Metal Roofing Calculator
A metal roofing calculator divides roof area by 36 sq ft per 3 × 12 ft panel and rounds up: a 600 sq ft roof section needs 17 panels, or $1,275 at $75 each.
Metal roofing calculator
A metal roofing calculator divides the roof area by the area one panel covers, 36 sq ft for a 3 × 12 ft panel, and rounds up. A 600 sq ft roof section needs 17 panels (16.7 rounded up), and at $75 each that is $1,275. Turn on the 10% waste box and the count rises to 19 panels, $1,425.
Panels are whole items, so the answer is always rounded up before pricing.
The formula in plain words
- Roof area = length × width of the sloped surface.
- Panels = area ÷ 36.
- With waste: panels = area ÷ 36 × 1.10.
- Round up to a whole panel.
- Cost = panels × price per panel.
Worked examples
A 30 × 20 ft roof plane at $75 a panel:
- Area: 30 × 20 = 600 sq ft
- Panels: 600 ÷ 36 = 16.67, rounded up to 17
- Cost: 17 × $75 = $1,275
- With waste: 16.67 × 1.10 = 18.33, rounded up to 19 panels, $1,425
A larger roof, 40 × 24 ft:
- Area: 960 sq ft
- Panels: 960 ÷ 36 = 26.67, rounded up to 27, $2,025
- With waste: 29.33, rounded up to 30 panels, $2,250
A building footprint of 30 × 20 ft with a 6 in 12 pitch needs sloped area:
- Sloped area: 600 × 1.118 = 670.8 sq ft
- Panels: 670.8 ÷ 36 = 18.63, rounded up to 19
Roof pitch changes your area
| Pitch | Area multiplier | 600 sq ft footprint becomes |
|---|---|---|
| 4 in 12 | 1.054 | 632 sq ft |
| 6 in 12 | 1.118 | 671 sq ft |
| 8 in 12 | 1.202 | 721 sq ft |
| 12 in 12 | 1.414 | 849 sq ft |
Use the roof pitch calculator if you do not know your pitch, and the roof area calculator to work out the sloped area from the footprint.
Cross-check by counting panels
The area method works well for a budget. For the order itself, count panels the way an installer does:
- Roof width divided by panel coverage width gives panels across. A 30 ft width at 3 ft coverage gives 10 panels.
- Eave to ridge length decides how long each panel is. A 14 ft slope needs 14 ft panels, which many suppliers cut to order.
- Multiply across by two sides for a gable roof.
If the two methods differ by more than one or two panels, measure again.
Why the count rounds up
You cannot buy two thirds of a panel. With 16.67 panels of coverage, the 17th panel is mostly a cut piece, but it still arrives as a full sheet. The extra length from that last panel often covers some of the waste you would otherwise need, which is why a roof section with a small remainder may not need the waste box at all.
When the decimal lands just above a whole number, such as 17.05, check whether one more panel is really needed or whether your measurement has a small error.
How to measure
- Measure each roof plane separately. Walk the eave for width and measure from eave to ridge for slope length.
- On a steep or high roof, measure from the ground using the building’s footprint and the pitch instead of climbing up.
- Add each plane. Include dormers and porch roofs as separate sections.
- Subtract large skylights and chimneys only if they are bigger than a panel.
Work on roofs carries real fall risk, so use the ground-based method or hire a roofer when in doubt.
Common mistakes
- Using the flat footprint on a steep roof, which can leave you short by a tenth or more.
- Ignoring panel length when the roof is longer than the panel you priced.
- Forgetting trim, ridge cap, closures and fasteners, which are priced separately from panels.
- Leaving out underlayment. The roofing underlayment calculator shows how many rolls the deck needs.
- Mixing panel prices per sheet with prices per square foot. Check which one your supplier quotes.
Related calculators
If you are comparing materials, the roofing shingle calculator gives bundle counts for asphalt shingles. Framing for a metal roof also needs closely spaced purlins, which the purlin spacing calculator counts.
Frequently asked questions
What panel size does this calculator assume?
Each panel is 3 ft wide by 12 ft long, which is 36 sq ft. The calculator divides your roof area by 36 and rounds up to a whole panel. If your panels have a different coverage width or length, scale the answer by the ratio of your panel area to 36.
Do I enter the footprint or the sloped roof size?
Enter the sloped surface. A roof with a 6 in 12 pitch has about 1.118 times the area of its footprint. A 30 × 20 ft footprint is 600 sq ft flat but about 671 sq ft on the slope, which is 19 panels rather than 17.
Are metal roofing panels sold in fixed lengths?
Many suppliers cut panels to length, often in whole feet, from eave to ridge. When your roof run is longer than 12 ft, count panels across the width and order each in the length you need. The area method is a good budget check, and your supplier's takeoff gives the final order.
How much overlap is built into the panel size?
None is added. The calculator treats 36 sq ft as the coverage per panel. Real panels often cover slightly less than their sheet width because of the ribs that overlap, so check the manufacturer's coverage width.
How much waste should I add for a metal roof?
Ten percent covers cut-offs at hips, valleys and the rake. Turn the waste box on for a roof with several planes. A simple gable roof cut to length may need less, because panels are made to size.
Estimates only. Verify quantities with your supplier before purchasing.