The Cartier Tank Must WSTA0138 is the current reference for the large SolarBeat — the solar-powered Tank on a black calfskin strap, and the number you'll find on paperwork from 2025 onward. It's also the lowest-maintenance Cartier ever made: light passing through the dial keeps it running for years on end, with no winding, no battery appointments, no service anxiety. Which leaves exactly one part of this watch that will ever ask anything of you — the strap.
That changes how you should think about strap buying on this reference. Here's the correct size, the reference numbers explained, and the straps worth owning over the long life of a SolarBeat.
Cartier Tank Must WSTA0138 Strap Size and Fit
The essentials:
- Reference: WSTA0138, Tank Must, Large model, SolarBeat photovoltaic movement
- Case dimensions: 33.7 × 25.5 mm, just 6.6 mm thin
- Correct strap size: 19 mm
On the product page, select the 19 mm option. The watch uses standard spring bars and a steel ardillon buckle, so any strap change takes a minute.
The buckle-end trap: the factory strap narrows to 16 mm at the buckle, and that smaller number sometimes ends up in spec sheets. The lug width — the number you order — is 19 mm.
The reference map: Cartier has used several numbers for the large SolarBeat on leather over the years (WSTA0059, WSTA0090, and now WSTA0138) — all the same 33.7 × 25.5 mm case, all 19 mm. The large SolarBeat on the steel bracelet (WSTA0122) is also 19 mm. Only the size changes the strap: the Small SolarBeat (29.5 × 22 mm) takes 16 mm, and the XL automatic takes 23 mm. If your case measures 25.5 mm across, you're a 19.
The Tank That Runs on Light
The Tank has been Cartier's defining rectangle since 1917, and the SolarBeat is its cleverest modern act: photovoltaic cells hidden beneath the dial, charging through micro-perforations so fine you can't see them at arm's length. The WSTA0138 wears the full classic signature — silvered dial, Roman numerals, blued-steel sword hands, the beaded crown with its blue synthetic spinel — while quietly running on whatever light your day contains, with an autonomy Cartier measures in years, not months.
Here's the ownership math that matters for this guide: a watch designed to run largely untouched for many years will comfortably outlive several leather straps. Its black calfskin is the only consumable on the entire watch. So the question isn't whether you'll buy straps for the WSTA0138 — it's which ones, and in what order. Below is the sequence that serves this watch best.
1. Black Leather – Your First Replacement, Before You Need It
The factory calfskin is worn every day on a watch that never rotates out — which is exactly the wear pattern that creases and darkens leather fastest. A black leather strap for the Cartier Tank is the like-for-like you'll reach for first: the factory look, restored in a minute, at a fraction of a boutique replacement. Buy it before the original wears out and rotate the two — each strap rested is a strap that lasts far longer. (If you want the factory's exact grained texture, a grained black option is available in the full Tank collection linked below.)
2. White Leather – Give the Light Back
There's a nice logic to this one: the SolarBeat's dial spends all day drinking light — a white strap is the piece that reflects it. On the silvered dial and steel case, white turns the quietest Tank into a bright, summery watch that feels completely different from the factory black, and it's the pairing almost no other Tank owner will have. The white leather strap is the seasonal half of your rotation.
3. Blue Leather – The Only Color the Watch Has
Every Tank Must hides one color in plain sight: the blue of the spinel in the crown and the blued-steel hands. A blue leather strap draws that thread out and makes it the watch's identity — subtle across a room, clearly intentional up close. The most popular way to personalize this reference without leaving Cartier's own palette.
4. Brown Leather – The Strap That Ages With It
A SolarBeat is a keeper — the kind of watch that's still on your wrist a decade from now. Brown is the one strap color that improves on that timeline: it softens, deepens and takes on character while black simply wears. A brown leather strap also gives the Tank its most classical look, the one it has carried for a century. The long-game pick.
One note for this size: crocodile straps for the Tank run only up to 17 mm, so the 19 mm large model stays with smooth leather — where, honestly, a 6.6 mm-thin case looks its best anyway. Every color above and more is in the full Cartier Tank strap collection, all available in 19 mm.
Keep It Slim
At 6.6 mm, the Tank Must is one of the thinnest watches Cartier makes, and it deserves straps cut to match. Thick, padded straps overwhelm a slim Tank and lift it off the wrist; every strap linked here is flat and slim, the proportion Louis Cartier designed around.
Final Thoughts
The WSTA0138 asks nothing of you — no winding, no batteries, no servicing worth mentioning. The strap is the entire ownership experience, and that makes it worth doing properly. Helvetus is one of the leading names in the aftermarket strap world, and our Tank straps are cut precisely for the 19 mm large case, buckle included. Rubber straps across our range carry a lifetime warranty, shipping is free worldwide with no import costs, and more than 50,000 watch owners already trust us with their straps.
Cartier built this Tank to run on light for years. Build it a strap wardrobe that lasts as long.
