The five Birkin sizes, measured
Every figure below is the published width × height × depth. Width is measured across the base, height is taken at the body rather than the handles, and depth is the base of the gusset. Two sizes sit close enough that buyers regularly confuse them at a glance: the 25 and the 30 differ by 5 cm of width but only by 3 cm of depth, so a photograph alone will not always separate them. The 35 is the size most often photographed on the arm, and the 40 reads as a travel or work bag once it is loaded, because a wider base carries more weight in the same panel of leather. When a description says only "Birkin", ask for the number: it is the fastest way to settle which one is in front of you.
What each size carries
The Mini 20 is a phone, a card holder and keys, and nothing bulky — with 12 cm of height and 6 cm of depth there is no room for a full-size wallet plus a case. The 25 adds a long wallet and a small water bottle without the shape distorting, which is why it is the size buyers most often choose as a daily bag. The 30 covers the same load with room left for a folded scarf or a compact camera. At the 35 you are carrying a laptop-sized flat object comfortably, and the 40 takes a 15-inch laptop plus a document folder, at the cost of weight once the bag is full. The practical line sits between the 25 and the 30 for most daily use.
Where the Birkin and Kelly separate
At the same named width the two bodies are close — a Birkin 25 is about 25 × 19 × 13 cm against a Kelly 25 at 25 × 18 × 10 cm — but the construction is not. The Birkin is open-top with two handles and no strap, so it is carried by hand or in the crook of the arm. The Kelly closes with a single top handle, a flap and a turn lock, and takes a detachable shoulder strap, so it can be worn on the shoulder or across the body. Depths also stay narrower on the Kelly through the range, from about 6 cm at the Mini to 15 cm at the 40, while the Birkin is the deeper of the two at every matching width. If you need your hands free, that difference matters more than the size number.
Describing a Birkin so the answer matches
Three details decide which bag someone shows you: the size number, the leather family, and the hardware colour. Lead with the number, because "Birkin" alone spans five widths. Then say the leather — smooth, grained or embossed — and the hardware in words (gold-tone, silver-tone or pale). If you are matching a bag you have seen, mention the stitching tone and whether the edges are painted a contrasting colour. On a photograph, check the proportions against the turn lock: on the Mini the lock is large relative to the body, and on the 35 and 40 it looks small. That single ratio separates the small end of the range from the large end faster than any measuring in a photo can.