Rarely, any Diamond or Gemstone is faceted exactly 1.000 carats or 7.00 MM For that reason trade goes by tolerances in size in both, mm and ct.
Diamond Physical MM Size:
MM's are considered in tolerance per quarters. For example, trade defination of 7 x 5 oval tolerance if 7.25 x 5.25 mm to 6.75 x 4.75 mm. While such difference is insignificant at eye level, it is often important to note if anyone is new with Diamonds and Gemstones. Trade standard tolerances are used daily worlwide and trade agreed range is always per 1/4 mm to either way +/-.
Diamond Physical CT Weight:
Diamond CT / Carat weight is based on also tolerance. Since rarely one will find any stone at exact 1.000 carats, trade uses tolerances for such. For example, 1/4 carat Diamond can be .23 carats to .27 carats. Mostly you will find specifc decimal listed in any sale offer. Below quick and most commonly used carat trade tolerances:
1/4 Carat: .23 to .27 Carats - but can be up to .30 carats also, while not below .23
1/3 Carat: .31 to .35 Carats - but can be up to .47 carats also, while not below .31
1/2 Carat: .48 to .52 Carats - but can be up to .72 carats also, while not below.48
3/4 Carat: .73 to .77 Carats - but can be up to .90 carats also, while not below .73
It is often wise to examine tolerances and never to assume automatically that every natural Diamond at 1/4 carats is always just .25 and nothing else.