Roofing Calculator: Squares, Bundles and Underlayment

A roofing calculator turns your roof size and pitch into squares, bundles, underlayment rolls and drip edge pieces. For most houses the quick answer is this: work out the sloped roof area, divide by 100 to get squares, then multiply squares by 3 for bundles of architectural shingles. The pitch step is the one people skip, and it is the step that leaves them short.

The formula

Measure each roof section as a flat plan rectangle, then correct it for slope.

Plan area = length x width, added up for every section

Pitch factor = square root of (1 + (rise / 12) squared)

Roof area = plan area x pitch factor

Squares = roof area x (1 + waste percent) / 100

Bundles = squares x bundles per square for your shingle

Underlayment rolls = roof area / coverage per roll, rounded up

Common pitch factors are worth keeping on paper:

Pitch	Pitch factor	Extra area vs flat plan
4/12	1.054	about 5 percent
5/12	1.083	about 8 percent
6/12	1.118	about 12 percent
8/12	1.202	about 20 percent
10/12	1.302	about 30 percent
12/12	1.414	about 41 percent

Most architectural shingles cover 33.3 sq ft per bundle, so 3 bundles make one square. GAF lists its Timberline HDZ bundle at 33.33 sq ft per bundle, which matches that rule. Heavy designer shingles can take 4 or 5 bundles per square, so read the coverage printed on the bundle before you order. A standard synthetic underlayment roll covers 1,000 sq ft, or 10 squares, before overlap. A roll of 15 lb felt covers about 400 sq ft, or 4 squares. Drip edge comes in 10 ft pieces and goes along eaves and rakes.

Worked example: a 40 by 30 ft house with a 6/12 pitch

Say the house footprint under the roof is 40 x 30 ft (12.2 x 9.1 m), the pitch is 6/12, the roof is a simple gable, and you pick architectural shingles at 3 bundles per square. You add 10 percent waste for cuts and ridge work.

Step 1, plan area: 40 x 30 = 1,200 sq ft.

Step 2, pitch factor for 6/12: square root of (1 + (6 / 12) squared) = square root of 1.25 = 1.118.

Step 3, roof area: 1,200 x 1.118 = 1,341.6 sq ft, call it 1,342 sq ft.

Step 4, add waste: 1,342 x 1.10 = 1,476 sq ft to cover.

Step 5, squares: 1,476 / 100 = 14.76 squares.

Step 6, bundles: 14.76 x 3 = 44.3 bundles. You cannot buy part of a bundle, so you round up to 45 bundles, and we rounded up on purpose like a buyer.

Step 7, underlayment: 1,342 / 1,000 = 1.34 rolls of synthetic underlayment, so you round up to 2 rolls. If you use 15 lb felt instead, 1,342 / 400 = 3.36, so you round up to 4 rolls.

Step 8, drip edge: eaves are 40 ft on each side, so 80 ft. Each rake is the sloped edge over half the width: 15 x 1.118 = 16.8 ft, and there are 4 rakes, so about 67 ft. Total is about 147 ft. At 10 ft per piece, 147 / 10 = 14.7, so you round up to 15 pieces.

We checked this example a second way. The slope length over the full 30 ft width is 30 x 1.118 = 33.5 ft, and 40 x 33.5 = 1,340 sq ft, which matches step 3 within rounding. For the full installed price on a job like this, see our guide to roof replacement cost.

How do you measure a roof safely?

Measure from the ground where you can. Get the house length and width at the foundation, add the overhang on each side, and write down each section on a simple sketch. A gable roof has two main sections. A hip roof, a garage roof and a porch roof each add sections, and each one gets its own length and width.

Get pitch without climbing if you can. A pitch gauge app on a phone held against a rafter tail or a gable end trim board gives a usable reading. You can also measure rise over a 12 in run with a level and tape in the attic against a rafter. Do not walk a steep roof to take a tape reading. Anything over a 7/12 pitch is hard to stand on, and a fall from one story can kill you. If the roof is steep, high or cut up with valleys, pay a roofer to measure it. Most give free quotes and their count is the one they will stand behind.

Count the extras while you measure. Write down total ridge and hip length and the number of vents. Ridge cap comes by the bundle or linear foot, and starter strip runs along every eave and rake.

What changes the bundle count?

Shingle type is the first lever. Standard 3-tab and most architectural shingles take 3 bundles per square. Thick luxury shingles take 4 or 5 bundles per square because each bundle would be too heavy at 3 per square. The bundle count changes, but the square count does not.

Roof shape is the second lever. A plain gable roof wastes about 10 percent on cuts, starter courses and ridge cap. A hip roof wastes near 15 percent, because every hip end is a diagonal cut. Valleys, dormers and chimneys push waste up again.

Pitch changes area, not waste. A 12/12 roof has 41 percent more surface than its floor plan. If you order from house size alone on a steep roof, you will be short by a third or more.

Common mistakes to avoid

Do not use the living area of the house as the roof area. The roof covers the garage, the porch and the overhangs, and slope adds more again. A 1,500 sq ft house often has 1,700 to 1,900 sq ft of roof.

Do not forget starter strip and ridge cap. Field shingles alone will not finish a roof. Budget starter along all eaves and rakes and cap along every ridge and hip.

Do not mix dye lots. Buy all bundles for one job at one time and check the lot or date code on the packs. A bundle bought two weeks later can be a shade off, and the patch will show from the street.

Do not skip ice and water shield where your code asks for it. Cold climate codes often require it along eaves, and it is smart in valleys in any climate. It is sold by the roll and is separate from the main underlayment count.

Frequently asked questions

How many bundles of shingles do I need for 1,000 sq ft?

At 3 bundles per square, 1,000 sq ft is 10 squares and needs 30 bundles before waste. With 10 percent waste, order 33 bundles.

How many square feet does one bundle cover?

A standard bundle covers about 33.3 sq ft. Three bundles cover one square, which is 100 sq ft. Check the bundle label, since heavy shingles cover less per bundle.

How much waste should I add for roofing?

Add 10 percent for a simple gable roof and 15 percent for a hip roof or a roof with valleys and dormers. Complex roofs with many small sections can need 20 percent.

How many rolls of underlayment do I need?

Divide roof area by roll coverage and round up. A synthetic roll covers 1,000 sq ft, so a 1,800 sq ft roof needs 2 rolls. A 15 lb felt roll covers about 400 sq ft, so the same roof needs 5 rolls.

Can I measure my roof from satellite images?

Yes, for a planning count. Satellite tools give plan size and pitch estimates that are close enough to price a job. Confirm with a ground measure or a roofer visit before you place a large order.

Next step

Run your own sections through the roofing calculator above, then check the money side with our guide to roof replacement cost before you call for quotes.