Roof Area Calculator
Calculate actual roof surface area from measured rectangular faces, horizontal plan dimensions with a known pitch, or an already-known actual area. Add named faces for a mono-pitch, gable or flat roof.
How the estimate works
Actual rectangular face: A = lengthM × spanM, where spanM is the actual sloping width. Horizontal rectangular projection: A = lengthM × spanM ÷ cos(pitchDegrees × π ÷ 180), where spanM is the horizontal run of that face and 0 ≤ pitchDegrees < 90. Known actual area: A = areaM2. Total actual area = sum of complete, non-overlapping face areas. No intermediate or numerical output rounding; only final display rounding applies.
Area arithmetic only, not a structural, installation, roof-access or safety plan. Rectangular dimensions cover a single constant-pitch face; irregular faces need their independently known actual area. A gable needs an explicit row for each face, not an assumed interpretation of total building span. No automatic overhangs, openings, waste, product quantities, stock, price or permissions. Do not access a roof to obtain measurements for this calculator.
Worked example
A gable roof has two rectangular faces. Enter Front as horizontal plan dimensions: length 8 m, this face’s horizontal run 3 m, pitch 30°. Enter Rear with the same values. Each actual area is 8 × 3 ÷ cos(30°) = 27.712812921102035 m²; their total is 55.42562584220407 m² (55.43 m² at two-decimal final display precision). A separate flat face with actual dimensions 2 m × 3 m adds 6 m². Do not apply pitch to dimensions already measured along the roof surface.
What to measure and check
Start with safely available measurements or reliable drawings for each roof face. Choose its method separately. For actual dimensions, enter the length and actual surface width; for a horizontal plan, enter the length and horizontal run of that one face plus its known pitch. A flat projected face still needs an explicit 0° pitch. Use known actual area for irregular faces or a reliably calculated area. Do not list the same surface twice.
Which dimensions should I enter?
Use safely available measurements or reliable drawings. Actual dimensions need the rectangular face length and its real surface width along the slope. Horizontal plan dimensions need the face length, that face’s horizontal run and an independently known pitch. Do not measure by accessing a roof for this tool.
How do I enter a gable roof?
Add one face row for each slope. Each plan row uses that face’s own horizontal run, not the entire building span. The calculator does not assume equal slopes, divide a span in half or invent a second face. A mono-pitch roof can be a single rectangular face.
Can I mix methods or enter a flat roof?
Yes. Every named face has its own method. A flat rectangular face can use actual dimensions, horizontal plan dimensions with an explicitly entered 0° pitch, or a known actual area. Unknown pitch is missing information, not an assumed zero.
Is pitch applied to an actual area or an actual sloping width?
No. Pitch is used exactly once, only for the horizontal-plan method. Pitch, dimensions or area fields that are inactive for the selected method are ignored, even if they still contain old values.
What about hips, valleys or irregular faces?
Dimension multiplication is for rectangular, constant-pitch faces only. Supply an independently known actual area for triangular, trapezoidal, curved or otherwise irregular faces, or split them into independently established non-overlapping areas. This calculator does not design roof geometry.
Are overhangs and openings included automatically?
No. Include an overhang only if it is explicitly within the supplied face measurements or known area. A known actual area may already account for explicitly measured exclusions belonging to that face. Rectangular dimension rows do not deduct openings. Nothing is added or deducted automatically, and a previously net area is not deducted again.
Does the result specify materials or structural work?
No. It is an actual surface-area basis, not a Materials List, purchasing quantity, cutting plan or installation instruction. Product coverage, overlaps, waste, structural design, roof access, compliance and permissions are outside scope.
What if one face is incomplete?
Completed faces remain visible as a clearly marked partial subtotal. Missing-method, missing-dimension or missing-pitch guidance identifies incomplete faces; a whole-roof total is shown only when every entered nonblank face is complete. Blank unused rows are ignored.
How are the formula and rounding derived?
For a constant-pitch rectangle, horizontal run = actual sloping width × cos(pitch), so actual width = horizontal run ÷ cos(pitch). Multiply that width by the face length, then sum non-overlapping actual areas. Full numeric precision is retained throughout; final display rounding does not change the summed values.
Where the numbers come from
Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.