Watch Amplitude Decay Calculator for Power Reserve and Balance Wheel Health

A single amplitude reading at full wind tells you where the movement is now. A before-and-after pair from the same movement at the same wind state tells you what the service accomplished. AWCI-trained watchmakers document pre-service amplitude as standard practice because it establishes the diagnostic baseline (how degraded was the movement when received) and the post-service verification (was the target amplitude range achieved after fresh lubricants and mainspring evaluation). If you receive a movement with 245° amplitude, service it, and it leaves with 287° amplitude, that 42° improvement is objective evidence of the service value. If post-service amplitude is still 245°, the mainspring, barrel arbor bearings, or escapement components require further investigation. As Adjusting Vintage Watches recommends, the target minimum for watches in good mechanical condition is 270° horizontal and 220° vertical; any serviced movement should achieve these benchmarks before leaving the bench.

While dial-up (DU) amplitude is the standard benchmark position for caliber specifications, crown-up (CU) and crown-down (CD) amplitude tell you more about the movement’s health under the conditions it actually experiences on the wrist. When the crown points up (the most common wrist position during wear for a right-hand wearer), gravity acts on the balance wheel and gear train in a way that exposes any friction increase, poise error, or pivot wear that the horizontal position masks. A movement that reads 285° in DU but only 190° in CU is showing a positional spread of 95°, far above the normal 40-60° range. That gap is a precise diagnostic indicator: values this extreme point to poise errors in the balance wheel (heavy side pulling down under gravity), worn or damaged pivots in the balance staff or gear train, or a bent balance staff causing the balance to run off-axis in vertical orientations. Rotate Watches (September 2025) documented this specifically, noting that positional spread above 60° warrants investigation and spread above 80° strongly suggests service needs that pure rate or horizontal amplitude readings would not reveal.

Watch manufacturers recommend service intervals in calendar time (every 3-5 years for modern automatic watches, every 3-7 years for vintage mechanical watches per AWCI guidelines), but calendar time is a crude proxy for the actual mechanical condition of the movement. A watch worn daily in a humid environment with a rotor spinning frequently develops lubricant degradation much faster than the same model worn occasionally in a controlled indoor environment. Amplitude decay rate gives a direct measurement of how well the movement is preserving its stored energy, independent of calendar time. A modern automatic with a decay rate above 4°/hr at one-third of its power reserve is showing signs of lubricant breakdown or mechanical wear regardless of when it was last serviced. A vintage movement with a decay rate below 2°/hr across its full 30-hour reserve is running cleanly regardless of the date on its last service record. Track amplitude at consistent intervals (say, every 8-12 hours) to build a decay curve for each watch in your collection or service rotation, and use that curve rather than the calendar to determine actual service timing.

Unlike beat error, which is a direct measurement independent of timegrapher settings, amplitude is a calculated estimate that depends critically on the lift angle entered on the timegrapher. The lift angle is a caliber-specific geometric property of the escapement (the angle the balance wheel rotates while the escape wheel tooth is lifting the pallet stone). Entering the wrong lift angle can cause your amplitude reading to be off by 15-25 degrees or more. For example, the Omega Co-Axial caliber 8500 uses a lift angle of approximately 38°, not the default 52°. Setting the timegrapher to 52° for this movement gives an amplitude reading roughly 18 degrees higher than the true value, potentially masking a low-amplitude condition. The caliber database in this calculator lists the correct lift angle for reference alongside the BPH. For movements not in the database, the manufacturer’s technical specification sheet is the authoritative source. If you cannot locate the specification, the practical check is to mark the balance arm and increase power until the balance just achieves 180°, then adjust the lift angle setting until the timegrapher reads 180°. This calibration method is documented in the WahaWatches lift angle guide (2022) and Escapement.watch (2026).

A common misunderstanding in watch enthusiast communities is that a high amplitude reading at full wind means the watch is in excellent condition overall. Amplitude at full wind is a necessary but insufficient indicator of health. What matters for practical accuracy is how amplitude behaves across the entire power reserve. As Kapoor Watch documented in April 2026, in movements with shorter effective reserves (due to degraded lubricants or a worn mainspring), amplitude drops rapidly in the second half of the reserve, causing the balance to oscillate at increasingly short arcs where the hairspring’s isochronism is less precise, leading to rate variation. The watch may read +2 s/day at full wind and +12 s/day at 75% reserve depletion, appearing to be a well-regulated watch to someone who only checks it after winding. Multi-session amplitude tracking, as supported by this calculator, reveals whether the movement is maintaining adequate amplitude across the full stated power reserve or whether it is effectively running out of power well before the manufacturer’s specification. The comparison of projected end amplitude against the 220° service threshold is the actionable metric for service decisions.

American railroad-grade pocket watches from Hamilton, Waltham, Illinois, Elgin, and Ball running at 18,000 BPH operate with different amplitude expectations than modern wristwatches. Their lower beat frequency means a longer half-period and a different mainspring torque-to-amplitude relationship. Where a healthy ETA 2824 might produce 290° at full wind, a healthy Hamilton 992B or Waltham Model 1908 typically produces 250-270°, and these lower numbers are appropriate and expected for these movements in good condition. AWCI training materials for vintage movement service specify that the minimum acceptable amplitude for a vintage pocket watch in horizontal position after service is 240°, substantially lower than the 270° minimum for a modern automatic. At the same time, amplitude decay rates for vintage pocket watches in original condition may legitimately be higher (4-8°/hr) than for freshly serviced modern movements (1-3°/hr) because vintage movements rarely achieve the lubrication quality of a fresh service and their barrel arbor bearings are often worn. When evaluating a vintage movement, always apply the vintage threshold mode in this calculator and compare the reading not against modern auto benchmarks but against the AWCI and NAWCC vintage movement service standards appropriate for the specific caliber. The NAWCC’s Martin Luther Archival Library at Columbia, Pennsylvania, maintains original factory service manuals for all major American pocket watch manufacturers.