Roofing
Metal Roofing Calculator
Panels and panel length for a metal roof, from eave length, pitch and panel coverage width.
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About this calculator
Metal panels run from eave to ridge, so the order is a count across the eave and a length along the slope. This calculator counts panels by coverage width — not panel width, so laps are not double-counted — and works out panel length from the pitch multiplier.
What this number means
Metal roofing is ordered by the panel, and panels run from eave to ridge — so the two numbers that matter are how many fit across the eave and how long each one must be. The panel count comes from coverage width, not panel width: the lap that weatherproofs the joint eats an inch or two of every panel, and counting full panel widths leaves you short.
The slope length is where the pitch multiplier earns its keep. A 6/12 pitch stretches a 15 ft horizontal run into 16.77 ft of panel, and that extra length is real material — panels are cut to order, so a short order cannot be stretched and a long one cannot be returned easily.
The formula
- Pitch multiplier = √(1 + (rise ÷ 12)²)
- Slope length = Horizontal run × pitch multiplier
- Panels per plane = ceil(Eave length ÷ coverage width)
- Total panels = Panels per plane × 2 planes × (1 + waste %), rounded up
- Roof area = 2 × eave length × slope length; Squares = roof area ÷ 100
Worked example
A gable roof: 40 ft eave, 15 ft run, 6/12 pitch, 36 in coverage, 10% waste.
- Pitch multiplier: √(1 + 0.5²) = 1.118
- Slope length: 15 × 1.118 = 16.77 ft — order 17 ft panels
- Panels per plane: 40 ÷ 3 = 13.33 → 14 panels
- Total: 14 × 2 = 28, with waste 28 × 1.10 = 30.8 → 31 panels
- Roof area: 2 × 40 × 16.77 = 1,341.6 sq ft (13.42 squares)
Assumptions and limitations
- A symmetric gable roof: two equal planes. For a hip roof, treat the result as a starting estimate and add the hip allowance in the waste figure — hip end planes are trapezoids, not rectangles.
- Panels are counted per plane across the eave; ridge cap, gable trim, valley flashing and fasteners are separate items, not included here.
- Panel length is rounded up to the next whole foot, as suppliers cut to order in whole-foot increments.
- Overhang beyond the eave and ridge is not included — add it to the run before calculating if your panels extend past the fascia.
Frequently asked questions
Coverage width is the exposed width after the side lap overlaps the next panel — for example a 38 in corrugated panel covers 36 in. Counting full panel widths would leave you about one panel short per 20 ft of eave.
The slope length: horizontal run × pitch multiplier, rounded up to the next whole foot. A 15 ft run at 6/12 pitch needs 16.77 ft panels, so order 17 ft. Panels longer than about 40 ft need expansion joints — check the manufacturer guidance.
It depends on the eave length, not just the area — panels are counted across the eave. A 40 ft eave at 36 in coverage needs 14 panels per plane regardless of slope; the slope changes their length, not their count.
Ten percent for a plain gable roof, 15% for a hip roof or one with dormers, valleys and skylights. Cut panels around penetrations rarely reuse elsewhere because each panel is a fixed length.
Only through panel length, not count: the count across the eave is fixed by coverage width. Steeper pitch means longer panels and more total square footage — a 12/12 pitch roof has 41% more surface than its footprint.
Ridge cap, gable and eave trim, valley flashing, underlayment, and screws with sealing washers (roughly 80 screws per square for exposed-fastener panels). This calculator covers panels and area; price the trims separately.