Equal Spacing Calculator

Frequently Asked Questions

How do I space N items evenly across a length?

For N items centered in a run of length L, on-center spacing = L ÷ N, with the first center at L ÷ (2N). For evenly spaced gaps between N items, gap = (L − N × item width) ÷ (N + 1).

What is the difference between on-center and gap spacing?

On-center measures center-to-center and ignores object width; gap spacing measures the clear space between objects and depends on their width. Picket fences use gap spacing; stud layout uses on-center.

Worked example for fence pickets?

For 10 pickets each 3.5 in wide across a 60 in opening: total picket width = 35 in, leaving 25 in for 11 gaps, so each gap = 25 ÷ 11 ≈ 2.27 in.

How do I lay out evenly spaced shelves or holes?

Divide the usable height by (N + 1) to get equal gaps between N shelves and the ends, or by (N − 1) if the top and bottom items sit at the very ends. Mark from a single reference to avoid cumulative error.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Evenly spacing slats, balusters, pickets, shelves, or fence boards across a run is one of those jobs where the eye is unforgiving - one slightly fat gap and the whole assembly looks wrong. The mistake most builders make is forgetting that the items themselves take up width, so you cannot simply divide the length by the count. This calculator subtracts the total thickness of all the items from the overall span, then distributes the remaining empty space into equal gaps. You choose whether you want a gap at each end too (items floating inside the run) or whether the first and last items sit flush with the ends (gaps only between items). It returns the clear gap between items and the on-center spacing - the centre-to-centre distance you actually mark with a tape - in inches or centimetres.

Formula / Method

Let span be the total length, N the item count, and t the thickness of one item. The wood occupies N × t, so the free space is span − N × t. That free space is split into equal gaps. If you want a gap at both ends, there are N + 1 gaps; if the end items sit flush, there are N − 1 gaps (the gaps only fall between items). So gap = free space ÷ gap count. The on-center distance - the practical measurement between item centres - is the centre of the last item minus the centre of the first, divided by N − 1. With flush ends that simplifies to (span − t) ÷ (N − 1). Centimetre inputs convert by ÷2.54 internally and back for display. The tool refuses a non-positive span, fewer than one item, negative thickness, or a case where the items are wider than the span.

Worked Example

You are mounting 5 shelves of ¾ inch stock evenly inside a 60 inch tall cabinet, with a gap at top and bottom too. Item wood = 5 × 0.75 = 3.75 inch; free space = 60 − 3.75 = 56.25 inch; gaps = 5 + 1 = 6, so each gap = 56.25 ÷ 6 = 9.375 inch. The on-center spacing works out to about 10.125 inch. Now switch to flush ends (top and bottom shelves butt the ends): gaps = 5 − 1 = 4, free space still 56.25, so each gap = 14.0625 inch and on-center = (60 − 0.75) ÷ 4 = 14.8125 inch. Mark from one end using the on-center figure and the run stays perfectly even.

Common Mistakes

Dividing the span by the item count and ignoring that the items have width - the gaps come out too large.

Confusing the clear gap with the on-center spacing; you mark to centres, but you size gaps by the clear opening.

Forgetting whether the design wants end gaps or flush ends - it changes the gap count by two.

Measuring nominal thickness instead of actual; a "1-inch" board is really ¾ inch and the gaps shift.

Letting the gap exceed a code limit for guardrails - baluster gaps usually must be under 4 inch.

Tips & FAQ

Should I mark gaps or centres? Mark centres with the on-center figure - it accumulates no error across the run, whereas measuring gap-by-gap lets small mistakes pile up. Flush ends or end gaps? Fences and slat panels usually look best with equal end gaps; built-in shelves often sit flush top and bottom. What about guardrail and deck codes? Many codes cap baluster openings at less than 4 inch so a sphere cannot pass - check your local rule and increase the count until the gap drops below the limit. How do I handle an odd remainder? Real boards vary slightly; nudge the count by one or split a fraction across the two end gaps where it shows least. Can I use this for tile or fence pickets? Yes - any repeated element of uniform thickness across a length. Does kerf matter? Only if you are ripping the items from one piece; account for saw kerf separately. Treat the output as a layout guide and dry-fit before fastening, since cumulative measuring and material variation always sneak in.

Frequently Asked Questions

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