Roof area calculator
Measure your roof. Plan your materials.
Roof preview
Materials estimate
Customize| Material | Unit | Quantity |
|---|---|---|
| Asphalt shingles | bundles | 50 |
| Underlayment | rolls | 5 |
| Ridge & hip caps | bundles | 2 |
How it’s calculated
We add the overhang, account for the slope, then apply your waste allowance. Every section is calculated separately.
Formula for Section A. All slopes in a hip section must have the same pitch.
Adjust waste & materialsMeasure once. Keep your whole estimate together.
Your sections carry over to the pitch and materials calculators. Your plan stays in this browser tab when you navigate or reload. Load a worked example to start over, and print or export your quantities before closing the tab.
Calculation breakdown — every section
Building lengths in ft; areas in sq ft. Overhang is horizontal and added on both ends of each dimension. Calculations use unrounded values; displayed figures may not add exactly.
| Section / shape | Wall dimensions | Overhang per side | Roof footprint | Pitch / multiplier | Surface area / share |
|---|---|---|---|---|---|
| Section Agable | 40.00 × 30.00 ft | 1.00 ft | 42.00 × 32.00 ft1,344.00 sq ft | 6.00:12× 1.11803 | 1,502.64 sq ft100.00% of total |
- Surface area before waste
- 1,502.64 sq ft
- Waste allowance (10%)
- 150.26 sq ft
- Order area including waste
- 1,652.90 sq ft · 16.53 roofing squares
Independent rectangular sections only. Overlaps, shared edges and valleys are not automatically deducted or constructed. Hip sections require equal pitch on every face. Confirm joined-roof measurements separately.
Package quantities round up after waste. Current assumptions: shingles 33.33 sq ft/bundle; underlayment 400.00 sq ft effective coverage/roll; cap 25.00 ft/bundle. These product settings use feet even when geometry is displayed in meters. Match the packaging and manufacturer’s overlap instructions before ordering.
Planning estimate only. Excludes labor, tax, removal, delivery, starter strips, flashing, ventilation, fasteners and deck repairs.
Calculation breakdown — every section
Building lengths in ft; areas in sq ft. Overhang is horizontal and added on both ends of each dimension. Calculations use unrounded values; displayed figures may not add exactly.
| Section / shape | Wall dimensions | Overhang per side | Roof footprint | Pitch / multiplier | Surface area / share |
|---|---|---|---|---|---|
| Section Agable | 40.00 × 30.00 ft | 1.00 ft | 42.00 × 32.00 ft1,344.00 sq ft | 6.00:12× 1.11803 | 1,502.64 sq ft100.00% of total |
- Surface area before waste
- 1,502.64 sq ft
- Waste allowance (10%)
- 150.26 sq ft
- Order area including waste
- 1,652.90 sq ft · 16.53 roofing squares
Independent rectangular sections only. Overlaps, shared edges and valleys are not automatically deducted or constructed. Hip sections require equal pitch on every face. Confirm joined-roof measurements separately.
Package quantities round up after waste. Current assumptions: shingles 33.33 sq ft/bundle; underlayment 400.00 sq ft effective coverage/roll; cap 25.00 ft/bundle. These product settings use feet even when geometry is displayed in meters. Match the packaging and manufacturer’s overlap instructions before ordering.
Planning estimate only. Excludes labor, tax, removal, delivery, starter strips, flashing, ventilation, fasteners and deck repairs.
Example arithmetic checked October 28, 2026 against independent geometry reference cases. Measurement method. This is a software calculation check, not a professional roof survey.
How to calculate roof square footage
Measure the horizontal footprint of each roof section at the exterior walls. Add the overhang to both ends of each dimension, then multiply the resulting footprint by the slope factor. This is roof surface area, before material waste.
Does roof pitch change the area?
Yes. A 6:12 roof has a slope factor of about 1.118. A 1,000 sq ft horizontal roof footprint therefore covers about 1,118 sq ft of sloped surface.
Can I calculate a hip or an L-shaped roof?
For a rectangular hip roof with equal pitch on every face, the footprint × slope-factor method gives the surface area. For an L shape, add separate non-overlapping sections. Valleys and shared edges require manual adjustment of trim lengths.
Are the diagram dimensions building or roof dimensions?
The form takes building dimensions measured at the exterior walls. The diagram shows roof dimensions after adding the horizontal overhang. The preview is schematic, not a construction drawing.
Why is a roofing square different from square feet?
One roofing square equals 100 square feet. We show the number of squares after your chosen waste allowance. Shingle quantities then round up using the coverage printed on your chosen bundle.
Roof area formula and worked example
For a section with a uniform pitch, multiply its horizontal roof footprint by the slope factor. Keep every length in the same unit. The result is square feet when lengths are in feet, or square meters when lengths are in meters.
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²)
Example: a 40 × 30 ft building with a 6:12 roof
- With 1 ft of overhang on every side, the roof footprint is 42 × 32 = 1,344 sq ft.
- A 6:12 pitch gives a slope factor of √1.25 ≈ 1.118.
- The sloped roof area is 1,344 × √1.25 ≈ 1,502.64 sq ft.
- With an example 10% waste allowance, the order area is about 1,652.90 sq ft, or 16.53 roofing squares.
Use the unrounded area for material calculations. For separate roof sections, calculate and add each area without counting overlapping footprints twice. Different slopes need separate sections. This calculator assumes the same horizontal overhang on every side of a section.
Roof footprint versus floor area
Living area is not roof footprint: a two-story house can have the same roof area as a one-story house with the same footprint. Include roofed garages and additions as separate sections when appropriate. A gable roof’s two slopes are already included in the footprint method; do not double the result.
Method: geometric roof planning estimates. Updated September 27, 2026. Verify measurements and product specifications before ordering.