The no symbol (also prohibition sign, no sign, circle-backslash symbol, or universal no) is a circle with a diagonal line through it (running from top left to bottom right), surrounding a pictogram used to indicate something is not permitted. The no symbol is usually colored red, but it is rarely colored other colors (usually black). It is also referred to as a slarcle (Slash + circle).
The Unicode code point for the prohibition sign (also called the no symbol) is
U+20E0;, Combining Enclosing Circle Backslash ( ⃠ ). It is a combining character, which means that it appears on top of the character before it, so putting d⃠ will show d⃠. (Note: This will appear only if the character exists in an available font.) It also appears in the Webdings and Wingdings 2 fonts.
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.