The rim ratio is an indication of how broad a rim is.

It is the proportion of the diameter occupied by the rim, or in mathematical terms:

2 x rim width ÷ overall diameter, expressed as a percentage.

Anything between 21% and 32% is regarded as a ‘normal’ rim.  For rim widths above or below this range the following terms are now generally accepted:

• Narrow rim: anything less than 17%, so long as actual rim width is <2cm.
• Semi-narrow rim: 17% to 21%
• Semi-broad rim: 32% to 36.9%
• Broad rim: 37% to 41.9%
• Ultra-broad rim: 42% or above 

Because area is proportional to the square of diameter, on an ultra-broad rim plate or dish over two-thirds of the surface area is taken up with the rim. Even a semi-broad rim occupies over half the total surface area.