Baluster Spacing Calculator

Lay out a level railing with equal gaps

This baluster spacing calculator finds the smallest whole number of uniform-width balusters that meets your chosen maximum clear gap, with equal gaps at both posts and between every baluster. It also gives center marks measured from one starting point, so you can transfer the layout to a rail without repeatedly adding a rounded spacing.

Use it for one level section at a time. The drawing updates with your measurements and shows widths to scale. Railing height is illustrative. Copy the results to keep the center positions, or print the layout and markout table. The starting measurements are an example, not a recommended railing specification.

Measure the opening and the actual baluster

1. Measure from the inside face of the left post to the inside face of the right post. Exclude the posts themselves.
2. Measure the baluster's actual width across that opening. A nominal lumber description is not an actual width measurement.
3. Enter your chosen maximum clear gap, in the same units. Confirm the requirements for the specific project before selecting that target.

The calculator supports inches and millimeters. Switching units converts the measurements you have already entered. For decimal inches, enter 1½ inches as 1.5 and 3¾ inches as 3.75.

If your posts or balusters vary in width, or your opening is not a level rectangular section, this model does not capture the full layout. In particular, it does not calculate the spacing measured along a sloping stair rail. The stair calculator and stringer calculator cover the step geometry separately.

Clear gap, center spacing and first mark

These three dimensions answer different questions. Clear gap measures empty space. Center spacing measures from the center of one baluster to the next. The first center mark starts at the left inside post face, so it contains one clear gap plus half a baluster width.

For a 72-inch clear opening, 1.5-inch balusters and a 3.75-inch maximum gap:

• Use 13 balusters, leaving 14 equal gaps.
• Balusters occupy 13 × 1.5 = 19.5 inches.
• Each gap is (72 − 19.5) ÷ 14 = 3.75 inches.
• Center-to-center spacing is 3.75 + 1.5 = 5.25 inches.
• The first center is 3.75 + 0.75 = 4.5 inches from the left inside face.

The same physical layout in millimeters uses a 1828.8 mm opening, 38.1 mm balusters and a 95.25 mm gap target. It still needs 13 balusters; its first center mark is 114.3 mm.

Formula and rounding

Let L be the clear opening, W the actual baluster width and G the chosen maximum gap. The minimum count is N = ceiling((L − G) ÷ (W + G)), with a lower bound of zero. The resulting gap is (L − N × W) ÷ (N + 1). The calculator rejects dimensions that cannot form this layout.

The equal-gap method can be checked against the baluster example on printed page 77 of the Calculated Industries Construction Master guide: a 156-inch opening with 1.5-inch balusters and a 4-inch gap target gives 28 balusters and 29 gaps. The unrounded gap is 114 ÷ 29, approximately 3.9310 inches.

Calculations retain full precision. The table displays decimal inches to four places or millimeters to two, plus an inch mark rounded to the nearest 1/16 inch. Measure each mark from the same origin. Repeatedly adding a rounded step can move the last baluster noticeably. Check both end gaps on the real assembly before fixing the balusters.

For a solid-wood rail or panel, seasonal width changes are a separate question; the wood movement calculator estimates movement across the grain. See our calculation methods or report an unclear result.