Great Clock facts

Clock dials

Number of clock dials: 4
Clock dials diameter: 7m
Length of hour figures: 60cm
Clock dial frames: cast iron
Glass in each clock dial: 324 pieces of pot opal glass
Illumination of each dial: 28 energy efficient bulbs at 85 watt each
Lifetime of each energy efficient bulb: 60,000 hours

Minute hands

Material: copper sheet
Weight: 100kg, including counterweights
Length: 4.2m
Distance travelled by minute hands per year: equivalent of 190km

Hour hands

Material: gun metal
Weight: 300kg including counterweights
Length: 2.7m

The hour figure of 4 o'clock is shown by the Roman numeral IV, rather than the usual IIII on other clocks.

The mechanism

Clock mechanism frame material: cast iron girder frame
Clock mechanism dimensions: 4.7m long and 1.4m wide
Clock mechanism weight: 5 tonnes
Pendulum length: 4.4m
Pendulum weight: 310kg
Duration of pendulum beat: 2 seconds

Pendulum adjustment: pre-decimal pennies are used to regulate the clock mechanism. Adding one penny causes the clock to gain two-fifths of a second in 24 hours.

'The Double Three-legged Gravity Escapement', designed by Edmund Beckett Denison, compensates for outside pressure (like the wind) on the clock hands and is crucial for accuracy.

The pendulum bob

Pendulum bob weight: 203kg
Material: concentric tubes of steel and zinc

Also in this section

  • View an animation of the Great Clock's mechanism

    How the Great Clock works

  • Keeping the Great Clock ticking

    From 11 August to 1 October, an electric system kept the clock moving, but Big Ben, the name for the Great Bell, and the quarter bells were quiet.

  • Great Clock stoppages

    The 2007 stoppage was the longest suspension of the hour strike (Big Ben) since 1990. The clock mechanism was also suspended for two days in October 2005 to allow inspection of the brake shaft. Previous stoppages of both the hour and quarter bells took place in 1934 (two months) and 1956 (six months).

  • Building the Great Clock's mechanism

    Charles Barry was a fine architect but he was not a specialist clockmaker. He sought advice from a friend, Benjamin Lewis Vulliamy, after adding a prominent clock tower to his design for rebuilding Parliament after the 1834 fire.


You can access a biography of

Edmund Beckett Denison

from the Oxford Dictionary of National Biography for free, online, using your local library card number (includes nine out of ten public libraries in the UK) or from within academic library and other subscribing networks.