Get The Fuck OutLayout & JoineryCustom arch by batten

How do you frame a perfect custom arch without calculating a radius?

You can absolutely work out the radius of an arch with a formula, a calculator and a string trammel. Or you can draw the thing full size on the floor, bend a strip of plywood through three points, and trace it. One of those takes four minutes.

⚡ Rule of thumb

Loft the arch full size, bend a thin plywood batten through your two spring points and the peak, and trace along it — the bent strip finds a fair curve on its own.

Starting point

Batten ≈ 3–6 mm (1/8–1/4 in) plywood strip through 3 points

Measured

Span = the chord between spring points · rise = peak height above that chord

Confidence

Creator technique

Applies to

Process: Rough carpentry / framing, general layout
Materials: Sheet stock for the template, dimensional lumber for the arch
Setup: Tape, pencil, flexible ply batten, jigsaw, blocks or nails to pin the points
Orientation: Laid out flat on the subfloor or on a sheet of plywood

Why it matters

A wrong radius doesn't look like a wrong number. It looks like a lumpy arch, forever, in someone's living room.

Lofting it full size skips the arithmetic entirely. The curve you trace is the curve that fits the opening in front of you, because you drew it against the opening in front of you.

What good looks like

One continuous curve — no flat spots, no kink at the peak
Template span and rise match the real opening, not the drawing
Flip the template end for end and both halves land on the same line

How to apply it

On the subfloor or a plywood sheet, strike a baseline equal to the opening width and mark the two spring points at its ends.
Find the centreline, measure up the rise, and mark the peak.
Tack small blocks or nails at the two ends and the peak so the batten has something to bear against.
Bend the batten so it touches all three points and hold or pin it there.
Trace along the batten, cut the line with a jigsaw, and use that template to mark and frame the arch.

Test it on your setup

Flip the cut template end for end about the centreline — both halves should match.
Sight down the cut edge for flat spots or a kink where the batten was pinned at the peak.

Exceptions & tradeoffs

A bent batten gives you a fair curve, roughly elliptical — not a true constant-radius arc. If this arch has to match other true-radius elements in the room, swing it with a trammel instead.

Very tight arches can out-bend the strip. Go thinner, or use a different method before you snap it across the room.

Common mistakes

Eyeballing the peak instead of measuring the rise on a marked centrelineMark the centreline first, measure the rise on it — that's what keeps both halves symmetric
Grabbing whatever scrap is nearest for the battenPick a strip that bends smoothly through all three points with no kink

My Supa LifeSource & visual reference

Lofting the arch full size and tracing a bent plywood batten through the end points and peak — How To Frame The Perfect Custom Arc ↗ at 0:16 (bend) · 0:44 (trace).

Summarized and independently edited by Start Here. The method — loft it full size, bend a batten, trace, cut — is clearly shown. The batten thickness range, the test steps and the trammel caveat are our editorial notes, not the creator's numbers.