Spacing calculator
Enter a run and the width of what you are spacing, and this gives the count, the on-center distance, the open space, and every mark as a cumulative tape read from the start. Three modes: cap the opening, spread a fixed count, or best-fit a desired on-center. For deck and stair balusters under the 4 in sphere rule, use the baluster spacing calculator; this page covers everything else: shelf pins, fence pickets, joists, slats, bolt patterns, and trim.
Spacing balusters? Baluster spacing calculator
Updated
Enter the run and the member width the way your tape reads: 92-1/2, 7 ft 8-1/2 in, or 0 for a point like a shelf pin or a bolt. Marks are measured from the start of the run, each rounded once to the nearest 1/16 in.
- On center
- 5-15/16 in (exact 5-59/64")
- Open space
- 3-11/16 in
Cumulative tape reads · Center
- 1. 4-13/16 in
- 2. 10-3/4 in
- 3. 16-5/8 in
- 4. 22-9/16 in
- 5. 28-1/2 in
- 6. 34-7/16 in
- 7. 40-5/16 in
- 8. 46-1/4 in
- 9. 52-3/16 in
- 10. 58-1/8 in
- 11. 64 in
- 12. 69-15/16 in
- 13. 75-7/8 in
- 14. 81-13/16 in
- 15. 87-11/16 in
How the layout works
Every mode ends up at the same geometry: a run, a count of members of one width, and equal openings between them. With no member at either end there is one more opening than members; with a member at both ends there is one fewer. The on-center distance is the run, adjusted for the end members, divided by the openings, and the open space is that on-center less one member width.
Limit the opening finds the fewest members whose open space stays at or under your maximum, the way a picket fence or a guard is laid out. Evenly space a count takes the count you give it and spreads it, for shelf pins or a fixed number of slats. Best fit takes a desired on-center such as 16 in and picks the member count whose actual on-center lands closest, so a run that is not a multiple of 16 still gets equal bays. An optional maximum gap flags a layout that opens too wide, and in best fit it raises the count until the gap is met.
Marks are measured from the start of the run, not stepped off from the previous mark. The calculator keeps each position as an exact fraction and rounds it once to the nearest 1/16 in, so the last mark lands on the run instead of drifting by a sixteenth or two. You can mark the leading edge, the center, or the trailing edge of each member.
A worked example: pickets on a rail
A 92-1/2 in rail with 2-1/4 in pickets and an opening capped at 4 in, no picket at either end. This is the app's own manual example.
| Pickets | 15 | ceil((92-1/2 + 2-1/4) / (2-1/4 + 4)) = 16 openings, 15 pickets |
| On center | 5-15/16 in | 94-3/4 / 16, rounded once to 1/16 |
| Exact on center | 5-59/64" | the fraction the marks are built from |
| Open space | 3-11/16 in | 5-59/64 - 2-1/4 |
| First mark (center) | 4-13/16 in | one on-center in, less half a picket |
| Last mark (center) | 87-11/16 in | fifteen on-centers in, less half a picket |
A second example: joists at 16 in on center
A 12 ft run of 1-1/2 in joists with a joist at each end, best-fit to 16 in on center. 12 ft is not a multiple of 16 in once the end joists are counted, so the calculator picks the count whose on-center lands closest and spreads the rest.
| Joists | 10 | 9 openings fit closer than 8 |
| On center | 15-13/16 in | (144 - 1-1/2) / 9 |
| Open space | 14-5/16 in | on center less 1-1/2 |
| Against the 16 in target | 3/16 in under | the achieved on-center is this much under 16 in |
| Centers | 3/4 in, 16-9/16 in, 32-7/16 in, 48-1/4 in, 64-1/16 in, 79-15/16 in, 95-3/4 in, 111-9/16 in, 127-7/16 in, 143-1/4 in | cumulative from the start |
Which mode to use
The job picks the mode. A few common layouts and where they land:
| Job | Mode | Why |
|---|---|---|
| Fence pickets, slats, a guard infill | Limit the opening | The gap has a maximum; the count follows from it. |
| Shelf pins, hooks, a fixed number of boards | Evenly space a count | You already know how many; you want them even. |
| Joists, studs, rafters, purlins | Best fit | A nominal on-center like 16 or 24 in, adjusted so the bays come out equal. |
| Bolt patterns, anchor layout | Evenly space a count | Count fixed by the hardware; zero width for a point. |
What the app adds
This page handles level runs in feet and inches. The app also takes a rake angle for a stair rail, so the marks are laid out along the rake while the opening is checked both along the rail and horizontally, accepts metric runs, keeps the exact fraction behind every number, lets you pick the tape resolution, and prints the mark schedule as a PDF you can tape to the work.
Common questions
- How do I space things evenly along a run?
- Add one member width to the run, divide by one more than the count, and that is the on-center distance when nothing sits at either end. Mark each position from the start of the run as a running total so the rounding does not build up. Five 1-1/2 in slats on a 36 in run go at 5-1/2 in, 11-3/4 in, 18 in, 24-1/4 in, 30-1/2 in on center. The calculator does the arithmetic and gives the marks to the nearest 1/16 in.
- What is the difference between on center and open space?
- On center is the distance from the center of one member to the center of the next. Open space is the clear gap between their faces, which is the on-center less one member width. Framing is specified on center; railings and fences are limited by the open space. The calculator shows both.
- How do I calculate joist spacing?
- Decide the nominal on-center, usually 16 in, and let the calculator best-fit it to the run with a joist at each end. For a 12 ft run of 1-1/2 in joists that is 10 joists at 15-13/16 in on center, a hair under 16 in so the bays come out equal. Span tables are written for the nominal spacing, so a layout that lands at or under it keeps the joist size you sized for.
- Why not just step off the same measurement with the tape?
- Because every step carries the rounding forward. A spacing of 5-59/64 in reads 5-15/16 on the tape, and stepping that off fifteen times puts the last mark almost 1/4 in past where it belongs. Marking from the start with each position rounded once lands the last mark where the math says. The calculator's cumulative reads are that list.
- Can I use this for deck or stair balusters?
- Use the baluster spacing calculator instead. It is built around the IRC 4 in sphere rule for guards and shows the pass or fail. This page is the general tool for anything else that needs equal spacing, including a rail where you want to set your own opening limit.
Mark it from the start, once
CutOnce lays out the spacing like this page, then handles raked stair rails, metric runs, and your tape resolution, and prints the mark schedule as a PDF, with no signal needed.
Free to download, with a 7-day trial of everything. This page gives the marks; the app prints them and keeps the layout with the job.
See also