200, 100, 80, or 8 billion?

Time, I thought this morning, to consider how many pebbles there are on Brighton Beach. I decided to make my own calculation before doing any online research or employing my AI friends.

This is how I did it. 

The beach is four miles long, lets say 6.5km - see How long is Brighton Beach

At the lowest tides, there is widespread sand, so this provides a boundary to the seaward edge of the beach and to the depth of pebbles onshore.

The width of the beach at low tide might average out along the length at, say 50m; and the depth of the pebbles might vary from 0 (at lowest tides) to, say, 10 metres - making an average depth of pebbles to be 5 metres.

The total volume of pebbles therefore is 6,500 x 50 x 5.

Average size of a single pebble. This is more difficult. The pebbles vary from shingle to fist-sized. So I’ll make two calculations, one based on large average pebble size, 0.04 m cubed (0.000064), and one based on a lower pebble size, 0.02 m cubed (0.000008).

Using the large pebble size I calculate the total number of pebbles at 25,390,625,000 i.e roughy 25 billion; using the small pebble size I get 203,125,000,000 i.e. roughly 200 billion.

Next I went to Perplexity, which gave me this calculation.

Beach volume: 6,400 m (length) x 50 m (width) x 1 m (depth) = 320,000 cubic meters,

Volume of a single pebble: (4/3) x π x (0.01 m)³ ≈ 0.000004189 cubic meters,

Estimated number of pebbles: 320,000 / 0.000004189 ≈ 76.4 billion,

Therefore, it says, a reasonable estimate for the number of pebbles would be in the range of 70-80 billion.

And to ChatGPT

The beach is about 5.5 km (5500 m) long.

The average width of the pebble-covered area is around 50 m.

The average depth of pebbles is 0.5 m.

Pebbles are packed with an estimated 50 percent void space (air gaps).

The average size of a pebble is around 2 cm in diameter, meaning each pebble takes up roughly 8 cubic cm (0.000008 cubic meters).

Total volume of pebbles: 5500 × 50 × 0.5=137500

Adjusting for void space: 137500 × 0.5=68750 of actual pebbles

Number of pebbles: 68750 ÷ 0.000008 = 8,593,750,000

And, finally, I did a quick search of the Argus website where I found an article dating back to 1999 about Dr Cornwell, 63-year-old physics lecturer, who had estimated the total number of pebbles as 100 billion. ‘It would take,’ he said ‘one man about 2,500 years counting at one a second to add them all up.’ He also noted, the article said, that there are ten trillion times as many atoms in a pebble as there are pebbles on Brighton Beach!

In summary, my highest estimate is 200 billion, a scientist’s calculation is 100 billion, and two AI’s give respectively 80 and 8 billion. Clearly, much depends on the original assumptions, particularly of average pebble volume and pebble depth. But 200 billion is a mind-boggling number.

By way of trying to visualise the number 200 billion here are some comparisons.

- If you laid £1 coins end-to-end, 200 billion coins would stretch for over 1,800,000 miles, nearly 75 times around the Earth.

- If you stacked £1 coins, the pile would reach over 13,000 miles high, almost halfway to the moon.

- 200 billion is a number comparable to the number of stars in a galaxy.