Short answer

Cartier has never sold a 21mm Clé de Cartier. The line launched in 2015 with three case sizes: 31mm, 35mm, and 40mm. When a replica listing says 21mm, one of three things is true. The seller measured the strap or the lug width, the seller measured the case with a ruler, or the number came from a different model.

What the 21mm number can measure

21mm appears in watch specs as a lug width, a bracelet width, or a strap width. It is not a Clé case diameter. Ask the seller for four numbers: case width across the case back at the 3 and 9 o'clock line, case width with the crown, lug width in mm, and total thickness. Case width is taken with the crown left out. A real 31mm Clé measures 31mm on that line.

Genuine Clé de Cartier sizes

  • 31mm small model. Several steel references run on quartz.
  • 35mm medium model. Automatic movement.
  • 40mm large model. Automatic movement.

Case metal options are steel, 18k pink gold, and 18k yellow gold. Diamond bezels exist on some references. The crown is cut into a key shape, which is where the name comes from, since clé is French for key. The crystal is sapphire on every genuine piece.

Checks before you pay

  1. Case diameter. Ask for a photo with calipers on the case back, crown excluded.
  2. Movement. Ask for the caliber number. Quartz ticks once per second. Automatic sweeps 6 to 8 times per second. A 21mm case cannot hold a large automatic movement, so a piece at that size is almost sure to be quartz.
  3. Crystal. Sapphire resists a steel key scratch. Mineral glass does not. Low price listings ship mineral glass and label it sapphire.
  4. Weight in grams on a scale. Plated brass and steel weigh close, so weight alone does not prove the case metal.
  5. Case back. Check engraving depth, font, and the reference number format.
  6. Water resistance. Treat any claim above 30m on a replica as marketing.

Price bands

Clé replica listings run from about USD 60 to USD 400. Movement type, case metal, and strap material set the spread. A quartz steel piece on a leather strap sits at the low end. A 35mm automatic with a plated gold case sits at the top.

Questions to send a seller

  • Case diameter in mm, measured with calipers, crown excluded.
  • Movement type and caliber number.
  • Crystal material.
  • Lug width and strap width in mm.
  • Photos of the case back and the crown.
  • Return window in days.

Bottom line

If you want a Clé replica that matches a real Cartier size, order 31mm, 35mm, or 40mm. A 21mm listing is a measurement error or a different watch. Ask for caliper photos before payment.