What is a roofing square?

One hundred square feet of roof surface. It is a unit of trade rather than a measurement standard, and it is why roofing is quoted in numbers that look far too small for the size of a house.

How do I find my pitch without climbing up?

Hold a level out horizontally from the roof surface, mark 12 inches along it, and measure straight down from that mark to the roof. That drop is the rise per 12 of run — the first number in the pitch.

How many bundles make a square?

Three is the norm across the major manufacturers for standard asphalt shingles, at about 33⅓ square feet a bundle. Thicker architectural and designer shingles are often packed four to the square, so the figure is a field on this page rather than an assumption inside it.

Why is the multiplier a square root?

Because the slope is the hypotenuse of a right triangle whose base is 12 and whose height is the pitch. Pythagoras gives the sloped length, and dividing by the base turns it into a multiplier you can apply to any footprint.

Home & construction

Roofing Calculator

Enter the footprint and the pitch to get the true sloped area, in square feet and in squares.

The link carries your figures, so it reopens on exactly these numbers.

What the pitch does to the surface

Same footprint, different slopes. A roof is always larger than the ground it covers, and the gap grows fast — guess the pitch low and you order short.

Pitch	Actual roof area (ft²)	Roofing squares	Bundles to buy
Flat (0/12)The surface equals the footprint.	1,320	13.2	40
4/12A shallow roof, still walkable.	1,391.4	13.91	42
6/12	1,475.8	14.76	45
9/12Steep enough to need staging.	1,650	16.5	50
12/12A 45 degree roof is 41% more surface than its footprint.	1,866.76	18.67	57

The formula

multiplier = √(1 + (pitch ÷ 12)²)

roof area = footprint × multiplier × (1 + waste ÷ 100)

squares = roof area ÷ 100

Worked example

A 40 ft × 30 ft footprint at 6/12 pitch, 10% waste

√(1 + (6 ÷ 12)²) = 1.1180
40 × 30 = 1,200 ft² of footprint
1,200 × 1.1180 = 1,341.6 ft² of roof
1,341.6 × 1.10 = 1,475.8 ft² → 14.76 squares

Where this goes wrong

Ordering shingles for the footprint instead of the slope

A roof is always bigger than the building under it. At 6/12 pitch it is 11.8% bigger; at 12/12 it is 41.4% bigger. Order for a 1,200 ft² footprint on a 12/12 roof and you are 500 ft² short — five squares, and a tarpaulin over the job while you wait.

Questions

What is a roofing square?: One hundred square feet of roof surface. It is a unit of trade rather than a measurement standard, and it is why roofing is quoted in numbers that look far too small for the size of a house.
How do I find my pitch without climbing up?: Hold a level out horizontally from the roof surface, mark 12 inches along it, and measure straight down from that mark to the roof. That drop is the rise per 12 of run — the first number in the pitch.
How many bundles make a square?: Three is the norm across the major manufacturers for standard asphalt shingles, at about 33⅓ square feet a bundle. Thicker architectural and designer shingles are often packed four to the square, so the figure is a field on this page rather than an assumption inside it.
Why is the multiplier a square root?: Because the slope is the hypotenuse of a right triangle whose base is 12 and whose height is the pitch. Pythagoras gives the sloped length, and dividing by the base turns it into a multiplier you can apply to any footprint.