Seiko Alpinist SPB121 vs SPB507 vs SPB155
The classic 39.5 mm Alpinist, the newer 6R55-powered SPB507 and the smaller 38 mm SPB155 ("Baby Alpinist").
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The short answer
The SPB155 is the smallest at 38 mm and 46 mm lug-to-lug. The SPB507 is the thinnest (12.7 mm) and the only one with the 6R55 (72 h). The SPB121 is the classic 39.5 mm size with the 6R35. All three take 20 mm straps and are rated to 200 m.
Naming: Seiko currently sells these under Prospex "Land". "Alpinist" and "Baby Alpinist" are the names buyers search for; we use them for clarity, not as Seiko's own model names.
Side-by-side specs
| Spec | Seiko SPB121 | Seiko SPB507 | Seiko SPB155 |
|---|---|---|---|
| Case diameter | 39.5 mm | 39.5 mm | 38 mm |
| Thickness | 13.2 mm | 12.7 mm | 12.9 mm |
| Lug-to-lug | Not published by Seiko | 46.4 mm | 46 mm |
| Lug width | 20 mm | 20 mm | 20 mm |
| Calibre | 6R35 | 6R55 | 6R35 |
| Movement | Automatic | Automatic | Automatic |
| Power reserve | 70 h | 72 h | 70 h |
| Hacking seconds | Not stated | Yes | Yes |
| Water resistance | 200 m | 200 m | 200 m |
| Crystal | Sapphire | Sapphire | Sapphire |
| Case material | Stainless steel | Stainless steel | Stainless steel |
| Ships on | Not stated | Cow leather | Bracelet |
| Weight | Not published by Seiko | 88 g | 144 g |
| US list price | $725 (US list) | $900 (US list) | $725 (US list) |
| Status | In production | In production | In production |
| Suits wrists of about our sizing guide | 15.7–17.7 cm | 15.7–17.7 cm | 15.5–17.0 cm |
Specs summarised in our own words from the brand's product pages: SPB121 source (checked 23 Oct 2026) SPB507 source (checked 23 Oct 2026) SPB155 source (checked 23 Oct 2026). "Not published" means the brand page we checked doesn't list it, so we leave it blank rather than guess.
Wrist fit
Lug-to-lug (tip of the top lugs to tip of the bottom lugs) decides whether a watch overhangs your wrist more than the diameter does. Enter your wrist to see how each one should wear.
- Seiko SPB121 –
- Seiko SPB507 –
- Seiko SPB155 –
Same rule of thumb as Watch Finder: case diameter of about 23.5% of your wrist circumference (−2 mm to +2.5 mm), and lug-to-lug no longer than about 29% of it plus 1 mm. A guide, not a rule; flat or round wrists differ.
Strap and lug-width fit
- Seiko SPB121: 20 mm lugs, ships on a strap or bracelet
- Seiko SPB507: 20 mm lugs, ships on cow leather
- Seiko SPB155: 20 mm lugs, ships on bracelet
All of these share the same lug width, so straps move between them freely. Seiko does not publish spring-bar diameter or a strap code, so check the hole size on any aftermarket strap. More in our Seiko Alpinist strap guide and the Seiko lug width chart.
Which should you buy?
38 mm with a published 46 mm lug-to-lug.
12.7 mm thick with a 72 h 6R55.
39.5 mm for $725 (US list), the same list price as the SPB155.
All three take 20 mm straps.
Prices and where to buy
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Questions
Which Alpinist is smallest?
The SPB155, at 38 mm across and 46 mm lug-to-lug.
What is the Alpinist lug width?
20 mm for the SPB121, SPB507 and SPB155 per Seiko. The 38 mm SPB249 is an exception at 19 mm.
What is the SPB121 lug-to-lug?
Seiko doesn't publish it on the product page we checked, so we don't show one.
Which movement is better?
The SPB507's 6R55 has the longer power reserve (72 h vs 70 h for the 6R35).
Watch.co is independent and not affiliated with any brand. We haven't handled these watches for this page: everything here comes from brand-published specs and our own analysis. Brand names are used only to identify products.