How do I calculate the square footage of a roof?

Measure the length and width of each rectangular section at the drip edge, multiply them to get the plan area, then multiply by the pitch factor for that section's pitch. Add the sections together to get the true roof area.

How many square feet are in a roofing square?

One roofing square is 100 sq ft of roof surface. Divide the true roof area by 100 to get the number of squares.

Can I use the house's square footage to size the roof?

Not directly. The living area leaves out the overhang, garages and porches, and counts every floor, while the roof depends on the footprint, the overhang and the pitch. Measure the roof itself.

How much waste should I add?

It depends on how cut up the roof is. A simple gable needs less than a roof with hips, valleys and dormers, so start low on simple roofs and raise it as the cuts add up.

How many bundles of shingles are in a square?

Typically 3 bundles of shingles cover one square, but some products use 4 or more. Check the coverage printed on the wrapper before you order.

Free calculator

Roof square footage calculator with squares, waste and bundles

Add each rectangular section of the roof with its pitch. You get the plan area, the true sloped area, roofing squares and a bundle count, in feet or metres.

The formula

Each section is worked out on its own, then the sections are added together.

Plan area: length × width Measured flat, at the drip edge, as if you were looking straight down.
Pitch factor: √(1 + (rise ÷ 12)²) Rounded to 3 decimals. A flat roof (0/12) has a factor of 1.
True area: plan area × pitch factor The sloped surface you actually cover, added up over every section.
Squares: true area ÷ 100 One roofing square covers 100 sq ft.
Squares with waste: squares × (1 + waste % ÷ 100) Covers cuts, starter, hip and ridge, and offcuts.
Bundles: squares with waste × 3 Typically 3 bundles per square; round up, and check the wrapper.

Worked example

A house with a 42 × 32 ft main roof at 6/12 and an attached 24 × 22 ft garage at 4/12, both measured at the drip edge, with 10% waste.

Main roof plan area: 42 × 32 = 1,344 sq ft
Garage plan area: 24 × 22 = 528 sq ft
Total plan area: 1,344 + 528 = 1,872 sq ft
Pitch factors: 6/12 → √(1 + 0.5²) = 1.118; 4/12 → √(1 + 0.333²) = 1.054
Main roof true area: 1,344 × 1.118 = 1,502.6 sq ft
Garage true area: 528 × 1.054 = 556.5 sq ft
Total true area: 1,502.6 + 556.5 = 2,059.1 sq ft
Squares: 2,059.1 ÷ 100 = 20.59
Squares with 10% waste: 20.59 × 1.10 = 22.65
Bundles: 22.65 × 3 = 67.95, rounded up to 68

About 2,059 sq ft of true roof area: 20.6 squares, 22.65 squares with waste, about 68 bundles.

Pitch factor table

How much a 1,000 sq ft plan area grows once the slope is counted.
Pitch	Pitch factor	1,000 sq ft plan becomes
3/12	1.031	1,031 sq ft
4/12	1.054	1,054 sq ft
5/12	1.083	1,083 sq ft
6/12	1.118	1,118 sq ft
7/12	1.158	1,158 sq ft
8/12	1.202	1,202 sq ft
9/12	1.250	1,250 sq ft
10/12	1.302	1,302 sq ft
11/12	1.357	1,357 sq ft
12/12	1.414	1,414 sq ft

Plan area vs true area

Plan area is the footprint of the roof seen from straight above: length times width. True area is the sloped surface you actually shingle. The steeper the roof, the bigger the gap between the two.

To get from one to the other, multiply the plan area by the pitch factor. A 6/12 roof has a factor of 1.118, so every 1,000 sq ft of plan becomes 1,118 sq ft of roof. Order from the true area, never from the footprint.

Measure at the drip edge, not the walls

Shingles run out to the drip edge, so that is where your length and width should start and stop. Measuring the walls from the ground misses the overhang on every side, and on a typical house that adds up to a lot of roof.

If walls are all you can reach, measure them and tick “I measured the walls”. Enter the overhang measured flat from the wall to the drip edge, and the calculator adds it to both ends of the length and the width. Check one rake by eye: overhangs on gable ends are often shorter than at the eaves.

Split an L-shaped or cut-up roof into rectangles

Most roofs are not one rectangle. Break the footprint into rectangles that do not overlap, and give each one its own pitch. An L-shaped house is two sections: the main block and the wing, with the wing measured only from where it leaves the main block.

A hip roof over a rectangle has the same area as a gable over the same rectangle at the same pitch, so you can enter it as one section. Dormers, porches and a garage at a different pitch each get their own section. Small bump-outs you skip will show up as a shortage on site, so add them or raise the waste.

When to measure the real roof instead

Rectangles are fine for a quick budget or a sanity check on a quote. They do not give you ridge, hip, valley, eave and rake lengths, and those drive starter, ridge cap, drip edge and valley material.

When the roof has several pitches, valleys or wings, or when you are writing a firm estimate, trace the actual roof. RoofStruct gives you free, unlimited self-serve measuring and PDF reports, no card: you trace each facet on satellite imagery and get every area, pitch and edge length.

Roof square footage questions

How do I calculate the square footage of a roof?: Measure the length and width of each rectangular section at the drip edge, multiply them to get the plan area, then multiply by the pitch factor for that section's pitch. Add the sections together to get the true roof area.
How many square feet are in a roofing square?: One roofing square is 100 sq ft of roof surface. Divide the true roof area by 100 to get the number of squares.
Can I use the house's square footage to size the roof?: Not directly. The living area leaves out the overhang, garages and porches, and counts every floor, while the roof depends on the footprint, the overhang and the pitch. Measure the roof itself.
How much waste should I add?: It depends on how cut up the roof is. A simple gable needs less than a roof with hips, valleys and dormers, so start low on simple roofs and raise it as the cuts add up.
How many bundles of shingles are in a square?: Typically 3 bundles of shingles cover one square, but some products use 4 or more. Check the coverage printed on the wrapper before you order.