Roofing calculator

Roofers measure in squares: one square is 100 sq ft of roof surface. To get there, the calculator starts from what you can measure on the ground, the building footprint, adds the eave and rake overhangs, and applies the pitch.

Inputs: roof shape (gable, hip, shed or pyramid), length and width, pitch as x/12 or in degrees, eave overhang, and rake overhang on gable ends.

Results:

• roof square footage and squares, plus the pitch multiplier
• rafter length, ridge height, ridge, hip, eave and rake lengths
• bundles of shingles (3 per square by default, editable)
• starter strip, ridge cap, drip edge in 10 ft pieces, underlayment rolls, ice and water shield along the eaves, and roofing nails in pounds

Every rate and allowance is editable, each line carries an estimated price you can overwrite, and the total updates as you type. A plan drawing shows ridge and hips. Export to PDF or Excel for free, with no account. It all runs in your browser; switch to metric if you prefer meters.

Pitch x/12 means the roof rises x inches for every 12 inches of horizontal run. The steeper the roof, the more surface sits over the same footprint:

roof area = footprint (with overhangs) × √(1 + (x/12)²)

If you only know the angle, enter degrees; the calculator converts both ways. On a hip roof every face has the same pitch, so the same multiplier applies to the whole footprint.

Example: a 40 × 30 ft house, gable roof, 6/12 pitch, 1 ft overhang at the eaves and the rakes, 10% waste, 3 bundles per square.

1. Footprint with overhangs: 42 × 32 = 1,344 sq ft
2. Roof area: 1,344 × 1.118 = 1,502.6 sq ft, or 15.03 squares
3. With 10% waste: 16.53 squares
4. Shingles: 16.53 × 3 = 49.6 → 50 bundles
5. Rafter length: 16 ft run × 1.118 = 17.9 ft; the ridge sits 15 × 6/12 = 7.5 ft above the wall plates
6. Ridge cap: 42 ft of ridge
7. Starter and drip edge: 84 ft of eaves + 71.6 ft of rakes = 155.6 ft → 16 pieces of 10 ft drip edge
8. Underlayment: 15.03 × 1.10 / 10 squares per roll = 1.65 → 2 rolls of synthetic underlayment (check the label; felt rolls cover only 2–4 squares, so you would need 5 or more)
9. Ice and water shield: 84 ft along the eaves; how far up the roof it must go depends on your local code
10. Nails: at about 320 nails per square and roughly 140 roofing nails per pound, 16.53 × 320 / 140 = 37.8 lb

A hip roof on the same house at the same pitch has a shorter ridge, four hips that also need ridge cap, and eaves on all four sides, so the starter and drip edge lengths change. The calculator handles those differences.

• Read the bundle label. Many shingles come 3 bundles per square, but heavier styles can be 4 or more. Change the rate to match.
• Waste depends on the roof. A plain gable needs less than a cut-up roof with valleys, dormers and skylights. Raise the waste for complex roofs.
• Split complex roofs. For an L-shaped house, run each rectangle separately and add them, then add valley material on top.
• Count the extras. Vents, flashing, pipe boots and step flashing depend on the layout; add them from the manufacturer's installation guide.
• Check the minimum slope. Shingles have a minimum pitch in the manufacturer's instructions; very low slopes need a different roofing system.

• Using the footprint as roof area. At 6/12 the roof is almost 12% larger than its footprint, and at 12/12 over 41% larger.
• Leaving out overhangs. In the example, 1 ft of overhang adds 144 sq ft to a 1,200 sq ft footprint.
• Ordering in squares, then forgetting bundles. Shingles are sold by the bundle; 16.53 squares is 50 bundles, not 17.
• Skipping the starter and ridge. They are separate products on most roofs, not cut from field shingles.

Insulating the attic too? Try the insulation calculator. Planning panels? See the solar panel calculator.