How to calculate the perimeter or girth of rectangular building with a set-back

A rectangular plan of a building with the length = 20,000mm and width = 9,000mm and wall thickness of 300mm.

It important to first note if the dimensions for a drawing are given from “out-to-out” or “in-to-in”. 

The formula for finding the Girth is 

2(L + W) ± 4(T) + 2r

If the dimensions are out-to-out then the formula turns

2(L + W) - 4(T) + 2r

If the dimensions are in-to-in then the formula turns

2(L + W) + 4(T) + 2r

Where L is the length (most times the longest side) of the building

            W is the width of the building

            T is the wall thickness

            r is the length of recess (is used only in the presence of a recess).

CALCULATION

In figure 3 we can see that the plan has a set-back corner at the bottom left corner. If the set-back corners are at 90 degrees to each other, we can assume it to be normal rectangle by drawing imaginary(dotted) lines to complete the rectangle and treat it like a rectangle plan

Based on the example the dimensions are both Out-in-out

Therefore, the girth can be calculated thus;

Note: convert dimensions to metres.

Thus:   L = 20,000mm = 20.00m

            W = 9,000mm = 9.00m

            T = 300mm = 0.3m

Thus,

Girth = 2(L + W) – 4(T) for out-to-out

            = 2(20.00 + 9.00) – 4 (0.30)

            = 2(29.00) – 1.2

            = 58.00 – 1.2

            = 56.8

Thus girth or perimeter is 56.8mm

Note: the girth is usually taken as the length of the centre line.