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Seiko’s new HFJ series is a four-model set of compact rectangular solar watches for Japan, scheduled for November 2026. The headline comparison is accurate only at the model level: the HFJ004P case is 27.4 × 18.0 mm and 5.9 mm thick, which is thinner than the 6.6 mm Cartier Tank Must Small WSTA0107. It is not a comparison with every Tank.
What Seiko is launching
Notebookcheck’s October 8, 2026 report names four Seiko Selection rectangular solar watches for Japan: HFJ001P, HFJ002P, HFJ003P and HFJ004P. Seiko’s official listing for HFJ004P confirms that reference, a planned release date of November 7, 2026, and that the model is shop-only. The other three configurations are described in the Notebookcheck report alone.
| Reference | Case finish and dial | Strap or bracelet | Reported Japanese price | Price source |
|---|---|---|---|---|
| HFJ001P | White dial | Steel bracelet | ¥38,500 | Notebookcheck report |
| HFJ002P | Gold-tone finish; dial not stated | Steel bracelet | ¥41,800 | Notebookcheck report |
| HFJ003P | Rose-gold-tone finish; dial not stated | Brown cowhide strap | ¥35,200 | Notebookcheck report |
| HFJ004P | Gold-tone case, black dial | Black cowhide strap | ¥35,200, tax included | Seiko official listing and Notebookcheck report |
Prices are Japanese retail prices in yen. No exchange-rate conversion has been verified, so they should not be read as dollar, euro or sterling prices.
Dimensions compared with the Cartier Tank Must Small
Seiko’s official HFJ004P listing gives a case of 27.4 × 18.0 mm and a thickness of 5.9 mm. Cartier’s official listing for the Tank Must Small WSTA0107 gives 29.5 × 22 mm and 6.6 mm. Notebookcheck reports that all four HFJ models share the same 27.4 × 18 mm footprint and 5.9 mm thickness; that lineup-wide figure has not been confirmed model by model on Seiko’s site.
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| Measurement | Seiko HFJ004P | Cartier Tank Must Small WSTA0107 | Difference |
|---|---|---|---|
| Length | 27.4 mm | 29.5 mm | HFJ is 2.1 mm shorter |
| Width | 18.0 mm | 22 mm | HFJ is 4.0 mm narrower |
| Thickness | 5.9 mm | 6.6 mm | HFJ is 0.7 mm thinner |
The width gap is the largest of the three, and it is easy to miss when the headline focuses on thickness. These are case measurements only. They do not establish how either watch sits on the wrist, how the case shape feels, or how the two designs compare in finishing; none of that was tested for this article.
Reading the headline correctly
“Thinner than a Cartier Tank” is accurate for one Cartier reference, the Tank Must Small WSTA0107, measured against the HFJ004P. The Tank line includes several sizes and case thicknesses, and the HFJ’s thickness advantage over one model does not carry over to the family. Any claim that the HFJ is smaller than the Tank in general should be treated as unsupported.
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Movement and everyday use
Solar movement and charging
- HFJ004P uses the V115 solar movement, according to Seiko’s official listing.
- Seiko rates average accuracy at ±15 seconds per month for HFJ004P.
- Seiko states about 12 months of operation from a full charge, with overcharge protection.
- Notebookcheck reports that all four models run two hands, with no seconds hand and no date. This applies to the lineup as reported, not to each reference’s official listing.
Glass, water resistance and magnetic resistance
- HFJ004P has Hardlex mineral glass, according to Seiko’s official listing.
- Seiko lists daily-life water resistance and magnetic resistance for HFJ004P. Notebookcheck describes water resistance for the series as daily-life only.
- The series is not suitable for swimming or diving on the evidence available.
Availability and what to check before buying
Seiko labels HFJ004P as a shop-only model. Notebookcheck reports that all four HFJ models are Japan-market, shop-only, and will not appear in Seiko’s own online store. No retailer stock, overseas availability or marketplace listing has been verified. Before buying:
- Confirm the exact reference (HFJ001P, HFJ002P, HFJ003P or HFJ004P) against Seiko’s listing, since the four differ in finish and strap.
- Check the Japanese retailer’s stated price and whether it includes tax, using HFJ004P’s ¥35,200 tax-inclusive price as the one officially confirmed figure.
- Confirm the release date with the seller. Seiko’s listing gives November 7, 2026 for HFJ004P only.
- Ask whether the store ships outside Japan before assuming international availability.
Readers searching for the series can use the model numbers “Seiko HFJ004P” and “Seiko Selection HFJ series” to find the correct listings.
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Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
