1. Calculate total perimeter of external walls, using outer face.

2. Assuming all corners of building are right-angled, calculate the total number of internal corners subtracting the total number of external corners (rectangular building has 4 internal corners, 0 external corners), and multiply this by the average width of the wall (outside face of external wall to inside face of inner wall)

3. Deduct the above from the original perimeter to give Mean Girth (otherwise known as centre-line length calculation)

2. Assuming all corners of building are right-angled, calculate the total number of internal corners subtracting the total number of external corners (rectangular building has 4 internal corners, 0 external corners), and multiply this by the average width of the wall (outside face of external wall to inside face of inner wall)

3. Deduct the above from the original perimeter to give Mean Girth (otherwise known as centre-line length calculation)