About the Baluster Spacing Calculator
This baluster spacing calculator works out how many balusters (spindles) fit between two posts and the exact gap that spaces them evenly, so the first and last gaps against the posts match the rest. Enter the clear distance between the posts, the width of your balusters and the largest gap you want to allow, and it returns the baluster count, the actual gap, the on-center spacing and where to mark the first baluster.
It is built for deck, porch and stair railing projects. US residential codes require that a 4-inch sphere cannot pass between balusters, so the gap must be less than 4 inches; many builders target 3½ to 3¾ inches to leave room for wood movement and slight layout error. If you have several railing sections of the same length, enter the number of sections to get the total count for your shopping list.
Measure between the faces of the posts, not center to center. Results are shown in decimal inches and to the nearest 1/16 inch for marking with a tape measure.
With the default inputs, the balusters per section is 13. Change any value above to recalculate instantly.
How to use the baluster spacing calculator
- 1Measure the clear distance between the inside faces of the two posts.
- 2Enter the width of one baluster and the maximum gap you will allow (under 4 in).
- 3Enter how many identical railing sections you have.
- 4Mark the first baluster center at the distance shown, then every on-center step.
- 5Check a few gaps with a 4 in block or spacer before fastening.
Formula and method
Between two posts there is always one more gap than there are balusters. If every gap were exactly the maximum g, then L = n × w + (n + 1) × g, so n = (L − g) ÷ (w + g). Rounding n up guarantees every gap is no larger than the maximum.
With the count fixed, the leftover space L − n × w is shared equally among the n + 1 gaps, giving the actual gap. The on-center spacing is the gap plus one baluster width, and the first baluster center sits one gap plus half a baluster width from the face of the post.
- L
- Clear distance between post faces (in)
- w
- Baluster width (in)
- g
- Maximum allowed gap (in)
- n
- Number of balusters
Worked examples
70 in deck section with 2×2 balusters
(70 − 3.75) ÷ (1.5 + 3.75) = 12.62, which rounds up to 13 balusters. They take 19.5 in, leaving 50.5 in shared by 14 gaps, or 3.607 in each (about 3-5/8 in). Centers are 5.107 in apart.
Four 8 ft sections with 2 in balusters
(96 − 3.5) ÷ 5.5 = 16.8, so 17 balusters per section. The 62 in of space left over is split into 18 gaps of 3.444 in. Four sections need 68 balusters.
Short 36 in stair landing section
(36 − 3.875) ÷ 5.375 = 5.98, so 6 balusters. They use 9 in, leaving 27 in for 7 gaps of 3.857 in — just under the 4 in limit.
Frequently asked questions
What is the maximum baluster spacing allowed?+
The US International Residential Code requires guard openings that do not allow a 4-inch diameter sphere to pass through. In practice the clear gap between balusters must be less than 4 inches.
How many balusters do I need per foot?+
With 1.5 in balusters and gaps around 3.5–3.75 in, you need roughly 2.3–2.4 balusters per linear foot of railing. Use the calculator for the exact count between each pair of posts.
Should I measure between posts center to center?+
No. Measure the clear distance between the faces of the posts, because the balusters only fill the open space. Using center-to-center would overestimate the space by one post width.
Why is my gap smaller than the maximum?+
You cannot install a fraction of a baluster, so the count is rounded up and the leftover space is shared equally. That makes every gap equal and slightly smaller than the maximum you entered.
Do stair balusters follow the same rule?+
Not exactly. The IRC relaxes the rule on the open sides of stairs so that a 4-3/8 in sphere must not pass between balusters, and the triangle formed by the riser, tread and bottom rail must not pass a 6 in sphere. Landings and level decks keep the 4 in limit, and many builders use 4 in everywhere for simplicity.