How this calculator works
Balusters are spaced so that every gap, including the two at the posts, is the same. If the largest gap you allow is g and a baluster is w wide, the number you need is the rail length less one gap, divided by one baluster plus one gap, rounded up. The exact gap is then what is left of the rail after the balusters, shared out over the n + 1 gaps.
Worked example: a 6 ft (72 inch) rail with 1½ inch balusters and a largest gap of 3¾ inches needs 13 balusters, with an exact gap of 3.750 inches and a center-to-center spacing of 5.250 inches. With 5/8 inch metal balusters it needs 16, and a 10 ft rail needs 23 of the 1½ inch ones.
Balusters for common rail lengths (1½ in balusters, gap under 3¾ in)
Rail length between the posts.
| Rail | Balusters | Exact gap | Center to center |
|---|---|---|---|
| 3 ft | 7 | 3.188 in | 4.688 in |
| 4 ft | 9 | 3.450 in | 4.950 in |
| 5 ft | 11 | 3.625 in | 5.125 in |
| 6 ft | 13 | 3.750 in | 5.250 in |
| 8 ft | 18 | 3.632 in | 5.132 in |
| 10 ft | 23 | 3.563 in | 5.063 in |
Tips before you buy
- Guards on decks and stairs usually have to be 36 to 42 inches high, and balusters must not leave a 4 inch sphere opening: check your local code before you buy.
- Make a story pole: a stick marked with the baluster positions, so that every section is the same.
- Cut a spacer block to the exact gap. It is quicker and more accurate than measuring each baluster.
- On stairs the balusters are set at an angle, and the gap is measured level, so they need more balusters than a level rail.
- Buy a few spares, since some will split or be cut wrong.
Tools & materials for this project
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Frequently asked questions
How many balusters do I need for a 6 foot rail?
13 balusters of 1½ inches with a gap of 3.750 inches, if the largest gap is 3¾ inches. With 5/8 inch metal balusters it is 16.
How far apart should deck balusters be?
Close enough that a 4 inch sphere cannot pass: so a gap of less than 4 inches, and most people aim for 3½ to 3¾ inches. That is a center-to-center spacing of 5.25 inches for 1½ inch balusters.
How do I space balusters evenly?
Divide the rail length less the width of all the balusters by the number of gaps (one more than the balusters): 3.750 inches here. Cut a spacer block to the gap and use it for each baluster.
How many balusters per foot of rail?
About 2.2 for 1½ inch balusters at a 3¾ inch maximum gap, and more for narrower balusters.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.