How do you divide a board into equal parts?

Lay a ruler at an angle across the board with 0 on one edge and a number your count divides evenly on the other edge. Mark at each even step and carry the marks square across. Five parts on a 9 in board: 0 to 10 in, a mark every 2 in.

Why does tilting the ruler work?

Similar triangles. The ruler and the width form a right triangle, and equal steps along the ruler project to equal steps across the width.

How many marks do I need for 12 equal spaces?

Eleven interior marks. The two edges are the first and last lines.

Does it work with a tape measure?

Yes, as long as the tape is pulled straight and flat. A stiff steel rule is easier to hold at an angle on short spans.

MeasuringLayout & Site WorkEqual spacing with a tilted ruler

How do you divide a space into equal parts without doing the math?

Twelve bends across a 9 5/16 in panel. Divide it out and you get 0.776 in per space, which isn't on anybody's tape. Round it, step it off twelve times, and the last space comes out as whatever's left over. Adam Fleisch skips the division: tilt the ruler until a friendly number spans the opening, and let geometry divide the space into equal parts.

⚡ Rule of thumb

Put 0 on one edge and angle a ruler across the space until a number that divides evenly by your count lands on the far edge. Mark at each even step, then carry the marks square to the edge. Twelve spaces across 9 5/16 in: 0 to 12 in, a mark every inch.

Starting point

12 spaces across 9 5/16 in (237 mm): 0 to 12 in, mark every 1 in (metric: 0 to 240 mm, every 20 mm)

Measured

Along the tilted ruler · marks projected square to the reference edge

Confidence

Widely accepted

▶ Source video — Need to evenly space an opening without doing the math? — Adam Fleischwatch on Facebook ↗

Applies to

Process: Sheet metal, fabrication and woodworking layout
Materials: Any flat stock
Setup: Steel rule or tape, marker or scribe, combination square
Best for: Bend lines, slats and ribs, hole rows, shelf-pin spacing
Counting: Count spaces, not marks: 12 spaces = 11 interior marks

Why it matters

9 5/16 in divided by 12 is 0.776 in (19.7 mm). You can't read that on a rule. You round it, and either way the error stacks: twelve spaces at 3/4 in comes up 5/16 in (8 mm) short, and twelve at 13/16 in runs 7/16 in (11 mm) long.

The tilted ruler turns the awkward division into whole numbers. It's similar triangles: the ruler is the long side, the panel width is the short side, and every inch mark on the ruler projects to exactly 1/12 of the width.

No calculator. No fractions.

What good looks like

0 and the end number sit exactly on the two edges
Spaces check equal with a caliper or rule
Lines carried across run square to the reference edge, not along the ruler's angle
The spaces add back up to the full width

How to apply it

Pick a ruler number longer than the width that your count divides evenly. Twelve spaces across 9 5/16 in: 12 in. In metric, 240 mm.
Put 0 on one edge and swing the ruler until that number sits on the opposite edge.
Mark at every step: every 1 in for 12 in and 12 spaces, every 20 mm for 240 mm.
Carry each mark across with a combination square registered on the reference edge.
Use the lines. In the clip they're bend lines, and the panel goes straight into the brake.

Test it on your setup

Spot-check two or three spaces with calipers. They should match within your marking accuracy.
Sum check: count × space should equal the full opening. 12 × 0.776 in = 9.31 in.
Flip check: run the ruler again from the other edge. The new marks should land on the old ones.

Exceptions & tradeoffs

The number has to be longer than the width, or the ruler can't reach across on a diagonal.

Steep angles make marks hard to place cleanly. Pick the first friendly number past the width when you can: 240 mm across a 237 mm opening sits nearly flat, while 360 mm works but tilts the ruler to almost 50°.

If the two edges aren't parallel, the spaces won't stay equal along the length. Divide at both ends and connect the marks, or square up the stock first.

Mind you, this works for any count. Five spaces across 9 in? Put 0 to 10 in across it and mark every 2 in.

Common mistakes

Counting marks instead of spaces12 spaces = 11 interior marks plus the two edges
Carrying the marks across along the ruler's angleProject each one square to the reference edge
Picking a number shorter than the opening because it divides nicelyIt has to be longer than the width — 240 mm across 237 mm, not 120 mm

Frequently asked questions

How do you divide a board into equal parts?Lay a ruler at an angle across the board with 0 on one edge and a number your count divides evenly on the other edge. Mark at each even step and carry the marks square across. Five parts on a 9 in board: 0 to 10 in, a mark every 2 in.
Why does tilting the ruler work?Similar triangles. The ruler and the width form a right triangle, and equal steps along the ruler project to equal steps across the width.
How many marks do I need for 12 equal spaces?Eleven interior marks. The two edges are the first and last lines.
Does it work with a tape measure?Yes, as long as the tape is pulled straight and flat. A stiff steel rule is easier to hold at an angle on short spans.

Adam FleischSource & visual reference

Tilting a rule so an evenly divisible number spans the opening, marking every unit, and carrying the marks across for brake bends — Need to evenly space an opening without doing the math? ↗ at 0:26–0:56.

Summarized and independently edited by Start Here from Adam Fleisch's Facebook clip, read off the frames and his caption with no audio. The 9 5/16 in panel, the 12 spaces and the 0-to-12 in ruler come straight from his caption. The metric version, the rounding math, the angle advice, the five-space example and the test steps are mine.