Deck Railing Calculator
Code-compliant deck railing comes down to spacing: get the baluster or cable spacing right and the rest is simple math. This calculator turns your railing run into a baluster, metal baluster, or cable count plus posts and total cost.
Covers curves, corners and offcuts. 5–15% is normal.
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Coverage & waste
Material profile
Enter your total railing run and a waste percentage to cover cuts and layout. The calculator divides that adjusted length by the on-center spacing for your chosen infill (about 5 in for wood or metal balusters, tighter for horizontal cable runs) to keep gaps under the standard 4 in code maximum. Posts are figured separately from your post spacing, then baluster or cable cost plus post cost gives you the total.
Baluster & Cable Spacing Reference
Most U.S. building codes require railing infill to stop a 4 in sphere from passing through. These are typical on-center spacings that satisfy that rule for common infill types.
| Infill Type | Typical On-Center Spacing | Notes |
|---|---|---|
| Wood balusters | ~5 in (2×2 baluster + ~3.5 in gap) | Most common, matches the 4 in sphere rule |
| Metal balusters | ~5 in on-center | Round or square, same code gap rule applies |
| Cable rail (horizontal) | ~3.5 in between runs | Vertical spacing between horizontal cable lines |
How to calculate deck railing materials
- Measure the total railing run, including stair rails if you’re including them.
- Add waste 5% covers trim cuts and adjustments around posts.
- Divide by spacing use the on-center spacing for your infill type, and round up.
- Add posts calculate separately from your post spacing along the same run.
- Total the cost multiply baluster or cable count and post count by their unit prices.
Worked example
Here’s the breakdown for a 40 ft railing run using wood balusters with 5% waste.
- Railing run40 ft
- Length with 5% waste42.0 ft
- Balusters needed100 balusters
- Posts needed (6 ft spacing)8 posts
- Baluster cost (100 × $6)$600
- Post cost (8 × $65)$520
- Total material cost$1,120
Meeting the 4-Inch Code Gap Rule
Most residential building codes (IRC included) require that a 4 in sphere cannot pass through any opening in a deck guard, and guards are required once a deck surface is more than 30 in above grade. Guard height itself is typically a 36 in minimum for residential decks, with commercial and some local codes requiring 42 in. Stair rails often have a stricter triangular-opening rule near the treads, so check your local amendment before finalizing spacing.
Always verify spacing and height requirements with your local building department before you order, since amendments to the base code are common and inspectors will check gap spacing before signing off.
Common mistakes
- Spacing balusters wider than code allowsalways confirm on-center spacing keeps the actual gap under 4 in, not just the on-center number.
- Forgetting stair rail sectionsstair railing often has different spacing rules and is easy to leave out of the total run.
- Skipping cable tensioning hardwarecable rail runs need turnbuckles and end fittings that aren’t included in the per-run material cost.
- Not planning post spacing around the layoutposts need to land at structurally sound points like the rim joist or framing, so a fixed spacing default may need adjusting on site.
What this calculator assumes
Adjust your result if your project differs from these.
- The railing run is a straight line or the sum of straight segments.
- Baluster and cable spacing default to typical code-compliant on-center distances.
- Posts are spaced evenly on-center at the spacing you enter.
- Waste percentage covers trim cuts, not stair transitions or corner blocking.
- Prices reflect the per-baluster/per-run and per-post costs you enter, not connector hardware.
Common questions
How many balusters do I need per foot of railing?
At the standard ~5 in on-center spacing used here, expect roughly 2.4 balusters per linear foot.
What's the maximum gap allowed between deck balusters?
Most codes require gaps small enough that a 4 in sphere can’t pass through, which usually works out to about 3.5-4 in of clear space.
Is cable railing spacing different from balusters?
Yes. Cable runs are horizontal lines spaced vertically, typically closer together (around 3.5 in) than baluster on-center spacing.
Does this calculator include post caps or cable hardware?
No. It covers baluster/cable and post counts and cost. Add caps, brackets, and cable tensioners separately.
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Project
| Shape | — |
| Dimensions | — |
| Total length | — |
| Material | — |
| Waste allowance | — |
Shopping list
| Units needed | — |
| Stakes | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm quantities with your supplier before ordering.