Carport Calculator: 9 Panels & 6 Posts for 20x12 ft

What does carport needed 9 panels and 6 posts mean?

It means nine roof panels and six posts roof a 20 ft by 12 ft carport, with room for cuts. The roof itself is 240 square feet, since 20 times 12 is 240; one 4 ft by 8 ft panel covers 32 square feet, so the exact need is 7.5 panels, and with 10% waste for cut ends and overlaps that becomes 8.25, rounded up to 9 whole panels because panels are sold by the panel, not by the fraction. The posts stand along the two long sides: at 10 ft spacing a 20 ft side needs three post positions, counting both ends, and two sides make six posts.

Output · formula · meaning

Three things to check before you trust the number

First, check the panel size you are actually buying: 4 ft by 8 ft is the common panel, but a smaller panel covers less roof each and raises this carport past 9 panels, while the roof area never changes. Second, remember the posts follow the length and the spacing, not the roof area: a wider carport needs more panels but no more posts, and a longer carport needs both. Third, judge the waste honestly: a plain flat roof fits 10%, but a roof cut around beams and edges can want 15%, since every cut takes panel. Beams, footings and the slab are bought separately — see the concrete driveway calculator for the slab under a carport like this one.

Worked example

20 ft long, 12 ft wide, 10 ft post spacing, 10% waste. Roof area: 20 × 12 = 240 sq ft. Panels exact: 240 ÷ 32 = 7.5. With waste: 7.5 × 1.10 = 8.25, rounded up to 9 panels. Posts: 20 ÷ 10 = 2 bays, plus the end post, is 3 positions a side, times 2 sides is 6 posts. In practice that means buying 9 panels and setting 6 posts, and expecting the spare panel to go on cut ends and overlaps. 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.

Every branch, checked

With a 24 ft long carport instead of 20 ft, the roof area rises to 288 sq ft and the need to 10 panels and 8 posts. With a 14 ft wide carport, the roof area rises to 280 sq ft and the need to 10 panels, but the posts stay at 6, since the length and spacing never moved. With 8 ft post spacing, the panels stay at 9 but the posts rise to 8, since a 20 ft side at 8 ft spacing needs 4 positions a side. With 5% waste, the exact need stays 7.5 panels but the buy falls to 8 panels, since 7.5 × 1.05 is 7.88 and still rounds up to 8. Length, width, spacing and waste each move the order on their own, so a wrong spacing figure is as costly as a wrong measurement.

The reverses, worked backwards

The length behind 9 panels is 20 ft: at 18 ft the need is 8 panels and 6 posts (18 ft × 12 ft carport, 10 ft post spacing, 216 sq ft roof, 6.75 panels exact, 10% waste); at 24 ft it is 10 panels and 8 posts (24 ft × 12 ft carport, 10 ft post spacing, 288 sq ft roof, 9 panels exact, 10% waste). The width behind 9 panels is 12 ft: at 10 ft the need is 7 panels and 6 posts (20 ft × 10 ft carport, 10 ft post spacing, 200 sq ft roof, 6.25 panels exact, 10% waste); at 14 ft it is 10 panels and 6 posts (20 ft × 14 ft carport, 10 ft post spacing, 280 sq ft roof, 8.75 panels exact, 10% waste). The post spacing behind 9 panels is 10 ft: at 8 ft the need is 9 panels and 8 posts (20 ft × 12 ft carport, 8 ft post spacing, 240 sq ft roof, 7.5 panels exact, 10% waste); at 12 ft it is 9 panels and 6 posts (20 ft × 12 ft carport, 12 ft post spacing, 240 sq ft roof, 7.5 panels exact, 10% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension, spacing or waste change.

What the defaults assume

The defaults assume a 20 ft by 12 ft rectangular carport, a flat roof area, 4 ft by 8 ft panels, 10 ft post spacing with posts on the two long sides only, and 10% waste for cuts and overlaps. They do not assume beams, footings, a slab or a sloped roof — a sloped roof has more area than its footprint, and its panels are counted off the slope, not off the ground.

Who uses this

• A homeowner roofing a carport: the 9-panel, 6-post order settles the yard call before the first panel is cut.
• A DIY builder framing a carport: the same maths on the carport alone gives the panels and the posts, not a whole garage.
• A contractor quoting a job: carport size, post spacing, waste — the quote follows the same steps, and the waste line shows the client why.
• A landlord budgeting a carport refit: the carport size off the plans, multiplied out, gives the panel and post lines on the refit budget.

Accuracy & limitations

The arithmetic is exact for the inputs entered; the judgement is the post spacing and the waste. Real carports have beams, slopes and edge trim, and the panel on the pallet overrides any planning figure; a carport measured gross over-orders, and a waste figure read off a plain roof under-orders one cut around beams. Treat 9 panels and 6 posts as the planning order for a 20 ft by 12 ft carport at 10 ft spacing and 10% waste, and allow 15% for a roof cut around beams and edges.

Cite this calculator

Bright Side Kit, “Carport Calculator: 9 Panels & 6 Posts for 20x12 ft”, https://brightoolbox.com/carport-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (20 ft × 12 ft carport, 10 ft post spacing, 10% waste) so the 9-panel, 6-post result can be checked.

Frequently asked questions