Measure the opening, then choose the balusters
Start with the Clear Length between the inside faces of the two Posts. For the worked example, that opening is 72 inches. Do not enter the distance between Post centers or the outside-to-outside length: those measurements include material that the balusters cannot occupy. Measure at the height where you will install the Rail, and check again near the other Rail. If the opening changes because the Posts are not parallel, correct or account for that condition before relying on one straight layout.
Measure the actual baluster width across the face that occupies the opening. The example uses 1.5 inches, a common actual dimension for a nominal 2×2. A product described as two inches wide does not necessarily occupy two inches. Measure several pieces from the batch, check for profiles that change width, and use the relevant installed face. A tapered or decorative baluster can leave a wider opening elsewhere than the straight rectangular face modeled here. Treat that geometry as a separate check.
Enter 72 in Bay 1 Clear Length, 1.5 in Member width, and 4 in Max Gap. Bare numbers are inches; explicit suffixes override that convention. A six-foot opening can also be entered as 6 ft or 1828.8mm. You can type a mixed measurement such as 8’ 3 1/2” without converting it first. The calculator converts to millimeters internally, retains the unrounded dimensions through the calculation, and only rounds the values shown on the screen.
Why the example needs 13 balusters
With n balusters between two Posts, there are n−1 interior gaps plus two End Gaps: n+1 gaps in total. If every gap is equal, the equation is Clear Length = n × Member width + (n+1) × Gap. Rearranging gives Gap = (Clear Length − n × Member width) ÷ (n+1). The choice is an integer count, so changing the count also changes the gap. An apparently minor difference of one baluster spreads across the entire opening.
In the default example, 13 balusters occupy 19.5 inches. Subtract that from the 72-inch opening and 52.5 inches remain for 14 gaps. Each gap is 3.75 inches, displayed as 3 3/4”. This includes the gap beside each Post. Twelve balusters would leave a gap greater than four inches, so the maximum-gap constraint rules out that smaller count. The result is a layout decision tied to the measured opening, not a recommendation about the strength or fastening of the railing.
The first left edge is 3.75 inches from the left Post’s inside face. Its center is another half baluster width away, at 4.5 inches. Each subsequent center advances by 5.25 inches, the sum of one baluster and one gap. The final baluster ends at 68.25 inches, leaving the same 3.75-inch gap at the right Post. The final boundary mark is 72 inches; it is not the center of the final baluster. Keep those two reference points distinct when checking your work.
Equal gaps and fixed gaps answer different questions
Equal mode asks which whole count produces a gap nearest your target while meeting the maximum. Leave Target Gap empty to use Max Gap as the target, or enter a smaller value for a denser appearance. A target is a preference. A maximum is a constraint. If the closest-looking arrangement violates the maximum, the calculator chooses a feasible count instead. The adjacent-count alternatives in the running-measurement details show other feasible choices, with the gap each would produce.
Fixed mode answers a different question: what happens if the interior gaps stay exactly at the target? With the same 72-inch opening, 1.5-inch balusters and four-inch target, 13 balusters and 12 interior gaps occupy 67.5 inches. The remaining 4.5 inches become two 2.25-inch End Gaps. Compare that result with the equal 3.75-inch gaps before choosing a layout. If either an interior gap or an End Gap violates the specified maximum, the calculator rejects the fixed arrangement and explains why.
Add each Bay independently
Use Add Bay for each separately measured opening. If a railing has openings of 72 and 80 inches, enter both lengths rather than 152 inches. The first needs 13 balusters at 3.75-inch equal gaps. The second needs 14 with an unrounded gap of about 3.933 inches, displayed as 3 15/16”. The combined installed count is 27. A single calculation across 152 inches would ignore the intervening Post and could put marks where solid material already exists.
The material estimate treats those two railing Bays as independent openings with two Posts each. If the openings share a Post or use existing Posts, subtract the duplicate or existing supports from your purchasing list. The railing tool does not infer how separate openings join. For a new fence with explicit shared Posts, Gates and Corners, use the picket fence planner. Its ordered Segment list models those connections and produces a separate Post-center table.
Transfer marks without accumulating rounding
Open the running-measurement details for the Bay you are building. Choose either left-edge marks or center marks and keep that choice consistent along the Rail. Hook the tape at the Bay’s left inside face and measure each printed position from that same origin. Do not measure each next position from the previous rounded mark. A repeating fractional pitch can accumulate error if it is rounded once and then stepped along the Rail. The displayed running positions are each independently rounded from the original formula.
For the 72-inch example, a quick dry check is to mark the first center at 4.5 inches, then inspect the final baluster and its right End Gap before fixing the full set. Lay a few actual balusters against the marks to confirm that the measured product width matches the input. Check both Rail faces if you transfer the pattern to a second Rail. The calculator assumes straight, parallel boundaries and uniform Member widths; it cannot see a bent Post, a damaged tape hook or an inconsistent batch of material.
Print, calibrate and share the result
Print marking strips opens a separate page containing true-scale lines for the selected dimensions. Choose Letter or A4 before printing and select the same paper in the print dialog. Use 100% or Actual size, disable fit-to-page, and turn off browser headers and footers. The first page includes a six-inch calibration line. Measure that line on the physical print with a ruler. If it is wrong, do not stretch, estimate or compensate by eye; correct the printer settings and print again.
The strips divide the Bay into numbered sheets. Cut at the vertical join lines and butt matching ends together without overlap. Solid marks identify left edges; dashed marks identify centers. Page labels retain the running measurement from the original Bay origin. Print one Bay first to check your workflow before producing all the sheets. Copy plan link saves the inputs in the address, so a second device can restore the same calculation without an account. Recheck dimensions after any field adjustment, then copy the updated link.
Keep layout arithmetic separate from guard acceptance
The default four-inch limit is a planning input associated with the residential guard-opening rule discussed in the baluster code guide. It is not a promise that a four-inch measured opening passes an inspection. Real construction needs tolerance, and the applicable rule, measurement location and exceptions depend on the locally adopted code and the actual assembly. This tool covers straight, level layouts. It does not evaluate stairs, loading, attachment strength or the full shape of decorative infill.
Before buying, compare the results with the product installation instructions and confirm local requirements. Before fixing the final baluster, verify the actual openings, including both ends and any changing profile. Read how we calculate for the full formulas, unit handling and assumptions. If an input produces a red error, correct that input rather than using an earlier count from memory. A valid numerical layout is one piece of the job, and the visible dimension checks give you a way to test that piece on site.