Enter your clock's drift and pendulum length — get the exact adjustment needed.
Observe the clock for a measured interval, then enter how much it drifted.
Track your adjustments over multiple sessions. Saved in your browser.
| Date | Drift/day | Old length | New length | Change | |
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A pendulum's period is governed by:
Because period is proportional to the square root of length, the corrected length is:
This formula is used by professional horologists and is derived from the relationship T ∝ √L documented in standard horology references and texts such as de Carle's Practical Clock Repairing.
Every tick and every tock counts as one beat. A "seconds pendulum" (period = 2 s) produces 3600 BPH. This must match the gear-train design of your movement exactly. The formula is:
Common values: longcase/grandfather clocks — 3600 BPH; Vienna regulators — 3600 BPH; French mantel clocks — commonly 3600 or 7200 BPH; American shelf clocks — often 3600 BPH.
Steel rods expand by about 11.7 parts per million per °C. For a 1-metre pendulum, a 1 °C rise lengthens it by 0.0117 mm, adding roughly 0.48 seconds per day. This explains why a clock regulated in summer may run slightly slow in winter. The temperature-correction panel in the results quantifies this effect for your rod material and the temperature change you enter.
Use the formula New Length = Current Length × ((86400 + drift_s) / 86400)² to correct a fast clock (gaining), or × ((86400 − drift_s) / 86400)² for a slow clock (losing), where drift_s is the daily drift in seconds. This calculator does all the arithmetic instantly.
Typically 1–2 minutes per day per full 360° turn, but it varies by clock. The best approach: make one turn, measure the rate change over 24 hours, and use that as your per-turn coefficient going forward. Some finer-threaded movements have far smaller per-turn effects (as little as 30 seconds per turn).
Period T = 2π√(L/g). Longer L → larger T → each escapement release takes longer → clock runs slow. Raising the bob (shortening effective length) makes the clock run faster.
BPH (beats per hour) counts every individual tick and tock. A clock movement's gear train is designed for a specific BPH. If the pendulum delivers the wrong BPH, the clock will gain or lose time no matter how the hands are set. Matching measured BPH to the design value is a precise way to regulate without waiting 24 hours per trial.
Yes — steel rods expand about 11.7 ppm/°C, lengthening the pendulum and slowing the clock. A 10 °C seasonal change can cause ~5 seconds/day drift on a steel-rod clock. Invar rods (1.2 ppm/°C) or compensated (gridiron/mercury) pendulums dramatically reduce this effect.
Effective length is measured from the pivot (suspension spring axis) to the centre of mass of the bob — not to the bottom of the bob or the tip of the rod. For most simple bobs this is close to the geometric centre. For complex or asymmetric bobs, the centre of mass must be calculated or determined by experiment.