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Posts, beams, rafters, top slats and footing concrete for a DIY wood pergola.
With these inputs:
| Result | Value |
|---|---|
| Concrete for the posts | 12 × 80 lb bags |
| 6x6 posts | 4 × 12 ft (11 ft each, 12 ft apart) |
| Beam boards | 4 × 14 ft (14 ft) |
| Rafters | 10 × 12 ft (12 ft) |
| Top slats | — |
| Concrete volume | 6.9 cu ft (1.73 per hole) |
| Rafters | ceil(12 × 12 ÷ 16) + 1 = 10 |
| Per hole | π × (12 ÷ 24)² × 36 ÷ 12 − post = 1.73 cu ft |
| Bags | ceil(6.9 ÷ 0.6) = 12 |
Bag yields are QUIKRETE Concrete Mix's (80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft). This is a materials list, not a structural design: beam and rafter sizes depend on the spans, your wind and snow loads and any roof you add — check them with your building department, and set posts below the local frost line.
A freestanding wood pergola is posts in concrete, a beam along each side, rafters across the beams and, if you want more shade, slats across the rafters. Enter the footprint, how many posts carry each beam, the post height, overhang and spacing. The calculator gives a cut list with the stock lengths to buy and the bags of concrete for the post holes, so you can price it at the lumber yard.
Posts, two beams (often doubled boards), rafters across the beams, optional slats across the rafters, concrete for the post holes and structural screws or bolts. A 12 × 10 ft pergola with four 6x6 posts, doubled beams and rafters 16 in apart needs 4 posts, 4 beam boards, 10 rafters and about 12 bags of 80 lb concrete.
A 12 in hole 36 in deep holds π × 0.5² × 3 = 2.36 cu ft; a 6x6 post takes up 0.63 cu ft of that, leaving 1.73 cu ft. QUIKRETE's 80 lb bag yields about 0.60 cu ft, so roughly 3 bags a post, or 12 for four posts.
Pergola rafters are commonly set 12 to 24 in apart; closer spacing gives more shade. The calculator counts them for 12, 16 or 24 in — the count is the length divided by the spacing, rounded up, plus one.
Deep enough to reach below your local frost line and to hold the posts against wind. Your building department sets the frost depth; enter it as the hole depth and the calculator sizes the posts and concrete to match.
6x6 posts are stiffer and give more bearing for doubled beams, which is why they're the usual choice for freestanding pergolas; 4x4s suit small or attached structures. Your plan or building department decides for your size and loads.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.