Results
| Posts along length | 3 |
| Posts along width | 3 |
| Total posts | 8 |
| Length spacing | 6.00 ft |
| Width spacing | 5.00 ft |
| Embed depth | 38.0 in |
| Concrete per hole | 3.25 cu ft |
| Total concrete | 28.64 cu ft |
| Bags needed | 48 |
Project inputs
Example data table
| Example inputs | Value | Example outputs | Value |
|---|---|---|---|
| Pergola size (L × W) | 12 ft × 10 ft | Total posts (perimeter) | 10 |
| Max spacing | 6 ft | Actual spacing (length/width) | 6.00 ft / 5.00 ft |
| Height above ground | 8 ft | Embed depth (rule + extra) | 38.0 in |
| Hole diameter, gravel | 14 in, 6 in | Concrete per hole (net) | ≈ 3.50 cu ft |
| Waste factor, bag size | 10%, 80 lb | Bags needed (estimate) | ≈ 65 |
Formula used
- Posts along a side = ceil(side length ÷ max spacing) + 1
- Perimeter total = 2·postsL + 2·postsW − 4
- Grid total = postsL · postsW
- Actual spacing = side length ÷ (posts − 1)
- Embed depth = max(frost depth, height ÷ 3) + extra
- Hole volume = π · (radius²) · depth
- Net concrete = hole volume − post displacement
- Total concrete = net per hole · posts · (1 + waste)
How to use this calculator
- Enter the pergola length, width, and post height.
- Set the maximum spacing you want between posts.
- Choose a layout: perimeter only, or a full grid.
- Provide footing inputs (frost depth, hole diameter, gravel base).
- Click Calculate to view results above.
- Use Download CSV or Download PDF for records.
Project guidance
Post layout planning
A pergola behaves like a simple frame: posts carry vertical weight and resist racking from wind. This calculator sizes the post layout from your length, width, and maximum spacing. For perimeter builds, it places posts along each edge and counts corners once. For grid builds, it adds interior posts at every intersection, which can reduce beam spans and improve stiffness for shade panels, vines, or heavier roofing.
Spacing and structural rhythm
Spacing affects both appearance and performance. For most gardens, spacing between 5 and 8 feet balances cost, sightlines, and beam deflection under load. When you enter a maximum spacing, the tool uses a ceiling step so real spacing never exceeds your limit. The actual spacing is distributed evenly: side length divided by posts minus one. This prevents “one short bay” that looks odd and concentrates load. If you prefer a specific module, adjust maximum spacing until the reported spacing matches your target.
Footing depth and drainage
Embed depth is estimated with a practical rule: the greater of frost depth or one‑third of above‑ground height, plus a safety extra. You can override it with a custom value when local practice requires deeper holes, expansive clay needs more bearing, or tall, slender posts demand extra resistance. A gravel base adds drainage and helps keep post bottoms from sitting in water.
Concrete and material takeoff
Concrete volume is calculated from a cylindrical hole (π·r²·depth) and then reduced by the embedded post’s displacement. A waste factor accounts for irregular holes, spillage, and over‑excavation. Bag counts are estimated from typical yields: about 0.60 cubic feet for an 80‑lb bag and 0.45 cubic feet for a 60‑lb bag. Always compare against your chosen product label.
Export-ready documentation
After calculating, export a CSV for quick purchasing and a clean PDF for site notes. Save the spacing, embed depth, hole size, and bag count with your project sketch. These outputs support repeatable builds across multiple pergolas, help compare perimeter versus grid options, and reduce change orders by making assumptions visible before you dig.
FAQs
1) How does the calculator choose the number of posts?
It divides each side by your maximum spacing, rounds up, then adds one for the end post. Perimeter totals avoid double-counting corners; grid totals multiply posts along length and width.
2) Should I use perimeter or full grid layout?
Perimeter suits light, open pergolas. A full grid adds interior support for longer spans, stronger wind resistance, and heavier features like dense shade slats, canopies, or mature vines.
3) What embed depth should I enter if codes differ?
Turn on custom embed depth and enter the value required locally. Use deeper holes for frost, tall posts, soft soil, or higher wind exposure, even if the rule-of-thumb seems adequate.
4) Why subtract post displacement from concrete volume?
The post occupies space inside the hole, so less concrete is needed to fill the remaining void. This improves cost estimates, especially with larger posts or deeper embed depths.
5) Are the bag yields exact?
They are typical field yields and vary by mix design and water content. For accuracy, check your product’s stated yield and increase the waste factor if your holes are rough or oversized.
6) Can I plan for perfectly even spacing?
Yes. The tool reports the evenly distributed spacing it creates. If you want a clean module like 5 ft or 2 m, adjust maximum spacing until the actual spacing matches closely.