Seiko
Seiko Prospex Alpinist (SPB121) Strap Size — 20mm Lug Width
The Seiko Prospex Alpinist (SPB121) has a 20mm lug width, so it takes a 20mm watch strap.
Lug width20mm
| Lug width | 20 mm |
|---|---|
| Case size | 39.5 mm |
| Lug-to-lug | 46.4 mm |
| Thickness | 13.2 mm |
| Spring bar Ø (typical) | Not confirmed |
| End type | Straight |
Which reference do you have?
Similar-looking models can take different straps. Your reference number is on the case back or your papers.
| Model | Case | Strap |
|---|---|---|
| Alpinist SPB121 (current) | 39.5mm | 20mm |
| Alpinist SARB017 (older) | 39mm | 20mm |
Strap styles that fit
- NATO / Zulu20mm fits if the strap's thickness clears the case under the bars.
- PerlonWoven pass-through, 20mm.
- 2-piece leatherStraight ends fit flush.
- Rubber / siliconeDivers & sports fit well.
- Milanese / meshStraight-end 20mm mesh fits.
- Metal braceletNeeds 20mm end-links shaped for this case.
▲ The current 39.5mm Alpinist; the older SARB017 is also 20mm.
Fit & sizing notes for the Prospex Alpinist (SPB121)
- Width: buy 20mm straps. Tapered straps listed as 20/18mm or 20/16mm fit too; only the first number has to match. See strap sizes explained.
- Spring bars: 20mm long. We haven't confirmed the diameter for this model, so measure the old bar before buying replacements (spring bar sizes).
- Wrist fit: at 46.4mm lug-to-lug it should sit within the top of wrists from about 15.5cm up (rule of thumb: lug-to-lug under ~30% of wrist size). That's longer than 31% of the watches in our index. See what size watch suits your wrist.
- Strap thickness: at 13.2mm thick it's a chunky case: thicker padded leather, rubber/FKM or a heavier NATO balance it visually; very thin straps can look undersized.
Sources & corrections
Lug width checked against: Serious Watches: SPB121J1. Last checked 24 October 2026.
Spec wrong for your watch? Send us a correction with your reference number.
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Seiko Prospex Alpinist (SPB121) — strap questions
What size strap does the Prospex Alpinist (SPB121) take?
The Seiko Prospex Alpinist (SPB121) has a 20mm lug width, so you need a 20mm watch strap.
Will any 20mm strap fit the Prospex Alpinist (SPB121)?
Most will. Width is the main thing to match, but also check that the strap's spring-bar holes suit the bars, and that the strap is thin enough to clear the case between the lugs. A metal bracelet needs 20mm end-links made for this model.
What spring bar size does the Prospex Alpinist (SPB121) use?
20mm long. We haven't confirmed the diameter for the Prospex Alpinist (SPB121), so measure the original bar with calipers before buying replacements.
Is the Prospex Alpinist (SPB121) good for strap swapping?
Yes. With standard 20mm spring-bar lugs, NATO, leather, rubber and mesh straps can all be fitted with a spring-bar tool.