How a pendulum keeps time
The period of a pendulum – the time for one full swing there and back – depends almost entirely on its length and gravity, not on its weight or (for small swings) how far it swings. That's why pendulum clocks were the most accurate timekeepers for nearly three centuries. A clock's escapement gives the pendulum a small push each swing and counts the swings; each "tick" or "tock" is one beat, half a period.
Length L = g × T² ÷ (4π²) · A "seconds pendulum" (60 beats per minute) is about 99.4 cm long
The formula describes an ideal pendulum with all its mass at one point. A real pendulum's effective length is measured to its center of oscillation, usually a little above the middle of the bob, so the rod is often slightly longer than the result.
Typical pendulum lengths
| Beats per minute | Theoretical length | Typical clock |
|---|
| 60 | 99.4 cm (39.1 in) | Grandfather / longcase, regulator |
| 80 | 55.9 cm (22 in) | Large wall clocks |
| 90 | 44.2 cm (17.4 in) | Wall and Vienna regulators |
| 120 | 24.8 cm (9.8 in) | Mantel and bracket clocks |
| 150 | 15.9 cm (6.3 in) | Small mantel clocks |
Regulating your clock
Most pendulums have a rating nut beneath the bob. Turning it down lowers the bob, lengthens the pendulum and slows the clock; turning it up shortens it and speeds the clock. Because the rate changes with twice the relative length change, tiny movements matter: on a seconds pendulum, 1 mm changes the rate by about 43 seconds per day. Measure the thread pitch so you know how far one turn moves the bob, adjust, then wait a full day before checking again.
Temperature and accuracy
Pendulum rods expand when warm, making the clock run slower. A steel rod changes the rate by about half a second per day for each degree Celsius; wood is much more stable, which is why many fine clocks use wooden rods. Precision regulators use compensating designs – gridiron rods of steel and brass or mercury jars – or low-expansion Invar to cancel the effect.
Care tips
- Set the clock level and in beat after every move.
- Stop the pendulum and secure it before transporting the clock.
- Keep clocks away from radiators, fireplaces and direct sunlight to avoid temperature swings.
- Have mechanical clocks cleaned and oiled by a clockmaker every 5 – 10 years.
Frequently asked questions
How long is a pendulum for a 1-second beat?
About 99.4 cm (39.1 in) – the classic 'seconds pendulum' used in grandfather clocks.
My clock runs fast – which way do I turn the nut?
Turn the rating nut down to lower the bob. A longer pendulum swings more slowly.
How much does 1 mm change a pendulum clock?
On a seconds pendulum, about 43 seconds per day. Shorter pendulums are even more sensitive.
Why does my clock run slow in summer?
Heat expands the pendulum rod, lengthening it slightly. Steel rods lose about half a second per day per °C.
What does 'in beat' mean?
The tick and tock are evenly spaced. A clock out of beat sounds uneven and may stop.
About BeatCheck
BeatCheck is an independent, free calculator for owners and restorers of pendulum clocks, based on the simple pendulum formula. Real pendulums differ from the ideal model, so make small adjustments and recheck. We are not affiliated with any clock maker.
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The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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