Which way do I turn the rating nut to speed up the clock?

Turn the nut so the bob rises (usually clockwise, viewed from below). A shorter pendulum swings faster, so raising the bob makes the clock gain time.

How much does one full turn change the clock?

On a typical metre-long pendulum it is roughly 30 to 60 seconds per day per full turn, depending on thread pitch and pendulum length. This calculator works out the exact figure when you supply those values.

How long should I wait between adjustments?

Run the clock for at least 24 hours against an accurate reference, then measure the error before adjusting. Re-check after each change because regulation often takes several days to dial in.

Pendulum Clock Regulation Calculator — Nut Turns to Set Time

1 · How is it running?

2 · The error you measured

Time off by

Unit

Measured over

Period

Tip: compare against your phone, let the clock run a full day, then note how far off it is.

3 · The pendulum Choose a pendulum type for the per-turn sensitivity:

Exact mode: enter pendulum length & rating-nut thread pitch

Pendulum length

mm, bob centre to suspension

Thread pitch

mm the bob moves per full turn

Adjustment needed

¾ turn

What this pendulum regulation calculator does

A swinging pendulum keeps a mechanical clock on time, and its length sets the speed. A longer pendulum swings more slowly, so the clock loses time; a shorter one swings faster, so the clock gains time. The little knurled rating nut beneath (or above) the bob raises or lowers it, changing the effective length by a tiny amount with each turn. This tool converts the daily error you measured into the exact direction and number of turns to make — so you stop guessing and twisting blindly.

How to use it

Set the clock to the correct time against a reliable reference (your phone or a radio-controlled clock).
Wait and measure. Let it run at least 24 hours, then note how many minutes or seconds it is fast or slow.
Pick "fast" or "slow" and enter the error and the period you measured it over.
Choose your pendulum type (or switch to exact mode and enter length + thread pitch).
Read the result: turn the rating nut the shown direction by the shown amount. Stop the pendulum, turn the nut, restart it, and re-check after another day.

Rule of thumb to remember: to speed a clock up, raise the bob; to slow it down, lower the bob. Most rating nuts sit under the bob, so turning them up (clockwise from below) shortens the pendulum and gains time.

The formula behind it

For a pendulum, the period is proportional to the square root of its length, so a small length change ΔL shifts the rate by half that fraction. Over one day (86,400 s) the daily error from a length change is:

error/day ≈ 86,400 × (ΔL ÷ 2L)

One full turn moves the bob by the thread pitch, so turns needed = (2 × L × dailyError) ÷ (86,400 × pitch). The "typical clock" presets bake in common length/pitch combinations; exact mode uses your own measurements for a precise answer.

Worked example

Say a clock gains 4 minutes (240 s) per day. With a ~994 mm pendulum and a 0.7 mm thread pitch, one turn is worth about 30 s/day, so you need roughly 240 ÷ 30 ≈ 8 turns — and because it's gaining, you must slow it down by lowering the bob. The calculator handles that direction logic for you.

Common mistakes

Adjusting too soon. Give it a full 24 hours; short samples exaggerate the error.
Turning the wrong way. On a few clocks the nut is above the bob, which reverses the up/down direction — check yours.
Forgetting to reset the time. The nut changes rate, not the current reading; you still set the hands by hand.
Confusing beat with rate. An uneven "tick-tock" (out of beat) stops a clock but is a separate fix from regulation.

Per-turn sensitivity at a glance

Pendulum type	Approx. change per full turn
Tall-case / grandfather (~1 m)	~15–25 sec/day
Typical mantel & wall clock	~30 sec/day
Short / lyre pendulum	~45–90 sec/day

These are general guides — exact mode with your real thread pitch is always more accurate.

Frequently asked questions

Which way speeds the clock up?: Make the pendulum shorter by raising the bob. With a nut under the bob that usually means turning it clockwise (thumb to the right). Lowering the bob slows the clock down.
How much does one full turn change things?: It depends on length and thread pitch, but for a typical metre-class pendulum it is roughly 30 to 60 seconds per day per turn. That's why small, patient adjustments work best.
How long should I wait between adjustments?: At least 24 hours per check. Fine regulation can take several days to a couple of weeks to settle, especially as temperature and humidity vary.
My bob is already at the top and it's still slow — now what?: You've run out of nut travel. The pendulum or its suspension leader needs to be physically shortened, or the suspension spring inspected — a job for a clock repairer.
Does temperature affect timekeeping?: Yes. Heat expands the pendulum rod and makes it run slow; cold makes it run fast. Compensated pendulums reduce this, but ordinary clocks may need seasonal tweaks.