Seiko · 5 Sports 5KX · Ref SRPD55
The cheapest way into a real Seiko automatic.
A 42.5mm steel automatic on the in-house 4R36, rated to 100m, with the day and date Seiko has been putting on this movement for years. It is the entry point most collections actually start at.
Movement
4R36
Seiko in-house automatic
Case
42.5mm
Stainless steel
Water resistance
100m
10 ATM, as rated by Seiko
Power reserve
41 hours
24 jewels, 21,600 vph
The 4R36
The 4R36 is the movement that makes this watch worth talking about. It is Seiko’s own automatic caliber, running at 21,600 vibrations per hour on 24 jewels, with a stated 41-hour power reserve. It hand-winds and it hacks, which at this end of the market is not a given.
It also carries the day and the date, so the calendar work is doing something rather than sitting there as a single window. Seiko has built this caliber in volume for a long time, which is why parts and watchmakers who know it are easy to find.
100m, honestly
Seiko rates the 5KX at 100m, or 10 ATM. That is a water-resistance rating, not a dive certification — Seiko sells its ISO-rated divers under Prospex, not under 5 Sports. Read 100m as swimming, showering and rain rather than as a licence to take it deep.
The crystal is Hardlex, Seiko’s own hardened mineral glass, not sapphire. It is more scratch-prone than sapphire and cheaper to replace, which is the trade the whole watch is built around.
Wearing 42.5mm
The case measures 42.5mm across in stainless steel. That is a genuinely large number on paper and the main thing to check before buying: if your other watches sit closer to 38 or 40mm, this will read as a step up in presence.
Made in Japan, per Seiko’s own designation for this reference. We research these figures from Seiko’s published specifications rather than from hands-on measurement.
Specs
- Reference
- SRPD55
- Case size
- 42.5mm
- Case material
- Stainless Steel
- Movement
- Automatic Caliber 4R36
- Caliber type
- Seiko In-House Automatic
- Crystal
- Hardlex Crystal
- Water resistance
- 100m (10 ATM)
- Power reserve
- 41 Hours
- Complications
- Day, Date
- Jewels
- 24 Jewels
- Frequency
- 21,600 vph (3 Hz)
- Origin
- Made in Japan
Specifications as published by Seiko. We research from manufacturer documentation and published sources rather than hands-on testing. Researched 2026-09-03.
Verdict
An in-house Seiko automatic with a 41-hour reserve and a 100m rating, at the price where most mechanical collections begin.
Strengths
- Seiko’s own 4R36 automatic, hacking and hand-winding
- 100m water resistance covers everyday water contact
- Day and date, not just a date window
- A caliber common enough that servicing is straightforward
Trade-offs
- Hardlex crystal rather than sapphire
- 42.5mm is a lot of case for a smaller wrist
- 21,600 vph is a slower beat than pricier Swiss automatics
- Rated 100m, but it is not an ISO-certified dive watch
Buy it if
A first mechanical watch, or a knockabout automatic you can wear in the rain without thinking about it.
Skip it if
You want sapphire, a sub-40mm case, or a certified dive watch — Seiko’s Prospex line is where those live.
FAQ
Seiko’s in-house 4R36 automatic. It runs at 21,600 vibrations per hour on 24 jewels with a stated 41-hour power reserve, and it drives both the day and the date. The 4R36 hand-winds and hacks, so you can set it to the second.
Sources
Researched and last updated 2026-09-03. We do not test watches in hand; every figure above is traceable to the manufacturer or the sources listed here.
Seiko 5 Sports 5KX
Reference SRPD55 · $250–$500
Prices and availability change; the band above is a research figure, so check the current listing.
Affiliate Disclosure
The Horologist is reader-supported. When you buy through links on our site, we may earn a commission at no extra cost to you. This never influences our editorial recommendations — every timepiece is selected on merit alone.
Complete the look
A fresh strap changes the whole watch.
Not ready today? Adding it to your cart keeps your pick saved.