dormers, and odd angles have more surface
and far more cutting waste — use the 15% waste setting for those, and get a pro
measurement before you order. Always buy a little extra: matching a shingle dye lot after
the fact is hard, and running short mid-job is worse than a leftover bundle.
Pick how you want to start — from the building footprint plus a roof pitch, or from a roof area you've already measured — and you'll get squares, bundles of architectural shingles, and underlayment rolls, with the waste factor applied.
Footprint is the flat ground area; pitch turns it into true roof area.
The actual sloped surface area — no pitch multiplier is applied to a direct area.
10% for a simple gable; use 15% for hip roofs or roofs with many valleys.
Roof area
Squares (with waste)
Squares to order (rounded up)
Waste applied
Bundles (3 per square)
Underlayment rolls (10 squares each)
The math, honestly
How the bundle count is figured
It starts with area. A square is the roofer's unit:
1 square = 100 square feet of roof. The catch is that a sloped roof has
more surface than the flat footprint it sits on, so you can't just use
length × width. You multiply the footprint area by a pitch
multiplier — √(1 + (rise/12)²) — to get the
true sloped area. A 6/12 roof multiplies by 1.118; a 12/12 by
1.414. If you already measured the sloped area, you skip this step.
From there: roof area ÷ 100 gives raw squares, and multiplying by
1 + waste/100 adds the waste factor. To get
bundles of architectural shingles, multiply squares by 3
and round up — there are 3 bundles per square. For
underlayment, one synthetic roll covers about 10 squares
(1,000 sq ft), so divide squares by 10 and round up.
Why the waste factor matters: shingles get cut at rakes, ridges,
valleys, and around penetrations, and the offcuts are usually scrap. A simple gable
wastes about 10%; a hip roof or a roof full of valleys wastes closer to 15%. The
worst outcome is running short and being unable to match the dye lot of the shingles
already on the roof — so this calculator rounds squares and bundles up.
Roof pitch multipliers
The pitch multiplier converts a flat footprint area into true sloped roof area. It's the
square root of 1 plus (rise/12) squared — the same figures the calculator uses.
Multiply your footprint square footage by the factor for your roof's slope.
Pitchrise in 12
Multiplier√(1 + (rise/12)²)
Slopecharacter
3/12
1.031
Low slope
4/12
1.054
Low
5/12
1.083
Moderate
6/12
1.118
Common residential
7/12
1.158
Moderate-steep
8/12
1.202
Steep
9/12
1.250
Steep
10/12
1.302
Steep (walkable limit)
12/12
1.414
Very steep (45°)
Multipliers are rounded to three decimals. Also useful: architectural shingles run
3 bundles per square, and one synthetic underlayment roll covers about
10 squares (1,000 sq ft). Confirm both against the wrappers you buy.
Common roof sizes
Worked examples for a few common footprints at a 6/12 pitch, computed with the same
formula and a 10% waste factor — so these match what the calculator gives you.
Bundles assume 3 per square; underlayment rolls assume 10 squares per roll.
Footprint @ 6/12
Roof areasq ft
Squares+10% waste
Bundles3 per square
Rollsunderlayment
30 × 20
671
7.38
23
1
40 × 30
1,342
14.76
45
2
50 × 30
1,677
18.45
56
2
60 × 40
2,683
29.52
89
3
The 40 × 30 row is the worked example: 1,200 sq ft footprint × 1.118 = 1,341.6
sq ft of roof, ÷ 100 = 13.42 squares, × 1.10 = 14.76 squares, which orders as
15 squares, 45 bundles, and 2 underlayment rolls.
Reading the result well
A square count is only useful if you act on it sensibly. Four things worth knowing before
you order.
Match the waste factor to the roof
Ten percent is right for a simple gable with two clean planes. A hip roof, or any roof with multiple valleys, dormers, or chimneys, forces a lot of angled cuts whose offcuts can't be reused — bump the waste to 15%. Steeper and more complex roofs lean toward the high end. This calculator defaults to 10%; change it to fit what you're actually covering.
Footprint is not roof area
The footprint is the flat ground the building covers. The roof is bigger because it slopes, and the steeper the pitch, the bigger the gap: a 12/12 roof has 41% more surface than its footprint. That's the whole job of the pitch multiplier. If you've physically measured the sloped planes, use the direct-area mode and skip the multiplier entirely.
Confirm bundles per square
This tool assumes 3 bundles per square, which covers most architectural (laminated) asphalt shingles and many three-tab products. But heavy designer and premium shingles can run 4 or even 5 bundles per square, which changes the bundle count significantly. Read the coverage printed on the shingle wrapper before you finalize the order.
Order whole units — and don't under-buy
You buy shingles by the bundle and underlayment by the roll, so the calculator rounds both up. Keep the extra until the job is done; a leftover bundle is cheap insurance. Running short mid-job means a second trip and the risk of a visible dye-lot mismatch where new shingles meet old. When in doubt, round up again.
Where to buy
Got your numbers? Here's where to pick up what you need:
The terms behind the calculator, in plain English. These are background definitions, not
installation specifications — follow your shingle manufacturer's instructions and
local building code for the real thing.
