Seiko
Seiko Marinemaster 300 Strap Size — 20mm Lug Width
The Seiko Marinemaster 300 has a 20mm lug width, so it takes a 20mm watch strap.
Lug width20mm
| Lug width | 20 mm |
|---|---|
| Case size | 44.3 mm |
| Lug-to-lug | 51 mm |
| Thickness | 15 mm |
| Spring bar Ø (typical) | 2.5 mm |
| End type | Straight |
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 / siliconePick straps with holes made for fat bars.
- Milanese / meshStraight-end 20mm mesh fits.
- Metal braceletNeeds 20mm end-links shaped for this case.
▲ Fat bars.
Fit & sizing notes for the Marinemaster 300
- 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, about 2.5mm diameter. These are fat bars, so pick straps with holes made for them, or swap to thinner 20mm bars for slim leather (spring bar sizes).
- Wrist fit: at 51mm lug-to-lug it should sit within the top of wrists from about 17.0cm up (rule of thumb: lug-to-lug under ~30% of wrist size). That's longer than 94% of the watches in our index. See what size watch suits your wrist.
- Strap thickness: at 15mm 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: WatchBase: SBDX017; StrapsCo: SBDX017. Last checked 24 October 2026.
Spec wrong for your watch? Send us a correction with your reference number.
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Seiko Marinemaster 300 — strap questions
What size strap does the Marinemaster 300 take?
The Seiko Marinemaster 300 has a 20mm lug width, so you need a 20mm watch strap.
Will any 20mm strap fit the Marinemaster 300?
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 Marinemaster 300 use?
20mm long and about 2.5mm in diameter (a typical figure; measure the original bar to be sure). These are thicker 'fat' bars, so heavier straps suit it best.
Is the Marinemaster 300 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.