The Great Clock at a glance
People say "the Big Ben clock", but the names are split. Big Ben is the Great Bell. The tower is the Elizabeth Tower, renamed in 2012 for the Diamond Jubilee of Elizabeth II. The clock itself is the Great Clock, and it sits inside the tower behind four dials.
This is an independent guide, not run by the UK Parliament or any tour operator. The figures below follow the published Parliament fact sheets. Where two figures exist for the same thing, both are given.
Dials and hands
The parts you can see from the street.
| Part | Figure |
|---|---|
| Number of dials | 4 |
| Glass per dial | 324 pieces of opal glass in a cast-iron frame |
| Numerals | 60 cm high |
| Dial diameter | 7 m on the technical fact page, 6.9 m on the tower tour page |
| Minute hand | 4.2 m long, 100 kg including counterweights |
| Hour hand | 2.7 m long, 300 kg including counterweights |
Both diameters come from Parliament. Treat the dial as about 7 m across.
Westminster tours and cruises near Big Ben
Walking tours, river cruises and sightseeing trips around Westminster, with live prices and availability.
The mechanism behind the dials
The movement sits in a cast-iron frame that weighs 5 tonnes and measures 4.7 by 1.4 m. It is driven by weights, not by electricity or a spring. A report with the clockmaker from December 2025 describes winding on Mondays, Wednesdays and Fridays.
The pendulum is 4.4 m long and weighs 310 kg, including a bob of 203 kg. Parliament gives the beat as 2 seconds. Do not mix up the beat with the full swing there and back, which is a separate number.
Pennies set the time
Old pre-decimal pennies sit on the pendulum to regulate the clock. Adding one penny makes the clock run faster by 0.4 seconds in 24 hours. That is the effect of the adjustment, not a measured daily error.
The escapement
The clock uses a double three-legged gravity escapement designed by Edmund Beckett Denison. It shields the pendulum from outside forces such as wind pressure on the hands, which matters on a clock face this large. The design is Denison's, and the clock was built by Edward John Dent.
How accurate is Big Ben?
The astronomer George Airy set the original requirement: the hour stroke could be off by no more than 1 second. That was the standard asked of the makers in the nineteenth century. It is a historical specification, not a promise about today.
What the tower tour adds
The Great Clock is the main reason many visitors book the tower tour. The tour includes the clock mechanism and the inside of the dials, and it also covers Big Ben itself. It lasts up to 90 minutes and climbs 334 steps of a narrow spiral staircase to the bell chamber, with planned rest stops. Westminster Hall can be visited afterwards.
Before you go up
Plan about 1 hour 45 minutes on site, including the lockers. Arrive at the Cromwell Green entrance at least 30 minutes before the start, because late entry is not possible. Bags, cameras and phones go into a secure locker, and the security check works like the one at an airport. Wear closed flat shoes. Ear protection is provided. Tickets are sold online only, released three months ahead on the second Wednesday of the month at 10:00. Confirm the time zone in the booking calendar.
Seeing the clock up close
From the street or from inside the tower.
From outside
The dials are visible from the street on any day. Westminster Bridge, Parliament Square and the South Bank all show the tower. A street view is not an entry to the parliamentary estate.
Best views of Big Ben →Inside the tower
The mechanism and the inside of the dials are part of the Elizabeth Tower tour. The tour lasts up to 90 minutes and climbs 334 steps to the bell chamber. Tickets for tours from 1 August 2026 cost £55 for adults and £35 for ages 11 to 17.
Tower tour guide →Who can climb
The tour is open to UK and international visitors from 11 years. The climb is a narrow spiral staircase, walking aids are not allowed, and the tour has no lift. Photography is banned in the whole tower.
Can you go inside? →More about the tower
The bells, the history and the full list of numbers.
Big Ben clock: frequently asked questions
How many dials does Big Ben have?
Four, one on each side of the Elizabeth Tower. Each dial has 324 pieces of opal glass in a cast-iron frame.
How big is the clock face?
Parliament gives about 7 m across on its technical page and 6.9 m on the tower tour page. The numerals are 60 cm high, the minute hand is 4.2 m long and the hour hand is 2.7 m long.
What are the pennies on the pendulum for?
They fine-tune the speed. One extra penny makes the clock run faster by 0.4 seconds in 24 hours.
Is the clock electric?
No. Parliament describes a weight-driven mechanism with a pendulum. A report from December 2025 mentions winding on Mondays, Wednesdays and Fridays.
Can I see the clockwork?
Only on the tower tour, which includes the mechanism and the inside of the dials. Tickets are bought online in advance. See the tour page for prices and the booking window.
Who designed the Great Clock?
The design is by Edmund Beckett Denison. The clock was built by Edward John Dent, to a specification from the astronomer George Airy.