Roofing Calculator — Roof Area, Squares and Shingles

Roof pitch multipliers
Pitch	Angle	Multiplier
2/12	9.5°	1.014
3/12	14.0°	1.031
4/12	18.4°	1.054
5/12	22.6°	1.083
6/12	26.6°	1.118
7/12	30.3°	1.158
8/12	33.7°	1.202
9/12	36.9°	1.250
10/12	39.8°	1.302
12/12	45.0°	1.414

Roof pitch multipliers

Pitch

Angle

Multiplier

2/12

9.5°

1.014

3/12

14.0°

1.031

4/12

18.4°

1.054

5/12

22.6°

1.083

6/12

26.6°

1.118

7/12

30.3°

1.158

8/12

33.7°

1.202

9/12

36.9°

1.250

10/12

39.8°

1.302

12/12

45.0°

1.414

How roof area is calculated

A sloped roof covers more area than the house beneath it. Start with the plan area — the footprint plus the overhang on every side — and multiply by the pitch multiplier, the length of the slope per foot of horizontal run:

roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²)

For a 40 × 30 ft house with a 12-inch overhang, the plan area is 42 × 32 = 1,344 ft². At a 6/12 pitch the multiplier is √(1 + 0.5²) = 1.118, so the roof is about 1,503 ft². The same multiplier works for gable and hip roofs as long as every plane has the same pitch; for roofs with several pitches, work out each section and add them up.

Pitch multipliers

Pitch	Angle	Multiplier
3/12	14.0°	1.031
4/12	18.4°	1.054
6/12	26.6°	1.118
8/12	33.7°	1.202
10/12	39.8°	1.302
12/12	45.0°	1.414

Pitch is the rise in inches for every 12 inches of horizontal run. To measure it, hold a level against the roof, mark 12 inches along it and measure the gap straight down to the roof surface. The multiplier is the slope length per foot of run, so it also gives rafter lengths: for any rise and run, the Pythagorean theorem calculator finds the slope side. Asphalt shingles have a minimum slope set by the building code and the manufacturer — commonly 2/12, with extra underlayment on low slopes — so check the installation instructions on the bundle before roofing a shallow pitch.

Squares, bundles and underlayment

Roofing is ordered by the square — 100 ft² of roof. Add a waste allowance for cutting at edges, hips and valleys: about 10% for a gable roof and 15% for a hip roof. With 10% waste, the 1,503 ft² roof needs 1,653 ft² of material: 17 squares, or 50 bundles at about 33.3 ft² per bundle. Underlayment comes in rolls too: a roll of #15 felt covers about 4 squares once the laps are allowed for, and a typical roll of synthetic underlayment covers 10 squares. Ridge cap and starter strip are ordered by the linear foot of ridge and eaves, on top of the field shingles.

From roof area to cost

Contractors quote by the square, so the estimate multiplies the squares with waste by your price per square; enter it to see the total. Ask whether the quote includes tear-off of the old roof, underlayment, flashing and disposal, since those are often priced separately. For the floor area of the house itself, use the square footage calculator; for siding or interior walls, the paint calculator and the drywall calculator.

Frequently asked questions

How do I calculate roof square footage?

Multiply the house footprint, including the overhang, by the pitch multiplier. A 42 × 32 ft footprint is 1,344 ft²; at a 6/12 pitch the multiplier is 1.118, so the roof is about 1,503 ft².

What is a roofing square?

100 square feet of roof. Shingles, felt and labor are priced by the square, so a 1,503 ft² roof is 15.03 squares, or 17 squares with 10% waste.

How many bundles of shingles are in a square?

Usually three, each covering about 33.3 ft², for 3-tab and most architectural shingles. Some heavy designer shingles need four or five bundles per square; check the wrapper.

How much waste should I add?

About 10% for a simple gable roof and 15% for a hip roof. Add more for valleys, dormers, skylights and steep or cut-up roofs, where more shingles are trimmed.

How do I find my roof pitch?

Hold a level against the roof with one end touching it, measure 12 inches along the level, then measure straight down to the roof. That rise in inches over 12 is the pitch, such as 6/12.

Does the calculator include the ridge cap and starter strip?

No. Ridge cap and starter shingles are sold by the linear foot of ridge and eave; order them separately from the field shingles.