Mind the gap
Proper deck spindle spacing follows building code, which requires that a 4-inch sphere can't pass between your balusters for safety.
Calculating the right number of spindles depends on your railing section length and the width of your balusters at their thinnest point.
Centered spindle placement requires finding the section's midpoint, then measuring outward using half the sum of one spindle width plus spacing.
Hiring a local deck building professional removes the guesswork from precise measurements and ensures your railing meets code for safety and support.
Whether you need to replace a few spindles, or balusters, to refinish a railing or replace all of them, you'll need to adhere to the rule stating that a 4-inch sphere can’t pass between balusters. This calculator will help you determine how many spindles you need and their placement.
To meet railing safety requirements for staircases and deck-building codes, precise measurements are necessary. To determine how many spindles you'll need, you must know the precise distance within each section of railing and the width of the thinnest part of your balusters.
The chart below provides the number of spindles you'll need that are 1-inch wide at their thinnest point for various section lengths between posts and walls.
| Section Width in Feet | Number of 1-Inch-Wide Spindles |
|---|---|
| 4 | 9 |
| 6 | 14 |
| 8 | 19 |
To determine the number of spindles you need for a section of railing, measure from the inside of the post or wall that it meets. Then add together how much space you desire between each spindle, keeping in mind that 4 inches is the maximum and the thinnest width of one of your spindles. Now, divide the section length by the sum of the spindle width and spacing value.
You'll likely come up with a fractional number. Round down to the nearest whole number. This is the quantity of spindles you'll need for the section. Repeat for each section and add the total number for all sections to determine the total count. This is useful when determining the cost of replacing an entire railing.
For stair railings, the measurement involves calculating the rise and run as you would when building a staircase or building a stair railing for it. However, if you simply determine and apply your measurements using only horizontal lengths, you can skip the additional math.
Spindle Quantity = Rail Section / (Spindle Width + Spacing Width)
The fraction value above the number of spindles you need represents that the spacing from the end spindle to the end of the section varies from the rest of the spacing. You'll want to center spindle placements within the section for the most attractive results.
The simplest way to measure for centered placement is to determine the center point of the section by dividing the total length by two. Use half of the sum value of one spindle plus the spacing width to determine the on-center location of the spindles nearest the center point.
Determining how many spindles you need is relatively straightforward. However, measuring for precise placement can get a bit tricky. While you can save money on the labor cost of installing a deck railing or stair rail, most homeowners choose to leave it to the pros and hire a deck builder or staircase expert for their needs.
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