A 5-Day Power Reserve describes a mechanical watch that runs for 120 hours on a single full wind. To reach that figure, brands almost always use two or three mainspring barrels, arranged in series or in parallel, which both extends the running time and releases torque more evenly than a single barrel ever could. Compared with the now-common 72-hour standard, a five-day watch shrugs off a long weekend completely: take it off Friday night, pick it up Wednesday, and it is still keeping accurate time. For anyone who rotates between several pieces, that is a genuine, everyday convenience rather than a spec-sheet brag.
The most accessible example is the Oris Calibre 400, an in-house automatic launched in 2020 that hits 120 hours via twin barrels storing roughly two-and-a-half days each, resists magnetism to 2,250 gauss, runs to -3/+5 seconds a day, and carries a 10-year warranty. The Aquis Date Calibre 400 now starts around $4,100 on bracelet. Christopher Ward's serially coupled SH21 brings a COSC-certified five-day reserve down to roughly $2,000. At the top, Blancpain's triple-barrel calibre 1315 powers the Fifty Fathoms north of $17,000, while F.P. Journe's calibre 1300.3 in the Octa Automatique Reserve climbs well above CHF 42,500 before tax in gold or platinum, technically capable of 160 hours but rated at 120 for stable chronometry.
When buying, look first at the barrel architecture: serially linked barrels give steadier torque but add height, so check the case thickness before you commit. Movements like the Oris Calibre 400, with a silicon escapement and a decade-long warranty, make the strongest value case for daily wearers, while infrequent wearers gain most from never having to reset and rewind from dead. The one caveat is that amplitude can sag in the final hours of a long reserve, so keep it topped up if absolute precision matters to you.
Do not confuse this category with eight-day (192-hour) hand-wound calibers; five days is the sweet spot where the automatic world balances genuine longevity against real-world practicality. Whether the watch is a diver, a pilot, or a dress piece, a 120-hour reserve is no longer a niche luxury but a mature engineering choice now available well into the mid-market.