A 3-Day Power Reserve describes a mechanical calibre that, fully wound, keeps running for about 72 hours, or three full days, without further input. That figure is no accident. Most brands settle on 72 hours because it maps neatly onto how people actually live with a watch: take it off on Friday evening, put it back on on Monday morning, and it is still going. The trade has a fond name for this, weekend-proof. There are two common routes to the spec. Either a single oversized mainspring barrel stores the energy, or, as Panerai favours, twin barrels work in series to release torque more evenly across those three days, which helps stability near the end of the run.
The clearest standard-bearer here is Panerai. Its in-house P.9000 and the slimmer P.9010 deliver 72 hours from dual barrels, beat at 28,800 vph, and place small seconds at nine and date at three. The micro-rotor P.4000 and the P.9002 share the same three-day autonomy. A 44mm steel Luminor Marina such as the PAM01312 lists around £7,600 new, though the secondary market sits closer to £3,800 as of 2026. Tudor takes a different angle: the MT5602 and the Black Bay Fifty-Eight's MT5402 are COSC-certified, run silicon balance springs, and are rated for 70 hours, with steel Fifty-Eight models around £3,650 to £3,950. Longines covers the value end, the Spirit Pilot pairing its L888.4 calibre with 72 hours, a silicon hairspring and COSC certification at roughly £2,150 to £2,450.
When buying, confirm the stated figure really is 70 to 72 hours and ideally verified by COSC or METAS, because many calibres lose amplitude, and therefore accuracy, in the final hours of a full wind. This category suits the collector who rotates several watches rather than wearing one daily, and anyone who swaps pieces over a weekend and resents resetting the time and date on Monday. The trade-off is honest: a big barrel or twin barrels can add a touch of case thickness, and 72 hours can look modest beside the 80-to-120-hour reserves now common. For real-world use, though, three days is the threshold most people genuinely need.
There are finer distinctions inside the category. Some movements are officially certified at 70 hours yet run close to 72 in practice, which is exactly what underpins Tudor's weekend-proof messaging. Higher-beat automatics like King Seiko's 8L45 pair 72 hours with titanium cases. At the entry level, the Tissot Powermatic family offers 72 to 80 hours at far more accessible money, proof that this once-premium feature has filtered all the way down the market.