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Seiko watch · Prospex · automatic

Seiko SRPA21

The Seiko SRPA21 measures 45 mm across and 13.4 mm thick, with 22 mm lugs.

Sources checked · How we source numbers
Case
45 mm1
Lug to lug
not published by Seiko
Thickness
13.4 mm1
Lug width
22 mm1
Seiko SRPA21: size drawingSeiko SRPA21, round case, top and side view to scale: case diameter 45 mm, lug to lug — (unknown), lug width 22 mm, thickness 13.4 mm, rotating bezel. Bezel, dial and crown size are schematic.lug width 22Ø 45lug to lug —TOPthickness 13.4SIDE
Seiko SRPA21, top and side, to scaleDrawing from data, not a photo · mm

Seiko SRPA21 specifications

Case diameter
45 mm1
Lug to lug
not published by Seiko
Thickness
13.4 mm1
Lug width
22 mm122 mm watches
Crystal
Hardlex1
Water resistance
200 m1
Movement
Seiko 4R361 · automaticSeiko 4R36 specs
Battery
none: the Seiko 4R36 is automatic
Years made
start not published
Case code
not in our data yet
Bezel
unidirectional rotating1
Line
Prospex1

On your wrist

No lug-to-lug or case length is published for the Seiko SRPA21, so there is no fit verdict yet.

Actual size on your screen

Calibrate once with a bank card and the drawing shows the Seiko SRPA21 at 1:1, next to a millimetre ruler.

1:1 · not calibrated: sizes are approximate
Seiko SRPA21: size drawingSeiko SRPA21, round case, top and side view to scale: case diameter 45 mm, lug to lug — (unknown), lug width 22 mm, thickness 13.4 mm, rotating bezel. Bezel, dial and crown size are schematic.lug width 22Ø 45lug to lug —TOPthickness 13.4SIDE
Drawing from data, not a photo · mm
01020304050

Seiko SRPA21 strap size

Lug width 22 mm1: a 22 mm strap with 22 mm spring bars fits between the lugs. 45 other models take 22 mm.

Attachment: not published in our sources.

Find your strap length in the watch strap size tool, with the Seiko SRPA21 filled in.

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

22 mm strap: strap diagramTwo-piece strap laid flat (22 mm strap). Lug width 22 mm. Short piece — (unknown), long piece — (unknown). Buckle, keepers and holes are schematic.short —long —lug 22lug 22strap length —22 mm strap
22 mm at the lugs; lengths depend on your wristDrawing from data, not a photo · mm

Movement: Seiko 4R36

The Seiko SRPA21 runs on the Seiko 4R36, an automatic movement. 47 other models in our base use it.

Beat rate
21,600 bph2
Power reserve
41 h2
2 sources disagree; show every value
  • manufacturer 41 h2
  • secondary source 40, 41 h4
Jewels
242
Accuracy
−35 to +45 s/day3
Hacking seconds
Yes2
Hand winding
Yes2

Battery

none: the Seiko 4R36 is automatic

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

Same size as the Seiko SRPA21

Same movement: Seiko 4R36

All 48 models on the Seiko 4R36

Parts documented for the Seiko SRPA21

1 claims for named aftermarket parts are in the catalogue. Each claim is tied to this exact model and its seller’s source.

Seiko SRPA21 questions

Does the Seiko SRPA21 need a battery?

No. The Seiko SRPA21 has no cell to replace: a wound mainspring drives its automatic Seiko 4R36.

What movement is in the Seiko SRPA21?

The Seiko SRPA21 runs on the Seiko 4R36, an automatic movement.

What strap size does the Seiko SRPA21 take?

The Seiko SRPA21 measures 22 mm between the lugs, so it takes a 22 mm strap with 22 mm spring bars. 45 other models in our base share that width.

Find the strap length for your wrist and the Seiko SRPA21 in the watch strap size tool.

More Seiko watches

Sources

Documents checked · Report a wrong number

  1. 1Seiko Watch Corporation — SRPA21 product page (Prospex), seikowatches.com ph-en · html · read
  2. 2Seiko Watch Corporation — Instructions: Seiko Automatic Diver's Watch Cal. 4R15/4R16/4R35/4R36/7S26/7S35/7S36 (English) · pdf · read
  3. 3Seiko Watch Corporation — Instructions: Diver's Watch for Air Diving, Cal. 4R35 / 4R36 (English) · pdf · read
  4. 4Grail Watch — Grail Watch Reference: Seiko Cal. 4R36 · html · read
  5. 5Seiko Watch Corporation — Instructions: Mechanical watch, Cal. 4R35 / 4R36 / 4R37 / 4R38 / 4R71 / 4R72 / 4R39 (English) · pdf · read