Roofing Calculator

Roof area is counted in squares. One square is 100 sq ft of roof surface, and common shingles come 3 bundles to a square. A sloped roof has more surface than the area you measure from directly above, and the pitch sets how much more.

How to use this calculator

1. Measure the roof's length and width in feet, from the outer edge of the overhang on each side rather than from the outside walls.
2. For a gable, hip or shed roof, enter the whole roof as one footprint. For an L-shaped or stepped roof, measure each section as its own rectangle, run the calculator once per section, and add the results.
3. Enter the pitch as the inches of rise for every foot of run. Use a level and a tape to measure how far the roof rises over a foot of horizontal distance. If you do not know the pitch yet, start with the roof pitch calculator.
4. Leave the waste extra at 10 percent for a plain roof. Open More options to raise it for a roof with hips, valleys or dormers.
5. The bundle count is already rounded up, so buy that number.
6. List ridge caps, starter strips, underlayment, drip edge and flashing as separate items, because the calculator does not count them.

Worked examples

Roofing squares for common footprints and pitches

Each cell: Roofing squares (squares). Inputs not in the table use the calculator's starting values.

Why a sloped roof needs more than its footprint

On a sloped roof, each foot of run carries more than a foot of roof, because the surface climbs as it runs out. The steeper the pitch, the bigger that difference.

The calculator turns the pitch into one slope factor, multiplies the footprint by it to get the roof surface, and adds the extra for waste. It then divides by the area of one square to get the squares, which show one decimal place. Bundles come from the squares, rounded up to whole bundles.

Waste, hips, valleys and dormers

The default extra of 10 percent comes from a shingle maker's estimating example. Treat it as a starting point, not a rule for every roof.

Hips, valleys and dormers need more. Each one adds angled cuts and offcuts that a plain rectangle does not, and the calculator does not set a figure for them, so raise the extra to suit the roof's shape.

Parts the estimate leaves out

The shingle count covers the roof area and nothing else. Ridge caps, starter strips, underlayment, drip edge and flashing are separate purchases. The calculator does not price them, and it does not estimate labor or tear-off costs. Check each product's label and your local building code for what the roof needs.

If you only want the shingle count, the shingle calculator is the page for that job. If the roof work includes new vents, size them with the attic ventilation calculator instead of relying on this estimate.

Mistakes to avoid

• Measuring the walls instead of the roof edge. The overhang adds surface, so measure out to the outer edge of the eaves and rakes.
• Using one pitch for a roof with several. Measure each section and enter its own pitch, or the squares will be off.
• Buying the roof area with no extra. Cuts at hips, valleys and dormers take more shingles than the open field, so keep the extra in the total.
• Entering a pitch below 2 in per 12 in. The calculator does not check slope, and a shingle maker sets that figure as the lowest slope for its asphalt shingles.

Can I measure my roof without climbing on it?

You can get the footprint from the ground by measuring the eave and rake lines, from house plans, or from a satellite map, then adding the overhang. For the pitch, hold a level against a rafter in the attic and measure the rise over a level run. Do not climb a steep or wet roof to measure it.

Common questions