Square
The standard roofing unit: 100 square feet of roof surface. Shingles, underlayment, and labor are all measured and priced per square. A roof with 1,500 sq ft of sloped area is 15 squares.
Footprint
The flat ground area a building covers — length × width. It's always smaller than the roof area because the roof slopes. The pitch multiplier converts a footprint into true roof area.
Pitch (slope)
How steep a roof is, given as rise in 12 — a 6/12 roof rises 6 inches for every 12 inches of horizontal run. Pitch sets the pitch multiplier: steeper roofs have more surface area over the same footprint.
Pitch multiplier
The factor that turns footprint area into sloped roof area: √(1 + (rise/12)²). A 4/12 pitch is 1.054, a 6/12 is 1.118, and a 12/12 is 1.414. Multiply your footprint square footage by it to size the roof.
Bundle
A wrapped pack of shingles. For most architectural (laminated) asphalt shingles, 3 bundles cover one square. Heavy designer shingles can run 4–5 bundles per square. The bundles-per-square figure is printed on the wrapper.
Architectural shingle
A laminated, multi-layer asphalt shingle — thicker and more dimensional than a flat three-tab. It's the most common residential shingle today and the basis for this calculator's 3-bundles-per-square assumption.
Underlayment
The water-resistant layer rolled over the roof deck before shingles go on. Synthetic underlayment rolls typically cover about 10 squares (1,000 sq ft) each; older felt rolls cover less. Code and shingle warranties usually require it under the full roof.
Waste factor
The extra material bought above the calculated roof area — typically 10% for a gable, 15% for a hip or cut-up roof — to cover cuts at rakes, ridges, valleys, and penetrations. It's cheap insurance against running short and facing a dye-lot mismatch.
Frequently asked
Start from your roof area in square feet, not the building footprint. Multiply the footprint by the pitch factor for your slope to get the true sloped area, divide by 100 to get squares, then add about 10% for waste. Architectural shingles run 3 bundles per square, so a 15-square roof needs roughly 45 bundles. Always round squares and bundles up to whole units — you buy shingles in full bundles. Try it in the calculator.
A roofing square is 100 square feet of roof surface. It's the standard unit roofers use to measure, quote, and order materials. A roof that measures 1,500 sq ft of sloped area is 15 squares. Thinking in squares keeps the math simple: shingles, underlayment, and labor are all priced per square.
Three bundles cover one square for most architectural (laminated) asphalt shingles, which is what this calculator assumes. Some lighter three-tab shingles also run 3 bundles per square, while a few heavy designer shingles run 4 or even 5 bundles per square. Always confirm the bundles-per-square figure printed on the shingle wrapper before ordering.
Pitch is why your roof area is bigger than your footprint. A steeper roof has more sloped surface over the same ground area, so it needs more shingles. The pitch multiplier is the square root of 1 plus (rise/12) squared: a 4/12 roof multiplies the footprint by 1.054, a 6/12 by 1.118, and a 12/12 by 1.414. So a 12/12 roof needs about 41% more shingles than the flat footprint suggests.
Use about 10% waste for a simple gable roof and 15% for a hip roof or any roof with many valleys, dormers, or cut-ups. Those features force many angled cuts, and the offcuts are usually too small to reuse, so more material ends up as scrap. This calculator defaults to 10%; bump it to 15% for complex roofs. Running short mid-job and having to match a dye lot later is worse than a few extra bundles.
One roll of synthetic roofing underlayment covers about 10 squares, which is 1,000 square feet. Divide your squares by 10 and round up — a 15-square roof needs 2 rolls. Coverage varies by brand and roll length, and felt underlayment rolls cover less per roll, so check the wrapper. Building code and shingle warranties usually require underlayment under the full roof.
Use the actual sloped roof area whenever you have it. The footprint is the flat ground area the building covers; the roof area is larger because the roof slopes. This calculator turns a footprint into roof area for you using the pitch multiplier, treating the footprint as a simple gable. If you've already measured the real sloped area, enter it directly in the calculator and skip the pitch step.
Common mistakes
Roofing shortfalls are painful — mid-project dye-lot mismatches can mean the entire slope needs to come off. These are the most common calculation errors.
Using the building footprint as the roof area
The footprint is the flat ground area; the actual roof surface is larger because the roof is sloped. A 6/12 pitch roof covers about 12% more area than the footprint below it, and a 12/12 pitch covers about 41% more. Enter the footprint and let the calculator apply the pitch multiplier — or enter your directly measured sloped area if you have it.
Using 10% waste for a hip roof or complex roof
A simple gable roof can work with 10% waste. Hip roofs, roofs with many valleys, dormers, and cut-up sections force far more angled cuts and generate proportionally more scrap — use 15% for those. Running short and needing to match a dye lot is far worse than a few extra bundles.
Assuming 3 bundles per square for every shingle type
Three bundles per square is correct for most architectural (laminated) asphalt shingles. Heavy designer shingles can require 4 or 5 bundles per square. Always confirm the bundles-per-square printed on the shingle wrapper before ordering — it is product-specific, not universal.
Forgetting underlayment
Shingle count gets the attention, but code and manufacturer warranties typically require underlayment under the full roof. One roll of synthetic underlayment covers about 10 squares; divide your square count by 10 and round up to get rolls. Don't skip this line on the materials list.