Roof Area / Square Footage Calculator
Calculate roof surface area from dimensions.
Roof sections
Add one rectangle for each roof plane. Enter its top-down dimensions, pitch, and any overhang.
Section 1
Use non-overlapping rectangular plan sections. For connected roof planes, include each shared area once and set overhangs on shared edges to 0. See the Roof Pitch Calculator if you need to find a pitch.
Combined roof area estimate
Complete each roof section
Enter a positive plan length, width, and pitch for every section. The combined estimate appears when all sections are valid.
Geometry estimate only. It does not include material waste, roof penetrations, flashing, or installation allowances. Confirm dimensions and material coverage before ordering.
Estimate roof area from rectangular plan sections
Enter the horizontal length, width, pitch, and optional overhang for each rectangular roof section. The calculator works out each section separately and adds the sloped areas. U.S. customary units are selected by default; metric units are also available. The calculation runs in your browser, and entered measurements are not sent to a server for processing.
How to use the calculator
- Choose units: Start with U.S. customary (feet and inches) or switch to metric (metres and centimetres).
- Add one section per plan rectangle: Enter its horizontal length and width. Use sections that do not overlap in plan view.
- Enter each section's pitch: Choose the x:12 rise/run ratio, angle, or slope percentage. Sections can use different pitches.
- Include horizontal overhang if needed: Use one amount for all four edges or enter each edge separately. Set a shared/interior edge to 0 to avoid counting it twice.
- Review the combined estimate: The result is available after every section has valid dimensions and pitch. Download options are PDF and PNG.
To calculate an unknown pitch, use the Roof Pitch Calculator.
What the results mean
- Plan footprint: The sum of the entered horizontal rectangles before overhang.
- Adjusted plan area: The plan rectangles after the entered edge overhangs are added.
- Estimated sloped roof area: Each adjusted section area is multiplied by its own slope factor, then the section results are summed.
- U.S. roofing squares: A U.S. roofing square is 100 square feet. Metric area is converted to square feet for this separate reference.
Formula and assumptions
For each rectangular section i, the calculator uses horizontal plan measurements and a single pitch across that section:
Adjusted dimensions
adjusted_length_i = length_i + length_start_overhang_i + length_end_overhang_iadjusted_width_i = width_i + width_start_overhang_i + width_end_overhang_iadjusted_plan_area_i = adjusted_length_i × adjusted_width_iPitch slope and factor
x:12 ratio (rise ÷ run): slope = x ÷ 12angle: slope = tan(angle)percentage: slope = percentage ÷ 100slope_factor_i = √(1 + slope_i²)Section and combined area
section_area_i = adjusted_plan_area_i × slope_factor_itotal_estimated_area = Σ section_area_iU.S. roofing_squares = total_estimated_area_in_sq_ft ÷ 100Overhang distances are horizontal plan distances. For the all-edge option, the same overhang is added to both ends of each dimension. For individual edges, each entered amount is applied to its corresponding dimension. Calculations keep full precision internally and round only displayed area to the nearest whole sq ft or m²; U.S. roofing squares are displayed to two decimal places.
Worked examples
One rectangular section
A 40 ft × 30 ft plan rectangle with a 6:12 pitch and no overhang:
- Plan area: 40 × 30 = 1,200 sq ft
- Slope factor: √(1 + (6 ÷ 12)²) ≈ 1.11803
- Estimated area: 1,200 × 1.11803 ≈ 1,342 sq ft
- U.S. roofing squares: 1,341.64 ÷ 100 ≈ 13.42
Two sections with different pitches
A 40 ft × 30 ft section at 6:12 plus a separate 20 ft × 20 ft section at 4:12, with no overhang:
- Section 1: 1,200 × √(1 + (6 ÷ 12)²) ≈ 1,341.64 sq ft
- Section 2: 400 × √(1 + (4 ÷ 12)²) ≈ 421.64 sq ft
- Combined estimate: 1,341.64 + 421.64 ≈ 1,763.28 sq ft
- U.S. roofing squares: 1,763.28 ÷ 100 ≈ 17.63
Limitations
- Rectangular plan approximation: Break an irregular plan into non-overlapping rectangles. This does not model curved surfaces, polygon outlines, intersections, valleys, dormers, or detailed roof-plane geometry.
- One pitch per section: Give a section its own pitch when pitches differ. The calculator does not average pitches.
- Shared boundaries: Enter each horizontal plan area once. Put 0 on shared/interior overhang edges so adjoining sections do not count the same extension twice.
- Area only: This estimate does not include material waste, shingle or panel quantities, flashing, penetrations, starter courses, ridge products, labor, or structural design.
Verify dimensions, pitch, overhang, and project geometry before ordering materials. For a separate shingle quantity estimate, use theRoofing Material Calculator.