How to cut baseboard corners

A square corner takes a 45 degree cut on each piece. Hardly any corner is square. Gary Katz, writing on baseboard for THISisCarpentry, measured one at 86 degrees and said a wall that far out is nothing unusual. There is a formula for the rest of them: set the saw to 90 minus half the corner angle. That 86 degree corner needs the saw at 47, not 43 and not 45. Here is how to read a corner, cut both joints, and close the ones that fight you.

What angle do you cut baseboard corners?

Set the saw to 90 minus half the corner angle. A square 90 degree corner gives 90 minus 45, so the saw reads 45. An 86 degree corner gives 90 minus 43, so the saw reads 47. One formula covers inside corners, outside corners, and bay windows.

Watch which way the numbers run. A tighter corner needs a bigger saw setting, and every 2 degrees the wall is off moves each cut by 1 degree, because the two pieces split the error.

The divide-by-two trap

Half the internet says to halve the corner and cut that. Half of 86 is 43, and 43 is a real number here: it is the angle the cut face makes with the length of the board. It is not the number on the saw. A miter scale counts up from a square crosscut, so 0 is a straight cut and 45 is a true 45. You want 90 minus 43, which is 47. At exactly 90 degrees both methods land on 45, which is why the bad version survives. On an 86 degree corner the shortcut sends you to 43, four degrees the wrong way, and the joint yawns open at the face.

Katz runs the same arithmetic and gets 47. Check that your saw goes there: a Makita LS1019L swings 60 degrees either way with detents at 0, 15, 22.5, 31.6, and 45, and a DeWalt DWS779 gives 60 right and 50 left. That odd 31.6 stop is not for base. It is the flat-cut miter for 38 degree spring crown molding.

How do you measure a corner that is not square?

Hold a digital angle finder into the corner with both legs flat on the walls and read the number, or set a sliding bevel to the corner and carry it to the saw. Do that at floor height, where the base actually lands, and do it at every corner in the room. Two corners on one wall rarely agree, so a single saw setting for the whole job is a bad bet.

One thing the angle finder cannot see is the mud. Tape and corner bead build the corner up, so a stiff board rides the high spot and pivots off the wall near the joint. Measure, cut to the number, then dry-fit. The last degree gets found on the wall, not on the saw.

How do you cut an inside baseboard corner?

Cope it. Run the first piece square into the corner, cut the second at a 45 degree inside miter, then saw away everything behind the profile line that miter exposes. The shaped end rides over the face of the first board, so a corner a few degrees out still closes tight.

The sequence goes like this. Cut piece one with the saw at 0 and push it into the corner. On piece two, set 45, cut an inside miter, and pencil the front edge of the cut so the profile line stands out. Follow that line with a coping saw, tilting the blade to take more off the back than the face. Fine Homebuilding describes the goal as leaving the face of the cut slightly proud of the back side, and trade practice keeps that tilt under about 10 degrees. A 15-tooth blade suits baseboard, and a jigsaw wearing a Collins Coping Foot does it faster once you trust it. Test-fit, rasp the high spots, repeat.

Fine Homebuilding gives two reasons for spending the extra minutes: a coped joint does not need the square corner a miter needs, and it is less likely to open after a few seasons of expansion and contraction. The NAHB guidelines side with the trade and ask for inside trim corners to be coped or mitered, with a butt joint allowed only on square-edge stock.

How do you cut an outside baseboard corner?

Miter both pieces, because there is no coped version of an outside corner. Wrap the angle finder around the outside of the corner, read it, and use the same 90 minus half rule. Then glue the joint and pin it, since an outside miter aims its seam straight into the room where everyone sees it.

The measuring flips too. On an inside corner the wall-to-wall length is the back of the board, the long point. On an outside corner you measure to where the walls break, and that is the short point at the back; the face runs past it by the thickness of the board. Cut long and sneak up on the fit.

Family Handyman's rule is the one to keep in your head. If the miter is open at the front, raise the setting toward 46 and recut both sides, taking a hair off each. A gap at the back rides against the wall and nobody sees it. A gap at the front is the whole job. Katz glues his outside corners and shoots 23 gauge pins through them, which holds the faces in line while the glue sets.

Splices, nails, and the runs between corners

Any wall longer than your stock needs a splice, and a butt joint will show a seam the first dry winter. Cut opposing 45s so one piece laps the other. This Old House calls for glue in the overlap and finish nails through it. Land the joint over a stud and face the lap away from the door you walk in through.

Two of Katz's numbers are worth stealing. Add 1/16 inch to your measurement on walls over 8 feet and 1/8 inch over 12 feet, then bow the piece and snap it in so it presses into both ends. Find the studs and drive a nail at least every 32 inches on center, angling the low nails into the bottom plate.

Bay windows and other odd corners

The formula holds when the corner stops looking like a corner. A bay turns 45 degrees off the wall, so the two walls meet at 135, half is 67.5, and each piece gets cut at 22.5.

Sprung trim is the one case that needs more than a miter. Base cap, cove, and crown lean out from the wall instead of sitting flat against it, so they take a bevel as well as a miter. Feed the corner you already measured plus the spring angle of the profile into the crown molding angle calculator and it returns both settings. For crown over those same walls, how to cut crown molding for odd corners covers measuring the ceiling line, and inside vs outside corners sorts out which way the saw swings.

Sources

• Gary Katz, "Installing Baseboard," THISisCarpentry: the 86 degree corner worked to a 43 degree miter and a 47 degree saw setting, the 1/16 and 1/8 inch length allowances, the 32 inch nail spacing, and glue with 23 gauge pins on outside corners (thisiscarpentry.com).
• Fine Homebuilding, "Coping Moldings": the back-bevel technique and why a coped joint beats a miter on an inside corner (finehomebuilding.com).
• Family Handyman, on cutting baseboard corners: raising the setting toward 46 when an outside miter is open at the front (familyhandyman.com).
• This Old House, "How To Install Baseboards": the glued 45 degree scarf joint (thisoldhouse.com).
• NAHB Residential Construction Performance Guidelines, Fourth Edition, sections 4-1-1, 10-3-3, and 10-3-4: the wall plumb tolerance, the coped-or-mitered rule, and the 1/8 inch mitered trim gap limit.
• Published Makita LS1019L and DeWalt DWS779 specifications: miter capacity and detent positions.