Balusters are the vertical pieces between the posts of a railing, and they are sold in boxes, so the real question is how many boxes. The answer comes from the rule that sets the gap between them. This guide shows the arithmetic, works one railing through, and lists what to check when a boxed lot arrives.
The spacing rule
Many building codes in the United States say a 4 inch sphere must not be able to pass through any opening in a guard. That is the common rule, but it is not the only one, and codes differ by place and by whether the railing is on a level deck or a stair. Treat the figure as something to confirm with your local building department before you buy, not as legal advice. We are describing the arithmetic, not your code.
Because the rule is a maximum, plan for a gap a little under it. Aiming for 3.5 inches leaves you room for a baluster that sits slightly off and a post that is slightly out of square.
The arithmetic
Between two posts there is always one more gap than there are balusters. With n balusters of width w, and a gap of g, the opening L is:
L = n × w + (n + 1) × g
Solve it for n and round up: n = (L - g) ÷ (w + g). Use the clear opening between the posts, not the post centers, and measure it rather than trusting the plan.
A worked example
Say the opening between two posts is 120 inches, the balusters are 1.5 inches wide, and you are aiming for gaps of no more than 3.5 inches.
Work out the count
(120 - 3.5) ÷ (1.5 + 3.5) = 116.5 ÷ 5 = 23.3. Round up to 24 balusters.
Work out the real gap
24 balusters take up 24 × 1.5 = 36 inches. That leaves 84 inches for 25 gaps, so each gap is 84 ÷ 25 = 3.36 inches. That is under 3.5, and under 4.
Count the boxes
Suppose the lot is a box of 20. One section needs 24, so one box is not enough. Three sections of the same size need 72, which is four boxes of 20, with 8 left over.
Round up and then recheck the gap, because a count that rounds down makes the gap larger than you planned. Add a few spares to the order for a bent or damaged piece. If the listing sells one box, remember that every unit here is one of a kind, so you cannot top up from the same lot later.
What to check in a boxed lot
- Count. Read the count on the listing, then count the pieces in the photos. The completeness score reflects what is missing from the box. Compare the count with your arithmetic above.
- Finish. Matte black, bronze and white can differ from one production run to the next. Buy everything for a rail from one lot, and look at the photos for chips, scratches and bare metal at the ends.
- Length. Balusters are cut to fit between a top rail and a bottom rail. Measure that opening on your railing and compare it with the length on the listing, allowing for the angle if your stairs are not level.
- Width and shape. A square baluster and a round one with the same label can have different widths, and the width goes straight into the formula. Use the figure on the listing.
- Hardware. Look for the screws, shoes, connectors or brackets that belong with the balusters. Missing hardware is a completeness question, and the report says what is there.
How our scores apply
Every inspected unit gets four scores, which are explained on how we rate condition. For balusters, two of them do most of the work. Cosmetic tells you about the finish: chips and scratches show here, and a score of 3 or lower always carries a photo of the flaw. Completeness tells you whether the full count and the hardware are in the box. Function reads N/A when an item has nothing to test, which is the usual case for a baluster, and packaging only matters if you are storing them for a while.
Before you order
Measure your openings, do the arithmetic for each section, check your local code, and then compare the total with the count on the listing. Shipping is one charge for the whole order and free over $199, and a few heavy or long items are quoted separately, with the listing saying so. The return window is 3 days from delivery and covers a unit that did not match its report. There is no change-of-mind return, so measure before you buy.
