Metal Roofing Calculator
Work out how many metal roofing panels a gable roof needs — slope length, panels per slope, and the total with wastage.
Metal roofing comes in long panels that run eave to ridge — so the two numbers that matter are the slope length and the panel count. This calculator works both out from the eave, the span and the pitch.
Order panels at least the slope length, and check the manufacturer's coverage width before ordering.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure the eave length and the rafter run (half the span).
- Enter the pitch and the panel coverage width from the manufacturer.
- Set wastage at 10%.
- Add a price per panel if you want a cost estimate.
Worked example
| Eave length | 40 ft |
|---|---|
| Rafter run | 15 ft |
| Pitch | 6/12 → rise 15 × 6 ÷ 12 = 7.5 ft |
| Slope length | √(15² + 7.5²) = 16.77 ft — order panels at least this long |
| Panels per slope (36 in panels) | ⌈40 ÷ 3⌉ = 14 panels |
| Total (2 slopes, 10% wastage) | ⌈14 × 2 × 1.10⌉ = 31 panels |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How long should metal roofing panels be?
As long as the slope — from eave to ridge, in one piece if possible. This calculator works the slope length out from the rafter run and pitch: 16.77 ft here. Order panels at least that long, plus a small overhang at the eave.
How many metal roofing panels do I need?
Divide the eave length by the panel coverage width, per slope, then double it for a gable roof and add wastage. A 40 ft eave with 36 in panels needs 31 panels for two slopes.
What is panel coverage width?
The width each panel covers once overlapped — usually less than the panel's full width. A 36 in coverage panel may be 38 in wide. Use the coverage figure from the manufacturer, not the full width.
Can I install metal roofing myself?
Metal roofing is a professional job for most people: panels are long, sharp and awkward, and a bad install leaks. The safety notice on this page applies — get a qualified roofer, especially for steep roofs.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.