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Seiko watch · Prospex · automatic

Seiko SRPD21

The Seiko SRPD21 measures 45 mm across, 47.7 mm lug to lug, and 13 mm thick, with 22 mm lugs. It fits wrists from ~17 cm (flat-fit estimate).

Sources checked · How we source numbers
Case
45 mm1
Lug to lug
47.7 mm1
Thickness
13 mm1
Lug width
22 mm1
Seiko SRPD21: size drawingSeiko SRPD21, round case, top and side view to scale: case diameter 45 mm, lug to lug 47.7 mm, lug width 22 mm, thickness 13 mm, rotating bezel. Bezel, dial and crown size are schematic.lug width 22Ø 45lug to lug 47.7TOPthickness 13SIDE
Seiko SRPD21, top and side, to scaleDrawing from data, not a photo · mm

Seiko SRPD21 specifications

Case diameter
45 mm1
Lug to lug
47.7 mm1
Thickness
13 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

The verdict compares the lug to lug with the flat top of your wrist. Balanced from ~17 cm, past the edges below ~15 cm.

= 6.7 in

Balanced on a 17 cm wrist: the 47.7 mm lug to lug covers 87% 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 SRPD21: wrist fitSeiko SRPD21 on a wrist, top view to scale. lug to lug 47.7 mm. Wrist 170 mm, estimated flat width 54.8 mm (circumference / 3.1). Ratio 0.87: balanced.wrist, flat topwrist flat ≈ 54.8lug to lug 47.7Balanced · 47.7 / 54.8 = 0.87estimate: wrist 170 / 3.1 = 54.8 flatcompactbalancededgeoverhang0.750.891
Seiko SRPD21 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 SRPD21 at 1:1, next to a millimetre ruler.

1:1 · not calibrated: sizes are approximate
Seiko SRPD21: size drawingSeiko SRPD21, round case, top and side view to scale: case diameter 45 mm, lug to lug 47.7 mm, lug width 22 mm, thickness 13 mm, rotating bezel. Bezel, dial and crown size are schematic.lug width 22Ø 45lug to lug 47.7TOPthickness 13SIDE
Drawing from data, not a photo · mm
01020304050

Seiko SRPD21 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 SRPD21 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 SRPD21 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 SRPD21

The models nearest to the Seiko SRPD21 in case width and lug to lug, drawn over it to scale; below, every model within 1 mm of its 47.7 mm.

Size comparison: Seiko SRPD21, Seiko SRP777, Casio MDV-106, Seiko SRPD23Top-view outlines to scale on one centre. A Seiko SRPD21: Ø 45, L2L 47.7, lug 22, T 13. B Seiko SRP777: W 45, L2L 48, lug 22, T 14 (vs A: W 0 L2L +0.3 lug 0 T +1). C Casio MDV-106: Ø 44.2, L2L 48, lug 22, T 12.1 (vs A: Ø −0.8 L2L +0.3 lug 0 T −0.9). D Seiko SRPD23: Ø 43.8, L2L 48.4, lug 22, T 13.4 (vs A: Ø −1.2 L2L +0.7 lug 0 T +0.4).D Ø 43.8C Ø 44.2A Ø 45B W 45A L2L 47.7B L2L 48C L2L 48D L2L 48.4A Seiko SRPD21Ø 45 · L2L 47.7 · lug 22 · T 13B Seiko SRP777W 45 · L2L 48 · lug 22 · T 14vs A: W 0 L2L +0.3 lug 0 T +1C Casio MDV-106Ø 44.2 · L2L 48 · lug 22 · T 12.1vs A: Ø −0.8 L2L +0.3 lug 0 T −0.9D Seiko SRPD23Ø 43.8 · L2L 48.4 · lug 22 · T 13.4vs A: Ø −1.2 L2L +0.7 lug 0 T +0.4
A Seiko SRPD21 and the 3 nearest in size, on one centre, to scaleDrawing from data, not a photo · mm

Nearest in size

  1. ASeiko SRPD21this watch
  2. BSeiko SRP777W same · L2L +0.3 · T +1 mm
  3. CCasio MDV-106W −0.8 · L2L +0.3 · T −0.9 mm
  4. DSeiko SRPD23W −1.2 · L2L +0.7 · T +0.4 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 movement: Seiko 4R36

All 48 models on the Seiko 4R36

Parts documented for the Seiko SRPD21

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

Seiko SRPD21 questions

How big is the Seiko SRPD21?

The Seiko SRPD21 measures 45 mm across, 47.7 mm lug to lug, 13 mm thick.

Will the Seiko SRPD21 fit a 7-inch wrist?

Balanced: the Seiko SRPD21's 47.7 mm lug to lug takes 83% of the ~57.4 mm flat top of a 7-inch (17.8 cm) wrist. By the same estimate (circumference ÷ 3.1) it sits balanced from about 17 cm and overhangs the flat top on wrists under about 15 cm.

Does the Seiko SRPD21 need a battery?

No. The Seiko SRPD21 has no cell to replace: a wound mainspring drives its automatic Seiko 4R36.

What movement is in the Seiko SRPD21?

The Seiko SRPD21 runs on the Seiko 4R36, an automatic movement.

What strap size does the Seiko SRPD21 take?

The Seiko SRPD21 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 SRPD21 in the watch strap size tool.

More Seiko watches

Sources

Documents checked · Report a wrong number

  1. 1Seiko Watch Corporation — SRPD21 product page (Prospex), seikowatches.com global-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