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How to Measure and Cut a Skirting Board for an Internal Wall Joint by Bisecting

Published : 10/01/2018 10:10:18
Categories : Video Guides

In this guide, we will showcase how to measure and cut skirting board for an internal wall joint by bisecting an angle. We have included a step by step guide and a video below:

When fitting skirting board you may find instances where you need to cut the boards to fit internal angles that are different from 90 degrees. This tutorial is designed to show you how to calculate the required angle for each cut, as well as how to make said cuts on a mitre saw.


Begin by using a sliding bevel to recreate the angle you will be cutting. Lock the angle in place when it is in the correct position to ensure it doesn’t move.

Marking angle with sliding bevel 

At this stage, you are essentially trying to identify the value of HALF of the angle you just measured with the sliding bevel. The reason for this is that to properly fit skirting boards around said angle, you will need to make cuts on TWO pieces of skirting, that when joined together recreate the total measured angle exactly.


The first step to calculating the necessary angle to cut is to take the bevel that is fixed in place at the overall internal angle of the wall, and transfer this angle to a scrap piece of wood or skirting board offcut. It will become clear why this is necessary in a moment, but essentially there are a few steps to take to ensure the angles are properly calculated and these steps require you to draw out a plan of the internal angle.

To transfer the known angle to the offcut, place the bevel onto the offcut so that the handle of the bevel is flush with the bottom edge of the offcut, and the actual sliding part of the tool (the metal edge in the case of our video tutorial) intersects the top edge of the offcut. This will make the measuring part of this exercise much easier than if, for example, the bevel intersected at one of the side edges of the offcut (see picture for our placement of the tooling).


With the bevel in position, use a pencil to mark a line along the full length of the metal part of the bevel, so you are left with a marking that runs from the top edge of the offcut to the bottom, and that looks like the angle on the wall that you are trying to replicate (see image)

Marking out bevel angle on skirting board

Once you have this initial marking drawn out, the next stage is to add further markings to the offcut that will enable you to accurately calculate the required cutting angle for the skirting.


For the next step you will need a pair of compasses and a pen or pencil. Place the pen or pencil into the compasses, and extend the arms so that they are at 50% of their maximum extension. Position the needle part of the compasses so that the needle resides at the base of the board (see picture). Place the pencil ‘arm’ of the compasses onto the board and rotate to make an arc that looks somewhat like a quarter circle. This first line should be drawn so that it cuts through the first angle line you drew a moment earlier.

mark out angles on skirting board

With the needle of the compasses remaining in the same position, move the pencil arm around so that when you draw another similar arc, it ‘cuts through’ the base of the board. This is tricky to describe so please refer to the still images for guidance on the compass placement.

 Compass mark on skirting board

You are about to create another set of markings on the offcut. It is important that these markings are able to cross each other so extend the reach of the compasses for these markings. Try 75% of the maximum reach of the compasses initially, but be prepared to extend them further if necessary.

 finding the bisect line

With the compasses now set in position, place the needle onto the ‘baseline’ of the offcut exactly at the point where the arc you drew a moment ago cuts through the baseline (refer to still image for clarification). With the needle in position, repeat the process of drawing a quarter circle arc.


Reposition the pair of compasses so that the needle is placed at the point where the first arc you drew cut through the line you made at the outset with the sliding bevel (again, see picture for further clarification). Once again use the pencil to draw an arc. All being well you will find that this arc intersects (crosses over) the arc you just made. If for some reason this is not the case you may need to further extend your compasses and draw both arcs again.

 2nd mark for bisecting angle

You should now be looking at an offcut with one straight line marking running from the baseline of the board to the top edge, and 4 quarter circle arcs. You are only now interested in the two arcs that form a cross. Using a straight edge and a pencil, create a new marking that is a continuous straight line that runs through the point at which the two arcs cross each other, and the point the original straight line you drew reaches the baseline of the board (see picture).

bisect line  

This final line is called the bisect line.

bisect line 

Looking at the board in front of you, you should be looking at a V shape made from the 2 solid straight lines you have drawn (ignoring the arcs at this point.) This V may be wide or narrow depending on the original angle of the wall. Your next job is to measure the sized of this angle with a protractor.

Place the protractor so that the left-hand arm of the V (the first line you drew) runs straight through the protractor at zero degrees. Now, look at where the second arm of the V runs through the protractor- this will give you the value of the angle inside the V. In the case of this example, the value of the angle is 67.5 degrees. Make a note of this angle size so you don’t forget it!

measuring bisectline


You are finally ready to calculate the angle required for the chop saw. To do this, you simply subtract the internal angle of the V from 90 degrees. In this example, 90 - 67.5 = 22.5 degrees. (see photo).

math bisect line


You can now cut the two pieces of skirting board that are to be joined together to form the required internal angle. Set up the mitre saw to the correct angle- 22.5 degrees in this case. Take an offcut or scrap piece of skirting board and cut it at this angle. Then, set the mitre saw to the same angle again but cutting in the opposite direction and cut another offcut or scrap piece at this angle. Take both pieces over to the wall and place them in position. You should find that they have made a perfect internal join. If you are happy that the join is tight, you can continue to make the cuts on the actual pieces of skirting board you are fitting.

set mitre saw for bisect cut

The pieces should correspond with the angle as shown below.

Bisect Skirting Board

About the Author

Kieron Miller is the Managing Director of Skirting 4 U and has been working in the skirting board and online retail industry since 2012. Kieron has been using his extensive product knowledge and problem-solving experience to help both trade and private consumers all over the country find the perfect skirting board products for their projects. He continues to innovate new solutions and product ideas as the market continues to grow.

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