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Cable Railing Calculator
Quick answer: Space cables at 3-1/8″ maximum — code bans a 4″ sphere passing through, and cable deflects under push. A 36″ guard takes about 10 to 11 runs. Keep posts at 4 ft where you can.
Cable runs, actual spacing, total cable footage, post count and fittings for a horizontal cable guard — with the code rule that governs the spacing.
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Cable spacing and the 4-inch sphere rule
The industry has settled on 3-1/8 inches (3.125″) as the practical maximum, which leaves margin for deflection between posts. Some jurisdictions require 3″ or less, and some require intermediate pickets on runs over a certain length. Check locally before you drill 40 posts.
Why post spacing matters more than anything else
Cable railing lives or dies on post spacing. Every cable is under 200–300 lb of tension, and that load pulls the end posts inward. Intermediate posts must resist deflection between them.
| Post spacing | Deflection risk | Verdict |
|---|---|---|
| Up to 4 ft | Minimal | Ideal; passes inspection everywhere |
| 4–5 ft | Low with intermediate supports | The common compromise |
| Over 5 ft | Significant | Needs intermediate pickets or spacer struts |
End and corner posts carry the accumulated tension of every cable and need to be substantially stronger than intermediate posts — typically 4x4 minimum in wood, or a structural steel post, bolted through the framing rather than surface-mounted.
Fittings
Each cable run needs a tensioner at one end and a fixed terminal at the other. Runs that turn a corner need a terminal at each corner post, because cable cannot bend around one under tension.
- Swage fittings — crimped with a hydraulic tool. Cleanest look, needs the tool.
- Swageless / mechanical — assembled with hand tools. More expensive per fitting, no special equipment.
- Turnbuckles — visible adjusters, easy to re-tension later.
- Threaded tensioners — hidden inside the post, tidier.
Tensioning
Tension from the middle outward, and expect to make several passes — tightening one cable slightly slackens its neighbours as the posts pull in. Cable stretches for the first few weeks, so plan to re-tension once after about a month. Over-tensioning is a real failure mode: it bows the end posts inward and no amount of adjustment fixes a post that has already moved.
Frequently asked questions
How far apart should cable railing be spaced?
3-1/8 inches (3.125 in) is the industry standard maximum. Code requires that a 4-inch sphere cannot pass through the guard, and cable deflects when pushed, so the installed spacing must be tighter than 4 inches to still pass under load.
How many cables do I need for a 36 inch railing?
Typically 10 to 11 runs, depending on top rail thickness and the spacing you choose. With a 2 in top rail and 3-1/8 in spacing, a 36 in guard gives about 10 cable runs.
How far apart can cable railing posts be?
Four feet is ideal. Up to five feet works with intermediate supports. Beyond five feet, cable deflection between posts becomes significant enough that most inspectors will require intermediate pickets or spacer struts.
Do I need to re-tension cable railing?
Yes. Stainless cable stretches over the first few weeks after installation. Plan to re-tension once at about a month, then check annually. Tension from the middle outward — tightening one cable slightly slackens its neighbours.