How to Bisect an Angle

How to bisect odd architrave and other carpentry angles for perfect miters

To bisect an angle is to cut or divide it into two equal angles.

There are several ways to go about equally dividing an angle up, I'll explain on this page how I do it with a bevel, or a compass, or on the wall and show you a tool I've bought to speed the process up when working on site.

Quite often during framing and finish carpentry work I encounter an area where an angle needs to be divided into two.

Some common examples include when installing architrave to angled doors (under stairs etc), fitting fascias and soffits, scribing skirting boards (in bay windows etc.) and when applying quadrant or other decorative moldings to stair strings for example.

You don't need fancy tools to bisect an angle but bisecting angles has become a lot quicker and far more accurate since I invested in a great little Trend angle fix tool. It's quick as all you do is push it up to the two edges to bisect and it automatically finds half. Then you can walk straight over to the saw and start setting out/cutting the work piece.

The old fashioned way is to use a bevel.

How to bisect an angle with a bevel

If you already have a Bevel you can use that to bisect an angle instead of buying a purpose made tool:

Step 1: To bisect an angel with a bevel, the first thing to do is set a bevel to the angle you want to bisect, then grab a scrap piece of timber. Butt the handle of the bevel up to the bottom edge, and mark a pencil line on both sides of the bevels blade like below.

bisect an angle

Step 2: Next, move the blade of the bevel to mark the blades width up from the bottom edge of the timber, intersecting the lines already marked.

bisect an angle

Step 3: Finally, draw a line from point to point like the purple one in the picture, and butt the bevel back up to the timber in it's original position and turn the blade to that new line.

bisect an angle

How to bisect an angle with a compass

Step 1: To bisect an angle with a compass, the first thing I do is again grab a scrap piece of timber. Mark the angle on there right from the corner like in the picture below, then take a compass and put the point in the bottom left hand corner. Set the compass to mark two lines like the red ones in the pic, as long as they are an identical distance from the corner it doesn't matter what the measurement is.

bisect an angle

Step 2: Next, put the compass at the point where the red lines marked/intersected the angles and mark two more lines further away, long enough to intersect each other and mark an 'X' opposite the angle being bisected.

bisect an angle

Step 3: Finally, draw a line from point to point again and re-set the bevel to the new bisected angle.

bisect an angle

Bisecting an angle on stair stringer

Sometimes, like in the picture below you can bisect an angle by putting each piece of the molding in place and marking a pencil line along the top. Then simply draw a line from point to point, where they intersect. Replace the molding and transfer the lines, then cut each on a mitre saw (I use a coping saw for tiny moldings like the one below).

You'll then need to check and adjust the cuts if necessary before fixing. For this reason, cut these bisected angle first whenever possible and once they fit cut the other end of the timber.

bisect an angle

bisect quadrant

Bisect architrave angles

The same can be done to bisect an angle with architraves. This is another area where the angle finder comes in handy. If you don't have one yet you can follow these simple steps instead,

Step 1: Mark the margin

Set a combination square to the size you want the margin to be at and mark this round the lining, especially where the two moldings will intersect (below left).

bisect an angle

Step 2: Mark the wall

The idea now is to put pieces of the architraves in place up to the margin one by one and draw around them, marking the wall where they intersect each other. Do this with each piece letting them sail past enough for the other angles to be cut later. Because I know the square corners will be 45 degree cuts, I leave them till last after I've fitted the more difficult bisected angles perfectly first.

Next, draw a line at each miter from point to point, from where the inside edges intersect to where the outside edges intersect (above right).

Step 3: Mark and cut

With an architrave like the one below, I'd start on the right hand side and again leave the right angle till last. Put the right hand architrave in place and mark it at both points where the margins intersect. Next, set up a bevel to the angle now marked on the wall. Set the miter saw to the angle and cut the architrave, keeping the saw blade to the side of the marks that isn't needed (waste side). Do the same for the next piece and then glue/nail the first one in place. Check the second piece fits tight to the first, adjust to suit and when happy mark the margin on the opposite end. Repeat the process fitting each end perfectly at a time before moving onto the next. Finally, you don't need to bisect an angle at the end but finish with a straight forward 45 cut.

Click here for help installing architraves

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