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Seiko watch · automatic

Seiko SPB121

The Seiko SPB121 measures 39.5 mm across, 46.5 mm lug to lug, and 13 mm thick, with 20 mm lugs. It fits wrists from ~16.5 cm (flat-fit estimate).

Sources checked · How we source numbers
Case
39.5 mm1
Lug to lug
46.5 mm1
Thickness
13 mm1
Lug width
20 mm2
Seiko SPB121: size drawingSeiko SPB121, round case, top and side view to scale: case diameter 39.5 mm, lug to lug 46.5 mm, lug width 20 mm, thickness 13 mm. Bezel, dial and crown size are schematic.lug width 20Ø 39.5lug to lug 46.5TOPthickness 13SIDE
Seiko SPB121, top and side, to scaleDrawing from data, not a photo · mm

Seiko SPB121 specifications

Case diameter
39.5 mm1seller listing
Lug to lug
46.5 mm1seller listing
Thickness
13 mm1seller listing
Lug width
20 mm2seller listing20 mm watches
Crystal
Sapphire1seller listing
Water resistance
200 m1seller listing
Movement
Seiko 6R351seller listing · automaticSeiko 6R35 specs
Battery
none: the Seiko 6R35 is automatic
Years made
start not published
Case code
not in our data yet

On your wrist

The verdict compares the lug to lug with the flat top of your wrist. Balanced from ~16.5 cm, past the edges below ~14.5 cm.

= 6.7 in

Balanced on a 17 cm wrist: the 46.5 mm lug to lug covers 85% of an estimated 54.8 mm flat top.

Flat top = circumference ÷ 3.1, a community convention, not a measurement of your arm. Compact ≤ 0.75 of the flat top, balanced ≤ 0.89, at the edge ≤ 1. Compare more watches in the watch size calculator.

Seiko SPB121: wrist fitSeiko SPB121 on a wrist, top view to scale. lug to lug 46.5 mm. Wrist 170 mm, estimated flat width 54.8 mm (circumference / 3.1). Ratio 0.85: balanced.wrist, flat topwrist flat ≈ 54.8lug to lug 46.5Balanced · 46.5 / 54.8 = 0.85estimate: wrist 170 / 3.1 = 54.8 flatcompactbalancededgeoverhang0.750.891
Seiko SPB121 on a 17 cm wristDrawing from data, not a photo · mm

Actual size on your screen

Calibrate once with a bank card and the drawing shows the Seiko SPB121 at 1:1, next to a millimetre ruler.

1:1 · not calibrated: sizes are approximate
Seiko SPB121: size drawingSeiko SPB121, round case, top and side view to scale: case diameter 39.5 mm, lug to lug 46.5 mm, lug width 20 mm, thickness 13 mm. Bezel, dial and crown size are schematic.lug width 20Ø 39.5lug to lug 46.5TOPthickness 13SIDE
Drawing from data, not a photo · mm
01020304050

Seiko SPB121 strap size

Lug width 20 mm2seller listing: a 20 mm strap with 20 mm spring bars fits between the lugs. 62 other models take 20 mm.

Attachment: not published in our sources.

Find your strap length in the watch strap size tool, with the Seiko SPB121 filled in.

On a bracelet? How to remove watch links to size it to your wrist.

20 mm strap: strap diagramTwo-piece strap laid flat (20 mm strap). Lug width 20 mm. Short piece — (unknown), long piece — (unknown). Buckle, keepers and holes are schematic.short —long —lug 20lug 20strap length —20 mm strap
20 mm at the lugs; lengths depend on your wristDrawing from data, not a photo · mm

Movement: Seiko 6R35

The Seiko SPB121 runs on the Seiko 6R35, an automatic movement. 17 other models in our base use it.

Beat rate
21,600 bph3
Power reserve
70 h3
Jewels
243
Accuracy
−15 to +25 s/day3
Hacking seconds
Yes3
Hand winding
Yes3

Battery

none: the Seiko 6R35 is automatic

A mechanical watch has no cell to replace; it needs a service instead.

