Deck Baluster Spacing and Count: The 4 Inch Rule and a Chart
Part of the project planner: Planning and building a deck.
Codes in most places say that a 4 inch sphere must not pass through any opening in a deck guard. For a 6 ft rail with 1½ inch (2×2) balusters and a gap of no more than 3¾ inches, that means 13 balusters with an even gap of 3.75 inches. The baluster calculator works out the count and the exact gap for your rail.
Balusters by rail length
The largest gap allowed is 3¾ inches, which leaves a margin under the 4 inch rule. The gap shown is the exact, equal gap between balusters and at the posts:
| Rail length (post to post) | 2×2 wood balusters | Exact gap | 5/8 in metal balusters | Exact gap |
|---|---|---|---|---|
| 3 ft | 7 | 3.19 in | 8 | 3.44 in |
| 4 ft | 9 | 3.45 in | 11 | 3.43 in |
| 5 ft | 11 | 3.63 in | 13 | 3.71 in |
| 6 ft | 13 | 3.75 in | 16 | 3.65 in |
| 8 ft | 18 | 3.63 in | 22 | 3.58 in |
| 10 ft | 23 | 3.56 in | 27 | 3.68 in |
How to space them
- Measure the clear distance between the posts, not the overall length of the rail.
- Work out the number and the gap with the calculator. Cut a scrap block to the gap.
- Fix the first baluster against the block at one post, then the next against the previous one, and so on. Check the last gap by eye before fixing.
- Keep every baluster plumb with a level, and fix it at both ends with screws or connectors.
- On stairs, the balusters are set at an angle, and the gap is measured level (horizontally), so you need to space them in line with the slope.
The code basics
- Opening size: no opening that lets a 4 inch sphere through, with a looser rule for the triangle at the stairs (a 6 inch sphere in many codes). Check the exact rule where you live.
- Height: 36 inches or more for a residential guard on a deck, 42 inches in some places and for commercial buildings.
- Strength: the guard has to resist a heavy load pushed outward at the top, so the posts and their fixings matter more than the balusters.
- Bottom gap: the gap between the deck and the bottom rail is also limited, usually to 4 inches.
Which balusters?
Wood balusters (2×2) are cheap and easy to fit, metal balusters are slim and need no painting, and glass or cable infill gives a clear view, each with its own rules for spacing. A narrower baluster needs more pieces: 16 instead of 13 on a 6 ft rail.
See also the deck materials calculator and the deck materials list. Estimates only: check your local code.
Frequently asked questions
How many balusters do I need for a 6 foot rail?
13 balusters of 1½ inch wood (2×2) with a gap of 3.75 inches, or 16 of 5/8 inch metal, if the largest gap you allow is 3¾ inches.
How far apart should balusters be?
Close enough that a 4 inch sphere cannot pass between them, so under 4 inches. Aiming for 3½ to 3¾ inches leaves a margin; with 1½ inch balusters that is 5.25 inches center to center.
How do I space balusters evenly?
Subtract the width of all the balusters from the rail length, and divide by the number of gaps (one more than the balusters): 3.75 inches here. Cut a spacer block to that size and use it for every baluster.
How many balusters per foot?
About 2.2 per foot with 1½ inch balusters at a 3¾ inch maximum gap, and about 2.7 with 5/8 inch metal balusters.
How high should a deck railing be?
At least 36 inches in most places for residential decks, and 42 inches for commercial ones. Decks more than about 30 inches above the ground need a guard. Check your local code.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.