Molding for Archway: Trim a Real Arch or Fake One
Molding for archway projects comes in two different jobs. A real arched opening between rooms needs flexible casing that bends to the curve of the drywall. A flat wall that you want to look arched needs a pre-cut arch frame applied to the surface. Pick the job first, because the products are not interchangeable.
That distinction is where most archway trim advice goes wrong: it shows a beautiful curved doorway, then recommends a product that only works on a flat wall, or the other way around. You probably have one of two situations, a plaster or drywall arch with bare edges, or a plain wall where you want the arched look without moving a stud. This guide gives you the arc-length math for trimming a real opening, a sizing table for faux arches, and the honest line on which of our kits fit. For the faux route, a molding for archway kit like our U-shaped arch arrives pre-cut with the curve and legs already joined. The Arched Wall Molding, Curved MDF Decorative Wall Panels is a ready-to-assemble kit in paintable MDF.
Quick answer: For a real arched opening, use flexible casing: each curved edge is 3.14 x half the opening width, so a 36 in arch needs about 57 in of inner curve, plus legs and 10% spare. For a faux arch on a flat wall, use a pre-cut arch kit, 22 to 38 in (56 to 96 cm) wide. For a pre-cut MDF version, see the Double Arched Wall Molding Set.
Key takeaways
- Rigid molding cannot follow a curved opening; real archways need flexible casing or a site-made curved piece.
- A semicircular arch's curved length is pi x half the width: 47 in for a 30 in opening, 75 in for a 48 in opening.
- Casing makes the outer curve longer than the inner one; add pi x the casing width to every outer edge.
- Our arch kits are flat-wall frames from 56 to 96 cm (22 to 37.8 in) wide, from $58.40 for one panel.
- A faux arch should be wider than the furniture under it by at least 4 in (10 cm) on each side.
Which molding for archway do you need: real opening or faux arch?
If your arch is a hole in the wall you can walk through, you are casing an opening, and the trim must wrap a three-dimensional curved edge. If your wall is flat and you want an arch shape on it, you are applying a frame. Casing a real opening is finish carpentry; applying a pre-cut arch frame is a one-afternoon DIY job. If you'd rather skip measuring and cutting, the Arch Wall Decor Molding ships pre-cut and ready to prime.
| Your situation | What to use | Difficulty | Our kits fit? |
|---|---|---|---|
| Drywall arch between rooms, bare corner bead | Flexible casing, or leave it as a clean plaster edge | High: curved miters and scribing | No |
| Square doorway you wish were arched | Reframe the opening, or paint a faux arch above it | Reframing is a contractor job | Only the faux version |
| Flat wall, want an arched niche look behind a console or bed | Pre-cut arch frame, painted inside a contrast color | Low | Yes |
| Hallway with a row of arches | Two or three matching arch frames | Low to medium | Yes, 2 and 3 panel sets |
| Wainscoting with arched tops | Half-wall arch panels under a chair rail | Low | Yes |
The honest caveat: we do not sell flexible casing for real arched openings. If that is your project, the math below still tells you how much to buy from a flexible molding maker, and you can compare it with what a faux arch on a nearby wall would cost.
How much curved molding does an arched opening need?
For a semicircular arch, the curved length is pi (3.14) times half the opening width. A 36 in (91 cm) opening needs about 57 in (144 cm) of curve on its inner edge. Because casing has width, the outer edge of each curved piece is longer, so add pi times the casing width.
Here is the method, then a ready table.
- Measure the opening width at the spring line, the height where the curve starts. Example: 36 in.
- Inner radius = width / 2 = 18 in.
- Inner arc = 3.14 x 18 = 56.5 in (143.6 cm).
- Outer radius = inner radius + casing width. With 2.25 in casing, 18 + 2.25 = 20.25 in.
- Outer arc = 3.14 x 20.25 = 63.6 in (161.6 cm). Buy flexible casing by the outer length, since that is the piece you cut.
- Add the legs: two straight pieces from floor or baseboard to the spring line, per side of the wall.
- Add 10% for trimming the joints where curve meets leg.
| Opening width | Inner curve | Outer curve, 2.25 in casing | Both faces of the wall (outer) |
|---|---|---|---|
| 30 in (76 cm) | 47.1 in (120 cm) | 54.2 in (138 cm) | 108 in (276 cm) |
| 36 in (91 cm) | 56.5 in (144 cm) | 63.6 in (162 cm) | 127 in (323 cm) |
| 48 in (122 cm) | 75.4 in (192 cm) | 82.5 in (209 cm) | 165 in (419 cm) |
| 60 in (152 cm) | 94.2 in (239 cm) | 101.3 in (257 cm) | 203 in (515 cm) |
| 72 in (183 cm) | 113.1 in (287 cm) | 120.2 in (305 cm) | 240 in (611 cm) |
Segmental arches (shallow curves)
Many archways are not half circles but a gentle curve. Measure the width (c) and the rise (h), the height from the spring line to the top of the curve. Radius r = c² / (8h) + h / 2. A 48 in opening with an 8 in rise: r = 2,304 / 64 + 4 = 40 in. The curve then spans an angle of 2 x asin(24 / 40) = 1.29 radians, so the inner curve is 40 x 1.29 = 51.5 in (131 cm). That is 24 in shorter than a full half circle of the same width, which matters when flexible casing is sold by the foot.
