How to Calculate a Carport Size in Five Simple Steps

Carport size in one line

Carport size is simple: roof area, divided by the area of one panel, plus waste, rounded up — and posts counted off the length and the spacing, not off the area. A 20 ft by 12 ft carport at 10 ft post spacing has 240 square feet of roof, one 4 ft by 8 ft panel covers 32 square feet, so the carport needs 7.5 panels exact, and with 10% waste for cuts that becomes 9 panels, standing on 6 posts. The rest of this guide is about getting those inputs honest: the length, the width, the spacing, and the waste.

Step 1: Measure the carport, length and width

Measure the carport in feet, length and width, on the ground itself. The worked example is 20 ft long and 12 ft wide, a common single-car carport, and the length sets the posts while both dimensions set the roof. If the carport is not a rectangle, measure each side separately and work out the roof area piece by piece: an L-shaped carport is several rectangles, and the panels follow the total roof area, not the longest side.

Step 2: Multiply length by width for the roof area

Multiply the length by the width for the roof area, since panels cover the roof, not the ground the car sits on. The example is 20 times 12, or 240 square feet. Do not skip this step: a carport measured by its posts alone has no area at all, and panels bought for the posts leave most of the roof bare. Width matters as much as length here — a foot wider on the same length is another 20 square feet of roof.

Step 3: Count the posts off the length and the spacing

Count the posts off the length and the spacing, not off the roof area. Divide the length by the spacing and round up for the bays, add one for the end post, and double for the two sides. The example is 20 divided by 10, or 2 bays, plus the end post, or 3 positions a side, times 2 sides, or 6 posts. Width never enters this step: a wider carport on the same length needs the same posts, only more panels.

Step 4: Divide the roof by the panel, add waste, round up

Divide the roof area by the area of one panel for the exact count, add the waste for cuts and overlaps, then round up to whole panels, since panels are sold by the panel. Ten percent on the example is 7.5 times 1.10, or 8.25, rounded up to 9 panels, and the spare panel is not wasted — it is the panel the cut ends and overlaps consume. Round up at the end, not at the start: rounding the roof area first and then dividing double-counts the rounding and over-orders.

Step 5: Buy, set and check the carport

Buy the panels and set the posts — 9 panels and 6 posts for the example — and check the carport before roofing it: every post plumb, the first row laid level. Then count what panels do not cover: beams, footings, the slab and the trim. For the slab under a carport like this one, the concrete driveway calculator counts the concrete, so the panels, the posts and the slab are ordered together, and the roof is not bought for a carport that was never footed.

Carport size in numbers

Suppose the carport is 20 ft long and 12 ft wide, the post spacing is 10 ft, the panels are 4 ft by 8 ft, and the waste is 10%. Roof area: 20 × 12 = 240 sq ft. Panels exact: 240 ÷ 32 = 7.5. Panels with waste: 7.5 × 1.10 = 8.25. Panels to buy: 9, rounded up. Posts: 20 ÷ 10 = 2 bays, plus 1, times 2 sides = 6 posts. Order: 9 panels and 6 posts.

Common mistakes

The most common mistake is counting the posts off the roof area. The second is forgetting the waste, which buys for a perfect roof and runs short at the first cut end. The third is measuring only the longest side and forgetting the width, which buys for a strip instead of a roof. The fourth is rounding up before adding waste, which double-counts the rounding and over-orders. The fifth is forgetting the beams and footings, which roofs a carport and leaves it standing on nothing.

Worked example, start to finish

A single-car carport, 20 ft long and 12 ft wide, 10 ft post spacing, 4 ft by 8 ft panels, 10% waste. Roof area: 240 sq ft. Panels exact: 7.5. Panels with waste: 8.25. Panels to buy: 9. Posts: 6. Result: buy 9 panels and set 6 posts, and buy beams, footings and a slab separately. If the same carport were 24 ft long instead, the need would rise to 10 panels and 8 posts, but the width would be unchanged.

What changes the answer most

The length changes the answer most, because it moves both the panels and the posts: moving from 20 ft to 24 ft lifts the order from 9 panels and 6 posts to 10 panels and 8 posts, while the width never changes. The width is second: every foot of width at 20 ft long is another 20 square feet of roof, and a wrong width rebuilds the panel order, though never the posts. The spacing is third, and it changes only the posts, never the panels — moving from 10 ft to 8 ft spacing still buys 9 panels, but sets 8 posts. The waste is fourth, and it changes only the top of the panel order, never the roof itself.

Frequently asked questions