The Audemars Piguet Royal Oak 15500ST is the successor to the 15400ST: both are 41 mm steel, time-and-date Royal Oaks, but the 15500ST (introduced in 2019) swapped the older calibre 3120 for the larger calibre 4302, gained a longer power reserve, and moved to a slightly thicker case with a revised dial. The 15400ST is the discontinued predecessor and usually the lower-priced way into a 41 mm steel Royal Oak; the 15500ST is the one to choose if you want the modern movement. Bizak & Co. is a sourcing desk, so every watch below is found to order and nothing here is a stock listing.
Last reviewed on October 28, 2026 by Filip Biczak. Sources: Audemars Piguet, Royal Oak collection and Monochrome Watches, 15500ST introduction. Figures vary slightly by source and production year, so treat the table as a guide and check the exact unit.
What is the difference between the 15400ST and the 15500ST?
The biggest difference is under the dial: the 15400ST runs calibre 3120 at 3 Hz with about 60 hours of reserve, while the 15500ST runs calibre 4302 at 4 Hz with 70 hours. The case diameter did not change, but the larger movement made the 15500ST a little thicker and forced a dial redesign. The table sets out the points that sources agree on.
| 15400ST | 15500ST | |
|---|---|---|
| Status | Discontinued predecessor | Successor, introduced 2019; itself later followed by the 15510ST |
| Case | 41 mm steel, about 9.8 mm thick | 41 mm steel, about 10.4 mm thick |
| Movement | Calibre 3120, selfwinding | Calibre 4302, selfwinding |
| Beat rate | 3 Hz (21,600 vph) | 4 Hz (28,800 vph) |
| Power reserve | About 60 hours | 70 hours |
| Water resistance | 50 m | 50 m |
| Date window | Smaller window, sits inside the 3 o'clock position with a marker beside it | Larger window with a white background; the 3 o'clock marker was removed |
| Bracelet | Integrated steel bracelet, folding clasp | Integrated steel bracelet, folding clasp |
Which movement does each watch use?
The 15400ST uses the in-house calibre 3120, and the 15500ST uses the in-house calibre 4302. The 3120 was the long-serving Royal Oak workhorse; the 4302 is wider (roughly 32 mm against about 26.6 mm) and raises the beat rate from 3 Hz to 4 Hz while extending the reserve from about 60 to 70 hours. In daily wear that means a 15500ST that has sat in a drawer over a long weekend is more likely to still be running. Audemars Piguet also describes the new calibre as an improvement in reliability. It is the single best reason to prefer the newer reference.
How do you tell a 15400ST from a 15500ST at a glance?
Look at the date window at 3 o'clock first. On the 15500ST the date sits against the minute track in a larger white window and the hour marker at 3 o'clock is gone; on the 15400ST the window is smaller and the marker is still part of the dial. The hour markers on the 15500ST are also shorter and broader applied white gold pieces, with more room for luminous material, and the Grande Tapisserie squares are spaced a little more widely. The reference on the caseback and warranty paperwork settles it: 15400 versus 15500.
- Date window: larger with no 3 o'clock marker means 15500ST.
- Thickness: the 15500ST is a fraction of a millimetre thicker, which you will feel more than see.
- Caseback and papers: the reference number and calibre will match 15400 or 15500.
Is the 15400ST discontinued?
Yes. The 15500ST replaced it as the main 41 mm steel time-and-date Royal Oak in 2019, and the 15500ST in turn was followed by the 15510ST. Production years for the 15400ST are quoted differently by different sources, so we do not rely on a single end date. For a closer look at the earlier watch, see the AP Royal Oak 15400 collection, and for the newer one the AP Royal Oak 15500 collection or the dedicated Audemars Piguet 15500ST page.
How do the prices compare?
Broadly, the 15500ST sits in a higher band than the 15400ST, though both are serious five-figure watches in steel and the gap moves with dial colour, condition and whether the full set is present. The 15400ST has generally been the entry route into a 41 mm steel Royal Oak, but market commentary notes that its prices have firmed as it became a discontinued reference, so do not assume a large discount. Treat any figure you see as a dated observation, because pre-owned values shift and a sourced unit may price differently. We quote against a real watch only after we have confirmed it, rather than from a list.
Which should you buy, the 15400ST or the 15500ST?
Buy the 15500ST if you want the newer calibre and longer reserve, and the 15400ST if you want the slimmer case and the classic dial layout at a lower band. Neither is a wrong answer, and condition matters more than the suffix.
- Choose the 15500ST for the 4 Hz calibre 4302, the 70-hour reserve and the cleaner, larger date window.
- Choose the 15400ST for a marginally slimmer profile, the earlier dial design and usually a lower asking band; accept that any manufacturer warranty will have lapsed.
- Either way, prioritize an unpolished case with sharp bevels, a complete set and a bracelet with its original length.
If you are weighing the wider Royal Oak line, the Audemars Piguet Royal Oak collection shows the sizes and configurations we can seek.
What should you check before buying either watch?
Check the full reference, the serial against the paperwork, the bezel screws, the case bevels and the bracelet link count. A polished case with rounded edges is the most common compromise on a pre-owned Royal Oak. Ask for close photographs of the dial, date window and caseback of the actual unit so the details above match what is on offer. Our authenticity page sets out what we verify.
Related reading: Rolex vs AP vs Patek vs Richard Mille compared · Royal Oak 15500ST vs 15510ST, Royal Oak 15400ST up close and Royal Oak 15450ST guide.
Quick answers
What is the difference between the 15400ST and the 15500ST?
The 15500ST uses calibre 4302 (4 Hz, 70 hours) instead of the 15400ST's calibre 3120 (3 Hz, about 60 hours), sits in a slightly thicker case and has a larger date window with no 3 o'clock marker. Both are 41 mm steel.
Is the 15500ST better than the 15400ST?
Mechanically, yes: the newer movement runs faster and holds more reserve. The 15400ST is slimmer and usually cheaper, so the better watch depends on your budget and which dial you prefer.
Is the 15400ST discontinued?
Yes. The 15500ST replaced it in 2019 as the main 41 mm steel time-and-date Royal Oak.
Are the 15400ST and 15500ST the same size?
They share a 41 mm case diameter. The 15500ST is about 0.6 mm thicker, at roughly 10.4 mm against 9.8 mm.
Get your Royal Oak sourced
Tell us the reference, dial colour, preferred year, condition and whether the warranty card is required, plus your budget and timing, through our sourcing enquiry form. We confirm the specific watch, photographs and final price before you commit. A $200 deposit, credited to the price, starts the search.
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