Metal roofing is ordered panel by panel, cut to length, and each panel covers a fixed width of roof. That makes the quantity easy to estimate if you know three numbers: how long the roof is along the eave, how long the slope is from eave to ridge, and the coverage width of the panel you are buying. This calculator also works out the ridge cap pieces and shows the total area the panels cover.
How to use the metal roofing calculator
- Enter the roof length along the eave.
- Enter the slope length from the eave to the ridge. If you only have the horizontal run and the pitch, work it out with the Rafter Length Calculator.
- Enter the panel coverage width and the overhang at the eave.
- Select how many roof planes are being covered, and add any extra panels you want to keep.
Metal roofing formula
Panel length = slope length + eave overhang + drip/ridge allowance
Total panels = panels per plane × number of planes
Ridge cap pieces = ⌈ ridge length ÷ (piece length − overlap) ⌉
Panels run vertically from the ridge to the eave, so each one has the full slope length. The last panel in each row is rarely a whole panel wide, but you still have to buy it and trim it, which is why the panel count is rounded up.
Worked example: 30 ft by 13.4 ft slope, gable roof, 36 in coverage
- Panels per plane: 360 in ÷ 36 in = 10. Two planes: 20 panels.
- Panel length: 13.4 ft slope + 1 ft overhang + 0.25 ft allowance = 14.65 ft.
- Total length: 20 × 14.65 = 293 linear ft, covering 879 sq ft of roof.
- Ridge cap with 10 ft pieces overlapping 6 in: 30 ÷ 9.5 = 3.2 → 4 pieces.
Panels for common gable roofs (36 in coverage, 1 ft overhang)
| Roof (eave × slope) | Panels | Panel length (ft) | Area covered (sq ft) | Ridge cap pieces |
|---|---|---|---|---|
| 20 × 10 ft | 14 | 11.25 | 472 | 3 |
| 30 × 12 ft | 20 | 13.25 | 795 | 4 |
| 36 × 14 ft | 24 | 15.25 | 1098 | 4 |
| 40 × 15 ft | 28 | 16.25 | 1365 | 5 |
| 50 × 16 ft | 34 | 17.25 | 1760 | 6 |
Panel types
- Corrugated and R-panel (exposed fastener): typical coverage widths are 36 inches; screws are driven through the flat of the panel.
- Standing seam (concealed fastener): narrower panels, often 12 to 18 inches, with clips holding them to the deck; roughly double the panel count for the same roof.
- Metal shingles and tiles: sold by the square (100 sq ft), not by panel; use the area from the Roofing Calculator.
Other things to order
You will also need underlayment (and ice and water membrane at the eaves; see the Ice and Water Shield Calculator), drip edge, rake trim, ridge vent or closure strips, inside and outside closures, screws with sealing washers, and butyl tape for laps. Valleys, hips, dormers and chimneys add custom flashings. Measure carefully, because panels cut to length are usually not returnable.
For roof geometry use the Roof Pitch Calculator. Calculate the deck beneath with the Roof Sheathing Calculator.
Frequently asked questions
How many metal roofing panels do I need?
Divide the roof length along the eave by the panel’s coverage width and round up; do that for each roof plane. A 30 ft eave with 36 inch coverage panels needs 10 panels per side.
What is the difference between panel width and coverage width?
A 3-foot corrugated or R-panel covers about 36 inches after the panels overlap, while its overall width is slightly larger. Standing seam panels list widths such as 12 to 24 inches. Always use the coverage width.
How long should panels be?
Panels usually run from the eave to the ridge in one piece. Add an overhang beyond the fascia, allowance for the drip edge and a little at the ridge. Panels can be ordered cut to length, so measure the slope carefully.
How much ridge cap do I need?
Divide the ridge length by the effective length of one piece (the piece length minus the overlap). With 10 ft pieces and a 6 inch overlap, each piece covers 9.5 ft.
What about screws, trim and underlayment?
These are bought separately: screws with neoprene washers (the manufacturer gives a count per square), drip edge, gable trim, closure strips and underlayment. The sheathing and underlayment calculators cover the deck.