Same size as the Seiko SPB121

The models nearest to the Seiko SPB121 in case width and lug to lug, drawn over it to scale; below, every model within 1 mm of its 46.5 mm.

Size comparison: Seiko SPB121, Seiko SPB209, Zelos Helmsman SS Meteorite, Seiko SPB167Top-view outlines to scale on one centre. A Seiko SPB121: Ø 39.5, L2L 46.5, lug 20, T 13. B Seiko SPB209: Ø 39.5, L2L 46.4, lug 20, T 13.2 (vs A: Ø 0 L2L −0.1 lug 0 T +0.2). C Zelos Helmsman SS Meteorite: Ø 40, L2L 46.5, lug 20, T 12.5 (vs A: Ø +0.5 L2L 0 lug 0 T −0.5). D Seiko SPB167: Ø 39.5, L2L 47, lug —, T 11 (vs A: Ø 0 L2L +0.5 T −2).A Ø 39.5B Ø 39.5D Ø 39.5C Ø 40B L2L 46.4A L2L 46.5C L2L 46.5D L2L 47A Seiko SPB121Ø 39.5 · L2L 46.5 · lug 20 · T 13B Seiko SPB209Ø 39.5 · L2L 46.4 · lug 20 · T 13.2vs A: Ø 0 L2L −0.1 lug 0 T +0.2C Zelos Helmsman SS MeteoriteØ 40 · L2L 46.5 · lug 20 · T 12.5vs A: Ø +0.5 L2L 0 lug 0 T −0.5D Seiko SPB167Ø 39.5 · L2L 47 · lug — · T 11vs A: Ø 0 L2L +0.5 T −2
A Seiko SPB121 and the 3 nearest in size, on one centre, to scaleDrawing from data, not a photo · mm

Nearest in size

  1. ASeiko SPB121this watch
  2. BSeiko SPB209W same · L2L −0.1 · T +0.2 mm
  3. CZelos Helmsman SS MeteoriteW +0.5 · L2L same · T −0.5 mm
  4. DSeiko SPB167W same · L2L +0.5 · T −2 mm

Differences against A in mm: W case width, L2L lug to lug, T thickness. Nearest by width and length together, from the published sizes.

Same published case dimensions (1)Seiko SBDC091

Same movement: Seiko 6R35

All 18 models on the Seiko 6R35

Seiko SPB121 questions

How big is the Seiko SPB121?

The Seiko SPB121 measures 39.5 mm across, 46.5 mm lug to lug, 13 mm thick.

Will the Seiko SPB121 fit a 6.5-inch wrist?

Balanced: the Seiko SPB121's 46.5 mm lug to lug takes 87% of the ~53.3 mm flat top of a 6.5-inch (16.5 cm) wrist. By the same estimate (circumference ÷ 3.1) it sits balanced from about 16.5 cm and overhangs the flat top on wrists under about 14.5 cm.

Does the Seiko SPB121 need a battery?

No. The Seiko SPB121 has no cell to replace: a wound mainspring drives its automatic Seiko 6R35.

What movement is in the Seiko SPB121?

The Seiko SPB121 runs on the Seiko 6R35, an automatic movement.

What strap size does the Seiko SPB121 take?

The Seiko SPB121 measures 20 mm between the lugs, so it takes a 20 mm strap with 20 mm spring bars. 62 other models in our base share that width.

Find the strap length for your wrist and the Seiko SPB121 in the watch strap size tool.

More Seiko watches

Sources

Documents checked · Report a wrong number

  1. 1Right Time International — SPB121 listing, Right Time (Denver), dealer spec block · html · read
  2. 2Right Time International — SBDC091 pre-owned listing, Right Time (Denver), dealer spec block · html · read
  3. 3Seiko Watch Corporation — Diver's watch for air diving instructions, Cal. 6R35 (EN) · pdf · read
  4. 4CaliberCorner — Seiko Caliber 6R35 watch movement page · html · read
  5. 5Grail Watch — Grail Watch Reference: Seiko 6R35 · html · read