How many stones were there in Hadrian's Wall?

 

We were musing on the amount of effort that went into building the wall. All the stones are cut by hand, unlike the later drystone walls. We decided to attempt the calculation of the number of stones as a Fermi question, using data to hand and reasoned estimates. I couldn't tell whether the middle was made of cut stones or filled with rubble. My poles in the picture are 1.3m long and contain about 35 stones. That's 70 counting the other side. But the wall was said to be 4m high by Bede who would have seen it only a couple of centuries after it was abandoned. That's a factor of 3 higher so that gives us 210 stones in 1.3 metres. It was 73 miles long which is 117km. There are 90000 x 1.3m sections giving nearly 20 million stones. However, some say only 3/5 was ever in stone bringing it down closer to 10 million. This says it was built by 15000 people in 6 years. That's just over 100 stones per person per year. Sounds well within reason.