This is a cutting problem, not a division
You cannot buy 14.2 metres of skirting. You buy lengths, usually 3.0, 3.6, 4.2 or 4.8 metres, and what decides your order is not the total at all. It is whether each individual wall can be cut from one length without a joint in it.
Take four walls of 2.4, 2.4, 2.4 and 3.9 metres. That is 11.1 metres of run. Divided by a 4.2 metre length it is three, and three is what nearly every calculator will tell you. Cut it properly and it is four, because the 3.9 metre wall takes almost a whole length on its own and each 2.4 metre wall leaves 1.8 metres that will not reach any of the others.
An offcut is only worth what it can be used for. A 1.1 metre piece represents real timber and is completely useless on a 2.4 metre wall. So this tool cuts your actual walls out of actual lengths, longest first, which is what you would do at the saw, and shows you the plan rather than a number.
The stock length changes the answer more than you would think
Longer is not automatically better. Three walls of 3 metres come out exactly right in 3 metre lengths with nothing wasted. The same three walls in 4.2 metre lengths waste 1.2 metres each, which is 3.6 metres of skirting in the skip.
The tool runs your walls against every stock size and tells you if a different one uses less timber overall. Worth checking the merchant actually carries it in your profile before you rely on it, because not all of them stock every size in every pattern.
An L-shaped room does not need less skirting
This one is genuinely counter-intuitive and it is the same fact the room area tool leads on, except here it costs you material.
Notching a corner out of a room reduces the floor area and leaves the perimeter exactly as it was, because the two faces of the notch replace precisely the wall length the notch removed. Because the area went down, people assume everything did, order against the smaller figure and come up short on the last wall.
Why the wastage allowance is not padding
Every external corner is two mitred cuts, and both are angled, so what comes off is a wedge that fits nothing anywhere else. Two external corners eat more than most people allow for.
Internal corners are usually scribed rather than mitred, where one board is cut to the profile of the other. That wastes less material and ruins more boards, because a scribe is a fiddly cut and the first one of the day generally goes in the bin. The allowance is for the cuts that go wrong, not for the arithmetic, which the plan has already done.
Doorways break a wall, they do not shorten the room
A doorway takes its own width out of the run, and that part is simple. What is less obvious is that it splits the wall into two pieces, and both pieces still have to be cut from a length.
So a 4 metre wall with a door in it is not 3.24 metres of skirting. It is a 1.6 metre piece and a 1.6 metre piece, and if you are unlucky with the cutting they come off two different lengths. If you have a wall like that, enter the two pieces separately and the plan will handle it properly.
Architrave is decided by the leg, and the arithmetic surprises people
A set is two legs and a head. A standard UK door is 1,981mm tall, and the leg runs past the head by about 150mm, so a leg is 2,131mm.
Two legs is therefore 4,262mm, which is more than a 4.2 metre length. That means every common stock size up to and including 4.2 metres gives exactly one leg and a remainder too short to be a second one. A single door needs two lengths for its legs whether you buy 3 metre or 4.2 metre, and only a 4.8 metre length gives you two.
Dividing total architrave by stock length misses that completely, and it is the reason people come home with either twice what they need or half of it.
Measure at the floor, wall by wall
Do not work from a floor plan. Skirting follows the wall, and walls bow: a run that measures 3.98 metres on the drawing can want 4.02 metres of timber to sit tight along its whole length.
And keep the offcuts over about 600mm. They are what a short return by a doorway, a boxing-in or a repair in five years gets cut from, and matching a discontinued profile later is a great deal more annoying than storing a metre of timber in the loft.
Common questions
How much skirting board do I need?
Not the total run divided by the length, which is what most calculators give you. Skirting is sold in fixed lengths, so what matters is whether each wall comes off one length in a single piece. Four walls totalling 11.1 metres divide into three 4.2 metre lengths on paper, and take four in practice, because the offcuts are all too short for the next wall.
Does an L-shaped room need less skirting?
No. The floor area drops but the perimeter does not move at all, because the two faces of the notch replace exactly the wall length the notch removed. Because the area went down, people assume the skirting did too and order short. It is the same trap as in the room area tool and this is where it costs you.
What length of skirting should I buy?
Whichever one your particular walls pack into best. Check rather than defaulting to the longest. Three 3 metre walls come out exactly right in 3 metre lengths and waste 1.2 metres each in 4.2 metre lengths. The tool works your walls against every stock size and says if a different one uses less timber.
Why do I need a wastage allowance if you have done a cutting plan?
Because every external corner is two mitred cuts and both are angled, so the offcut is a wedge that fits nothing. Internal corners are usually scribed instead, which wastes less material and ruins more boards, since the first scribe of the day generally goes in the bin. The allowance covers the cuts that go wrong, not the arithmetic.
How much architrave does a door need?
Two legs and a head, and the leg is what decides the order. A leg is 2,131mm including the run past the head, so two legs is 4,262mm, which is more than even a 4.2 metre length. Every common stock size up to 4.2 metres gives exactly one leg and a remainder too short for another, so a single door needs two lengths for its legs whether you buy 3 metre or 4.2 metre. Only a 4.8 metre length gives two.
What if a wall is longer than the skirting I can buy?
It has to be joined, and the joint should be a 45 degree scarf rather than a butt. A butt joint opens as the timber moves and shows as a black line, and it will be on the longest and most visible wall in the room. Put the joint away from the door rather than in the middle where the eye goes.