Page "No symbol" Paragraph 4

According to the ISO standard ( and also under a UK Statutory Instrument ), the red area must take up at least 35 % of the total area of the sign within the outer circumference of the " prohibition sign ".

So 35 % of everything within the outer edge of the " no symbol " must be the symbol itself.

Additionally, for printed signs under the UK rules, the width of a " no symbol " is set at 80 % the height of the area it is printed to.