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Roofing & Siding

Roof Shingle Calculator

Squares, bundles and underlayment for a reroof, worked from the footprint and the pitch rather than from a measured slope.

Your measurements

Measure the building footprint on the ground. The pitch converts it to actual roof area.

Roof
A 6 in rise over 12 in of run is a 6:12 roof. Measure it from inside the loft if you cannot get on the roof safely.
Material
Printed on the wrapper. Three tab is almost always three; heavier laminates need four or five.
Ten percent for a simple gable. Fifteen or more for hips, valleys and dormers, which cut heavily.

Your result

Sample values

14.76

Squares

1,476

Roof area, ft²

45

Bundles

4

Rolls of underlayment

40 × 30 ft footprint at 6:12, 10% waste, 3 bundles per square.

What to order

Item Quantity
Shingle bundles 45 bundles
Squares covered 14.76 squares
#15 underlayment, 400 ft² rolls 4 rolls
Starter strip, 100 ft rolls 2 rolls
Ridge cap, linear feet 40 ft

How to use this calculator

Measure the footprint of the building on the ground rather than trying to measure the roof itself, then enter the pitch. Pitch is what turns a flat footprint into real roof area, and on a steep roof the difference is large — a 12:12 roof has over forty percent more surface than the ground it covers.

Check the bundles per square on the wrapper before ordering. Three tab shingles are almost always three bundles to the square, but architectural and designer laminates run to four or five, and getting it wrong is a third of the order.

Formula

Slope factor = √(1 + (Pitch ÷ 12)²)

Roof area = Footprint × Slope factor × (1 + Waste)

Squares = Roof area ÷ 100

Bundles = ceil(Squares × Bundles per square)

Square
100 ft² of roof — the unit roofing is sold and quoted in
Slope factor
1.000 at flat, 1.118 at 6:12, 1.414 at 12:12

Underlayment is estimated at 400 ft² per roll of #15 felt.

Worked example

A 40 ft by 30 ft footprint at 6:12, three tab shingles, 10% waste.

  1. 40 ft × 30 ft = 1,200 ft² of footprint
  2. Pitch 6 ÷ 12 = 0.5 ratio
  3. Slope factor √(1 + 0.25) = 1.1180
  4. 1,200 × 1.1180 = 1,341.6 ft² of actual roof
  5. 1,341.6 × 1.10 = 1,475.8 ft² with waste
  6. 1,475.8 ÷ 100 = 14.76 squares, × 3 = 45 bundles

14.76 squares, or 45 bundles

Understanding your results

The square is the unit everything in roofing is quoted in, so it is the figure to take to a supplier. One square is 100 square feet of finished roof, not of ground.

Waste is not padding. A simple gable genuinely wastes close to ten percent in starter courses and ridge cuts; a roof with hips, valleys and dormers wastes far more because every cut through a valley produces an offcut too small to use. If the roof is complicated, fifteen percent is a fairer allowance than ten.

Questions

About this calculator

Ask something else

From inside the loft, hold a level horizontally against a rafter, mark 12 inches along it, and measure straight up from that mark to the rafter underside. That vertical measurement is the rise per 12 of run, which is exactly what this calculator wants.

A square is 100 square feet of roof surface. It is the trade unit because shingles are packaged and priced to cover it, so quotes, bundles and labour rates all reference squares rather than feet.

Not in the bundle count. Both are separate items and the amounts depend on ridge and eave lengths rather than area, so they are listed in the order table for you to measure. Some roofers cut cap from a spare bundle instead of buying it.

Most codes allow a maximum of two layers of asphalt shingles. Beyond that the deck cannot take the weight and nails no longer reach sound timber. If there are already two, the calculation for a tear off is the same but you should also budget removal and disposal.

Keep going

All Roofing & Siding tools
Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.