Enter Deck Details
Example Data Table
| Deck (L × W) | Max spacing | Overhang | Edge offset | Layout | Estimated posts |
|---|---|---|---|---|---|
| 16 ft × 12 ft | 6 ft | 0.5 ft | 1 ft | Grid | 9 |
| 20 ft × 14 ft | 6 ft | 1 ft | 1 ft | Grid | 12 |
| 10 ft × 8 ft | 5 ft | 0.5 ft | 0.5 ft | Perimeter | 8 |
Examples assume a rectangular deck and evenly spaced post lines.
Formula Used
The calculator sizes a support rectangle by subtracting overhang from each side: SupportLength = L − 2×Overhang, SupportWidth = W − 2×Overhang.
It then applies an inward edge offset: EffLength = SupportLength − 2×EdgeOffset, EffWidth = SupportWidth − 2×EdgeOffset.
The number of post lines is computed using your maximum spacing: Intervals = ceil(Eff / Spacing), Lines = max(MinSide, Intervals + 1).
Grid layout uses Total = LinesLength × LinesWidth. Perimeter layout uses Total = 2×LinesLength + 2×LinesWidth − 4.
How to Use This Calculator
- Choose the unit system you will enter.
- Enter deck length and width from your plan.
- Set maximum post spacing based on your design.
- Add overhang if joists cantilever past outer posts.
- Set an edge offset if posts must sit inward.
- Select grid or perimeter layout and calculate.
- Download CSV or PDF for quotes and records.
This tool gives an estimate. Verify spans, loads, frost depth, and local building requirements with a qualified professional.
Why post spacing controls deck performance
Post spacing governs beam spans, joist reactions, and how loads travel into soil. When posts are too far apart, beams deflect more, fasteners work harder, and decking can feel bouncy. A conservative spacing limit also helps when you add planters, pergolas, or wet soil loads near garden edges. Use this calculator to test spacing scenarios quickly, then pick a safer layout.
Converting deck size into a support rectangle
Decks often include cantilevered overhang beyond the outer beam line. The calculator subtracts your overhang allowance from both sides to estimate the support rectangle where posts and footings sit. An optional edge offset moves post lines inward when you need clearance from borders, edging stones, or planting beds. This keeps the layout practical while honoring a maximum spacing rule.
Choosing grid versus perimeter-only layouts
A perimeter-only layout counts posts around the boundary and suits small platforms, stairs landings, and simple framed decks. Grid layout adds interior lines and is common for larger decks or higher live loads. Grid counts rise faster, but they reduce beam spans and distribute loads more evenly. If your deck will hold heavy garden pots or built‑in seating, grid layouts usually feel stiffer.
Reading the spacing achieved and coordinates
The calculator reports “spacing achieved” because evenly dividing the effective rectangle rarely lands exactly on your limit. Actual spacing is usually smaller, which is safer and helps standardize framing. The coordinate preview shows sample post locations measured from one deck corner, accounting for overhang and offsets. Use these values to sketch a footing plan and avoid clashes with drains or tree roots.
Field checks before you dig and pour
Before construction, confirm frost depth, soil bearing, drainage, and buried utilities. Validate beam sizing, joist span tables, and ledger attachment if the deck connects to a structure. On sloped sites, step footings or use adjustable post bases to keep beams level. Follow code requirements and manufacturer instructions for connectors, preservative treatments, and concrete curing.
FAQs
1) Does this replace a structural design?
No. It estimates post quantities from spacing rules. Confirm beam sizes, joist spans, connections, and live loads using code tables or an engineer, especially for elevated decks or heavy garden features.
2) What spacing value should I enter?
Start with the maximum spacing allowed by your framing plan or local guidance. Many residential decks target 6 ft or less between posts, but beam size, species, and load assumptions can require tighter spacing.
3) What does overhang mean here?
Overhang represents cantilever beyond the outer post line. If your joists or beams extend past the last support, enter that distance so the calculator places posts under the remaining support rectangle.
4) When should I use perimeter-only mode?
Use it for small platforms where beams run around the edge and interior supports are unnecessary. For larger decks, tall decks, or heavy planters, grid layouts usually reduce deflection and feel more rigid.
5) Why is “spacing achieved” smaller than my limit?
Because the calculator divides the effective span into equal intervals. To keep spacing within your maximum, it adds enough post lines, which often produces a slightly smaller, safer spacing.
6) Can I use this for non-rectangular decks?
You can approximate by splitting the deck into rectangles, calculating each area, and combining plans. For curves or complex shapes, lay out primary beams first, then use the calculator to size supports for each zone.