Equal gap
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Shop estimate. Measure your stock and cut a test on scrap.
How to use it
- Choose shelves or holes.
- For shelves, enter the opening, how many shelves, and the thickness of each. The gap above, between, and below comes out equal.
- For holes, enter the span, how many holes, and the margin from each end. One hole is centered.
- Mark the starts or the centers from the same end you measured.
- If the shelves are thicker than the opening, or the margins meet, the page stops instead of printing a negative gap.
The formula
Shelf gap = (span − count × thickness) ÷ (count + 1). Hole spacing = (span − 2 × margin) ÷ (count − 1).
- Count + 1. The gaps when there is air at both ends and between each shelf.
- Margin. The distance from the end of the rail to the first and last center.
- Start. For a shelf, the distance from the start of the opening to the top of that shelf.
Worked example
Three shelves, each 20 mm thick, in a 700 mm opening, with a gap above the first and below the last, need a gap of 160 mm. The shelf tops start at 160, 340, 520 mm from the top of the opening.
Three hole centers on a 100 mm rail, 10 mm in from each end, are 40 mm apart, at 10, 50, 90 mm. One hole is placed in the middle of the span.
What it will not decide
- It does not know how much weight a shelf can carry. Spacing is not a span rating.
- Shelf pins drilled on a system line, such as 32 mm, will not match an arbitrary equal gap. If the pin line is fixed, fit the shelves to the line.
- A single hole ignores the margin and sits in the middle, and it says so.
Questions
Why is the gap not just the opening divided by the number of shelves?
Because the shelves have thickness, and because you usually want air above the top shelf and below the bottom one. Three shelves make four gaps.
Can I use this for balusters?
Use holes if you are marking centers. Use shelves if the baluster has a width and you want equal daylight, including the daylight at the posts. The code’s maximum gap is your problem to check. This page will happily space a gap that a railing code would reject.
Does the order of marks start at the top?
It starts at the end you called the start when you measured the span. Stay with that end. Mixing ends is how a row of pins walks off the stile.