Baluster Spacing Calculator

Evenly spaced balusters make a railing look professional and keep it safe. Enter the clear distance between your posts, the width of each baluster (spindle) and the largest gap your code allows, and the calculator gives the number of balusters, the exact equal gap, the on-centre spacing and a list of marks to lay out on your rail — no trial and error.

The formula

To space balusters evenly with equal gaps at both posts: number of balusters = (length − gap) ÷ (baluster width + gap), rounded up to a whole number. Then the exact gap = (length − number × width) ÷ (number + 1), and the on-centre spacing is gap + width. Rounding up the number of balusters guarantees the actual gap is never bigger than your maximum.

The 4-inch rule

Most US residential building codes require guards (railings) where a deck, porch or landing is more than 30 inches above the ground, and state that a 4-inch sphere must not pass through any opening. That is why many builders aim for gaps just under 4 inches — 3⅞ or 3¾ inches — to leave a margin for wood movement and imperfect installation. Common US code provisions allow a larger 4⅜-inch sphere between balusters on the open side of stairs, and a 6-inch sphere in the triangle formed by the tread, riser and bottom rail. Other countries use different figures (for example 100 mm in the UK), so always check your local code.

Worked example

A deck section is 96 inches between posts, balusters are 1½ inches wide and the maximum gap is 3⅞ inches. Number = (96 − 3.875) ÷ (1.5 + 3.875) = 17.1, rounded up to 18 balusters. Exact gap = (96 − 18 × 1.5) ÷ 19 = 3.632 inches (about 3⅝ in), on-centre spacing 5.132 inches. For four identical sections you need 72 balusters — buy about 76 to allow for splits and mistakes.

Laying out the balusters

1. Measure the clear distance between post faces at the bottom rail and again at the top — they should match.
2. Use the calculator’s exact gap; cut a spacer block to that width.
3. Mark the first baluster’s left edge at one gap from the post, then every on-centre distance.
4. Alternatively, mark centre points and centre each baluster on its mark.
5. Fix the first and last balusters, check the remaining space, then fill in using the spacer.
6. Check plumb every few balusters with a level.

A spacer block is faster and more consistent than measuring each gap: fix one baluster, hold the block against it, fix the next.

Stair balusters

On stairs, balusters follow the slope of the rail. Measure the horizontal run between posts (not along the slope) for layout from above, or measure along the rail and use the same calculator — the gap measured square to the balusters is what matters for the sphere rule. Many stair designs use two or three balusters per tread; check that the gap between them meets your code’s stair limit.

Choosing balusters

Narrow metal balusters need more pieces for the same length, because the gap limit stays the same while each baluster covers less width.

Common mistakes

• Measuring to the outside of posts instead of the clear space between them.
• Rounding the baluster count down, which makes gaps too wide.
• Forgetting that wood balusters can twist or shrink, enlarging gaps.
• Using the same count for sections of different lengths.
• Not checking the gap between the bottom rail and the deck surface (also limited by code in many places).

Planning the whole deck

Measure every section separately — posts are rarely exactly the same distance apart — and run the calculator for each. Keep the same maximum gap everywhere so the spacing looks consistent; the actual gaps will vary slightly from section to section, which is not noticeable once installed. Add up the balusters for all sections, including stairs, and buy a few extra: wood balusters can have knots, splits or twists that make some unusable.

Metric layouts

Switch the units to millimetres for metric materials. For a 2,400 mm span with 40 mm balusters and a 95 mm maximum gap, you need 18 balusters with gaps of about 88.4 mm. In the UK and many other places, the 100 mm sphere rule for domestic balustrades is common — check the regulations that apply to your project.

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