Calculator
Example data table
| Scenario | Length | Width | Layout | Max Spacing | Cantilever | Estimated Posts |
|---|---|---|---|---|---|---|
| Small garden deck | 12 ft | 10 ft | Grid | 6 ft | 0 ft | 6–9 |
| Raised planter platform | 4 m | 3 m | Perimeter-only | 1.5 m | 0.2 m | 10–14 |
| Wide entertaining deck | 20 ft | 16 ft | Grid | 5 ft | 1 ft | 20–30 |
Examples are illustrative. Your loads and soil conditions may change post requirements.
Formula used
- SupportedLength = Length − 2 × Cantilever and same for width.
- Spaces = ceil(SupportedLength / MaxSpacing) gives required spans.
- PostsAlong = Spaces + 1 ensures both ends are supported.
- ActualSpacing = SupportedLength / (PostsAlong − 1) fits evenly.
- Grid total posts: PostsAlongLength × PostsAlongWidth.
- Perimeter-only posts: edges plus corners counted once.
- Footing volume: V = π × (d/2)² × depth, multiplied by total posts.
How to use this calculator
- Select units that match your tape measure and plan.
- Enter deck length and width from your layout sketch.
- Set maximum spacing based on beam and joist spans.
- Add a cantilever offset if posts sit inside edges.
- Choose grid for stronger support on larger decks.
- Enable footing estimates if you want concrete planning.
- Press Calculate to see counts and coordinates above.
- Download CSV for staking, or PDF for quick sharing.
Support lines and post roles
Deck framing works best when loads travel in a predictable line from surface boards to joists, then to beams, and finally into posts and footings. This calculator assigns each point as Perimeter or Interior, helping you decide where beams should run and where bracing may be needed. Increasing post count reduces tributary area per post, which generally improves stiffness and reduces bounce.
Spacing targets you can control
The maximum spacing inputs create a clean grid by converting the supported span into equal intervals. For example, a supported length of 15 ft with a 6 ft limit produces 4 spaces and 5 posts along that direction, with an actual spacing of 3.75 ft. This approach ensures no bay exceeds your target and helps keep beam segments consistent for material takeoffs and cuts.
Cantilever offset and edge performance
Cantilever is modeled as an inward offset on all sides, so the post grid sits on the supported footprint. A 1 ft cantilever on a 12 ft width yields 10 ft of supported width, which changes both spacing and post locations. Use this to keep posts clear of planting beds, edging, or irrigation while still maintaining repeatable beam lines for garden decks and platforms.
Footing volume and bag planning
When enabled, each footing is treated as a cylinder using V = πr²h. The total volume is multiplied by the post count and reported in ft³ and m³. Bag counts use typical yields (about 0.60 ft³ for 80 lb and 0.45 ft³ for 60 lb mixes). Add 5–10% for waste, soil overbreak, and uneven holes.
Stake-out data for fast layout
The coordinate table gives X and Y distances from the deck corner, making it easy to snap string lines and square the grid. Download the CSV to print a field list, then mark corners first and work inward. If you choose perimeter-only, the output avoids duplicate corners and provides a clean outline for beam placement, planter ledgers, and garden-facing edges.
FAQs
1) Does this replace a building code check?
No. It estimates post count and spacing geometry. Always confirm beam spans, joist spans, soil bearing, frost depth, and attachment details with your local requirements or a qualified professional.
2) Why does actual spacing differ from my maximum spacing?
The calculator distributes posts evenly across the supported footprint. It uses your maximum as a limit, then adjusts to a smaller, uniform spacing so the grid aligns cleanly at both ends.
3) When should I use perimeter-only?
Perimeter-only suits small platforms or designs with strong perimeter beams. Larger decks often need interior supports to reduce beam spans and deflection, especially with heavier finishes or frequent traffic.
4) How should I enter footing sizes in metric?
Choose metric units, then enter deck dimensions in meters. Footing diameter and depth fields accept centimeters in metric mode, letting you plan common hole sizes without extra conversions.
5) Are bag estimates exact for concrete?
They are planning-level estimates. Yield varies by product, compaction, water content, and hole shape. Use the estimate for ordering, then keep a small buffer for field adjustments.
6) What if I need posts aligned to existing beams?
Use the grid output as a starting point, then shift spacing to match beam lines or obstacles. Re-run with updated maximum spacing or cantilever to keep all bays within your target limits.