The size is a strap length, not a body measurement
Gucci defines the size of a belt as the distance from the outer edge of the buckle to the third hole of the strap, and that single sentence explains most of the confusion around the style. A belt stamped 90 is 90 cm along that line, so it does not sit at a 90 cm waist; it sits wherever 90 cm from the buckle edge to the middle hole lands on the wearer. Two neighbours on the ladder are 5 cm apart, which at the waist is roughly one trouser size on the women's chart and one on the men's, so the step is fine enough to correct in one move if the first length is wrong. Measuring a belt you already wear, from the buckle edge to your most-used hole, is the most useful number to send.
Two ladders, 65–120 and 75–120
The women's ladder starts at 65 cm and runs to 120 cm, paired with trouser sizes from US 23/00 to US 34/20 and jeans from 20 to 42. The men's ladder starts at 75 cm and runs to 120 cm, paired with US 26 to 44 and jeans 26 to 44. The same centimetres therefore carry different US trouser figures depending on the chart: 90 cm is US 28 on the women's ladder and US 32 on the men's. Both ladders print Italian, European and UK trouser equivalents alongside the US column, and both are numbered in 5 cm steps from top to bottom, so the number on the strap is comparable across the two even when the US reading is not.
Four published widths, printed in inches
Gucci publishes the strap width in inches rather than centimetres, and there are four tiers. The 1.5 inch strap — about 3.8 cm — carries the Double G on the GG Marmont wide versions for men and women, the GG Marmont matelassé wide, the Gucci Signature leather and the GG Supreme canvas straps, where the buckle becomes an interlocking G. The 1.2 inch strap is about 3 cm and appears on the women's thin GG Marmont. The 1 inch strap is about 2.5 cm and belongs to the men's slim Double G. The thinnest women's strap is 0.8 inch, about 2 cm. Width is the second number to send after length, because it is what makes two belts with the same buckle read as different products.
What the buckle adds, and how to check a belt remotely
The buckle changes scale with the strap. On the 1.5 inch wide strap the buckle is published at 3 in × 2.4 in, also written as 3.03 in × 2.36 in, roughly twice the strap width. On the 1 inch slim strap it is 2.4 in × 2 in, or 2.36 in × 1.57 in. On the 0.8 inch thin strap it is 2.36 in × 1.85 in, close to three times the strap width. Finishes vary per version rather than by tier: gold-toned and light gold-toned on the classic versions, palladium-toned on others, antiqued brass or antique palladium on some women's styles, and ruthenium or black resin on specific straps. To check a belt remotely, ask for the strap laid flat with a tape from the buckle edge to the third hole, the full strap with all holes in frame, the strap width against a ruler, and a close-up of the keeper loop and the edges.