How to size a faux archway on a flat wall
A faux arch looks right when it is wider than whatever sits under it and tall enough to read as a doorway shape. Leave at least 4 in (10 cm) of wall on each side of a console, bench or headboard inside the arch, and keep the top of the curve at least 12 in (30 cm) below the ceiling so the shape does not look squeezed.
Our arch kits come in fixed widths, which makes planning simple. Use the widths below against your furniture.
| Kit width | Fits furniture up to | Typical use | Price, 1 panel |
|---|---|---|---|
| 56 cm (22 in), MDF arch kit | 14 in (36 cm) | Narrow hallway, plant stand, sconce niche | $58.40 |
| 70 cm (27.6 in), U-shaped 70 x 135 cm | 19.6 in (50 cm) | Entry bench, side table, mirror frame | $81.40 |
| 76 cm (29.9 in), MDF arch kit | 22 in (56 cm) | Small console, reading chair | $73.60 |
| 96 cm (37.8 in), U-shaped 96 x 148 cm | 29.8 in (76 cm) | Console table, desk, single bed headboard | $88.50 |
For the full painted-arch effect, paint the inside of the arch one shade darker than the wall, or the same color in a higher sheen. The molding then frames a "doorway" of color. Two or three arches side by side make a hallway or dining wall feel like an arcade; our 2 and 3 panel sets keep the curves identical.
Caveat: these are MDF kits for dry interior rooms. A faux arch next to a shower or in an unheated porch is the wrong place for them.
How to install a pre-cut arch frame
A pre-cut arch goes up in about an hour of work plus curing time: mark a center line and the spring point, dry-fit, glue, fill and paint. Our arch kits arrive with the curve already cut to its radius and factory-cut joints where the curve meets the straight legs, so there is no bending or kerfing on site.
- Mark the center line of the arch with a level, then mark the spring point, the height where the curve meets the legs.
- Dry-fit the curved section against the legs on the floor to confirm the joint closes.
- Glue the curve first, centered on your line, with construction adhesive. Tape it in place while the adhesive grabs.
- Add the legs, checking each with a level. The legs set the look: if they lean, the whole arch reads crooked.
- Fill the two joints, caulk both edges, sand lightly.
- Prime the cut ends. These are raw, sanded MDF kits, so prime the cut edges before your top coat.
- Paint the frame, then the inside of the arch if you are doing a contrast fill.
Pro tips
- Trace the arch on kraft paper first and tape the template to the wall for a day. Arches look larger on the wall than on a screen.
- Line the arch up with something real: the center of a console, a pendant light or the window across the room. A faux arch floating at random looks like a sticker.
- Rest the bottom of the legs on top of the baseboard, not on the floor, so the arch looks like it belongs to the room's trim.
- For a real opening, let flexible casing sit in the room for a day before fitting; it bends more evenly at room temperature.
- On a segmental archway, make a cardboard template of the curve from the wall itself. Openings are rarely the exact radius on the drawing.
- For a row of faux arches, keep the gap between arches equal to the side margins at the ends of the wall.
Common mistakes with archway trim
- Buying straight casing for a curved opening. Rigid molding will not bend to an arch and kerf-cutting it rarely looks clean. Fix: flexible casing, or a clean plaster edge with no casing.
- Measuring only the inner curve. The outer edge of 2.25 in casing on a 36 in arch is 7 in longer. Fix: buy by the outer arc in the table above.
- A faux arch narrower than the furniture. A 30 in console under a 30 in arch looks jammed. Fix: arch width at least furniture width plus 8 in (20 cm).
- Leaning legs. The eye checks verticals first. Fix: level each leg, not just the curve.
Shop the look
- MDF Arched Wall Molding Kit: three widths, 56, 76 or 96 cm, from $58.40; the most flexible sizing for faux arches.
- Arched Wall Molding, Medium or Large: 70 or 96 cm wide, from $83.40, sold in 1 to 3 panel sets for a row of arches.
- Half Wall Arched Molding: arched-top panels for wainscoting under a chair rail, 1 to 5 panels from $73.60.
Frequently asked questions
What kind of molding is used for archways?
Real arched openings use flexible casing, usually a polyurethane or polymer molding that bends to the curve, or a custom curved piece made on site. Flat walls with an arch design use pre-cut arch frames that are glued on.
Can you put crown molding on an archway?
Not in the usual sense. Crown molding is made for the straight joint between wall and ceiling. An archway gets casing that follows the curve; any crown in the room simply stops at the wall on either side.
How do you make a flat doorway look arched?
Without reframing, apply an arch frame on the wall above or around the doorway and paint inside it a contrast color, or reframe the head of the opening with a curved header, which is a contractor job.
Is flexible molding hard to install on an archway?
It is harder than straight trim: the curve must be scribed to the wall, the joint where curve meets leg must be fitted, and the piece glued and pinned while it holds the bend. Budget extra time and material.
What size should a faux arch be?
Wide enough to leave 4 in (10 cm) of wall on each side of the furniture inside it, and with its top at least 12 in (30 cm) below the ceiling. For a console up to 29.8 in (76 cm) wide, a 96 cm (37.8 in) arch fits.
See more curved layouts in our arched wall molding ideas. Ready to choose? Browse all Arched Wall Molding kits, or plan your wall with live pricing in our Wall Designer. Need a size we do not list? Contact our support team with your wall measurements.
Last updated: October 7, 2026
