Metal Roof Panel Calculator: Panels per Slope & Total
Enter the eave length of one slope, the panel coverage width and the number of slopes to get the panels per slope and the total panels. Panels run from eave to ridge, so the panel length is usually your rafter length.
Calculator
An eave of 40 ft with 36 in coverage needs 14 panels/slope (28 for 2 slopes), each 16.77 ft long.
Standing-seam and exposed-fastener metal roofing comes in long panels that run the full slope from eave to ridge. Because a panel spans top to bottom in one piece, the number of panels depends on how wide each one covers and how wide the slope is along the eave — not on the roof area directly.
The key number is the net coverage width: the finished width one panel covers after the sidelap or seam that joins it to the next. A common exposed-fastener panel nets 36 inches (3 feet), but standing-seam profiles vary from about 12 to 18 inches, so always use the coverage width from your profile, not the raw panel width. The panel length is normally your rafter length; find it with the rafter length calculator if you do not have it yet.
Formula
Panels per slope is the eave width over the coverage width; total multiplies by the slope count:
panels_per_slope = ⌈ eave_ft × 12 ÷ coverage_in ⌉ total_panels = panels_per_slope × slopes
- eave_ft × 12 — the slope width converted to inches.
- coverage_in — net width one panel covers (≈ 36 in for a 3 ft panel).
- ⌈ ⌉ — round up: a partial run still needs a full panel, trimmed.
Worked example
Cover a gable roof: 40 ft eave per slope, 36 in coverage, 2 slopes, panels 16.77 ft long:
- Per slope: 40 × 12 ÷ 36 = 13.33, rounded up to 14 panels.
- Total: 14 × 2 = 28 panels.
- Panel area: 28 × 16.77 ft × 3 ft = about 1,409 sq ft of panel.
Twenty-eight 16.77-foot panels cover both slopes. Panels are usually cut or ordered to the exact slope length, so confirm your rafter length before you place a factory order — most metal is made to size.
Metal panel notes
Metal-roofing quantities in practice:
- Coverage width, not panel width. Always size from the net coverage after the seam or sidelap. Using the raw panel width under-counts the panels you need.
- Panels are cut to length. Because each panel runs eave-to-ridge in one piece, most metal is ordered to the exact slope length. That eliminates horizontal end-laps but makes the rafter length critical.
- Trim and accessories are separate. Ridge cap, hip flashing, eave and rake trim, closures and fasteners are ordered on their own — this tool counts field panels only.
- Complex roofs cost more. Hips, valleys and dormers force diagonal cuts that waste panel material; a simple gable is the most efficient shape. Add extra for cut-up roofs.
Reference table
Panels per slope by eave length at 36 in coverage (total for 2 slopes):
| Eave length | Per slope | Total |
|---|---|---|
| 20 ft | 7 | 14 |
| 30 ft | 10 | 20 |
| 40 ft | 14 | 28 |
| 50 ft | 17 | 34 |
| 60 ft | 20 | 40 |
Frequently asked questions
How many metal roofing panels do I need?
Divide the eave width of a slope (in inches) by the panel coverage width, round up, then multiply by the number of slopes. A 40 ft eave with 36 in panels is 40 × 12 ÷ 36 = 13.33 → 14 panels per slope, or 28 panels for a two-slope gable roof.
What is panel coverage width?
It is the finished width one panel covers after the sidelap or seam that joins it to the next panel — always less than the raw sheet width. Exposed-fastener panels often net 36 in (3 ft); standing-seam profiles range from about 12 to 18 in. Use the coverage width from your specific profile.
How long should metal roof panels be?
Usually the rafter length — the eave-to-ridge distance along the slope — so each panel runs in one continuous piece with no horizontal seam. Find it with the rafter length calculator, then order panels cut to that length.
Do I count trim and ridge cap as panels?
No. This calculator counts field panels only. Ridge cap, hip and valley flashing, eave and rake trim, closures and fasteners are separate line items sized by linear foot — add them to your order after the panel count.
How much waste should I expect on a metal roof?
A simple gable roof wastes very little because panels run straight up the slope. Hips, valleys and dormers force diagonal cuts that waste more material, so add extra panels for a cut-up roof and confirm the exact cut list with your supplier.