Calculators · Metal Roofing Calculator

Roofing

Metal Roofing Calculator

Enter each roof plane and a panel coverage width to get the number of metal panels, the length of each and the roof area.

Plane 1

Width along the eave

Length up the slope

Pitch (optional)

Only if you measured the horizontal run. Type the rise, like 4 for 4/12.

Identical planes

Panel coverage width

The width one panel covers once installed. Check the data sheet of your panel.

Eave overhang (optional)

Added to the length of every panel. Empty means 0.

Spare panels (%)

Optional. Empty means 0.

How to calculate how many metal roofing panels you need

Metal panels run from the eave to the ridge and sit side by side, so the count depends on how wide the roof is and how wide one panel is once installed. That installed width is called the coverage. It is smaller than the width of the sheet, because the side lap is taken out.

Step	Formula
1. Panels across one plane	plane width (in) ÷ coverage (in), rounded up
2. Length of each panel	slope length + eave overhang
3. Total panels	panels across × number of identical planes
4. Panels with spares	total × (1 + spare %), rounded up
5. Roof area	plane width × slope length, added for every plane

If you measured the horizontal run of the roof instead of the slope, type the pitch in the plane row. The calculator multiplies the run by the pitch factor, which is the square root of 1 plus (rise ÷ 12) squared. Use the roof pitch calculator to find a pitch from a rise and a run, and the roofing calculator to find the roof area from a footprint. Measurements can be typed as 13'6" or 13 ft 6 in.

Example: a gable roof with 36 inch panels

A gable roof has two equal planes. Each one is 40 ft wide along the eave and 13 ft long up the slope, with 1 ft of overhang at the eave. The panel has a 36 inch coverage width and you want 10% spares.

Step	Operation	Result
Panels across one plane	40 × 12 ÷ 36 = 13.33, rounded up	14
Length of each panel	13 + 1	14 ft
Total panels	14 × 2 planes	28
Total panel length	28 × 14	392 linear ft
Roof area	2 × (40 × 13)	1,040 sq ft (10.4 squares)
With 10% spares	28 × 1.10 = 30.8, rounded up	31 panels

The last panel of each row is cut to fit, so part of it is waste. If you can plan the width of the roof around the coverage, you waste less. For instance, a 36 ft wide plane takes exactly 12 panels of 36 inch coverage.

Coverage width by panel

These widths come from the manufacturers’ own data sheets, which we read in October 2026. The calculator uses the coverage figure, never the overall width of the sheet.

Panel	Type	Coverage	Lengths and slope in the sheet	Source
Metal Sales Classic Rib	Exposed fastener rib	36 in	5 ft to 45 ft, minimum slope 3:12	Data sheet
Fabral Grandrib 3	Exposed fastener rib	36 in	1 ft to 40 ft, recommended pitch 2½:12 and above	Data sheet
Western States 7/8 in corrugated	Exposed fastener	34.67 in	1 ft to 52 ft, recommended minimum slope 3/12	Data sheet
Western States Western Lock	Snap-lock standing seam	18 in standard, 16, 14 or 12 in	1 ft to 52 ft, recommended minimum slope 3/12	Data sheet
Metal Sales Magna-Loc	Mechanically seamed standing seam	16 in or 18 in	Minimum slope 1/2:12	Specification data

The Fabral sheet says the lengths, profile configuration and colors can vary by region. Other manufacturers sell panels with other coverage widths. If yours is not on the list, choose “Custom coverage width” and type the figure from your supplier. We could not verify the coverage of other manufacturers, so the list does not include them.

Panels needed by plane width and coverage

Plane width	36 in	34.67 in	18 in	16 in	12 in
10 ft	4	4	7	8	10
20 ft	7	7	14	15	20
30 ft	10	11	20	23	30
40 ft	14	14	27	30	40

Panels across one plane, rounded up. Multiply by the number of identical planes.

Tips for measuring

Measure the length of the slope from the drip edge to the ridge, or measure the horizontal run and type the pitch.
Check the maximum panel length in the data sheet. If your slope is longer, ask your supplier how to handle it, because that case is not covered here.
Add one plane for every roof section. A shed roof is one plane, a gable roof is two, and a section with another pitch is its own row.
Order a little longer rather than a little short. Mid-Michigan Metal Sales says it is more cost-effective to add a few inches than to repurchase new panels if they fall short (checked October 2026).

What this calculator does not do

It counts panels, panel lengths and roof area from the numbers you type. It does not give prices. It does not count ridge cap, eave and rake trim, closures, fasteners, underlayment or sealant, because the amount depends on the profile and on the manufacturer’s system. Ask your supplier for the trim list of your panel.

Frequently asked questions

How do I calculate how much metal roofing I need?

Divide the width of each roof plane by the coverage width of the panel and round up. That is the number of panels across. Each panel runs from the eave to the ridge, so its length is the length of the slope plus any overhang at the eave. A 40 ft wide plane with 36 inch coverage needs 480 ÷ 36 = 13.33, so 14 panels.

What is the coverage width of a metal roofing panel?

Coverage is the width one panel covers once it is installed, after the side lap is taken out. It is smaller than the width of the sheet. The data sheets we checked in October 2026 give 36 inches for Metal Sales Classic Rib and Fabral Grandrib 3, 34.67 inches for a 7/8 inch corrugated panel from Western States, 18 inches as the standard width of a Western Lock standing seam panel and 16 or 18 inches for Metal Sales Magna-Loc.

How do I calculate a metal roof for a pole barn?

A simple gable pole barn is two equal planes. Enter the length of the building as the width of the plane, enter the length of the slope, and type 2 in the box for identical planes. If you only know the horizontal run, type the pitch and the calculator finds the slope length.

How long should the panels be?

Measure from the drip edge up to the ridge on the slope and add the overhang you want at the eave. Mid-Michigan Metal Sales notes that it is more cost-effective to add a few inches than to buy new panels when they fall short. Panels also have a maximum length that depends on the product, so check the data sheet of yours.

How many squares of metal roofing do I need?

A roofing square is 100 square feet of roof. The calculator adds the area of every plane measured on the slope and shows it in squares. For example, two planes of 40 ft by 13 ft make 1,040 square feet, which is 10.4 squares.

How much extra should I order?

It depends on the roof, so ask your supplier. This calculator already rounds the panels across each plane up, and the spare box adds a percentage on top of the total count. Leave it empty to add nothing.

How much does a metal roof cost?

Prices change by region, panel profile, material and finish, so this calculator does not show them. Take the number of panels and the length of each panel from the result and ask suppliers to quote the same panels.