Wt. of 1CF Adobe Cob ?

I priced a Tulikivi for my house and gave them  all of its specs .. it came to $15,000 and close to six ton of soapstone. That I decided was too costly and I found this site.

I am trying to get 4 to 5.5 tons of mass for my Rocket Mass Fire Place with an eight foot bench and heated wall. From charts for dirt on the net .. it looks like 100 lbs/cf for the weight of adobe is close.

Bench 22" seat x 18" Ht x 8' long .. std chair height. Bench would catch pass #2 at the base.. 2,196 lbs.

Wall immediately behind bench seat .. catching pass #1 at base and pass #3 in the middle .. wall will be 20" wide and 3' tall and 8' long.. 4,800 lbs. I didn't know how thin I could safely make an adobe cob wall.

Less 30' of 6" vent .. x .2 = 600 lbs lost weight.
4,800 + 2,196 - 600 = 6,396

To correct to 5,500 lbs

8/6400 as X/5500 = 6.9 ft. for length of wall. I am trying to create 150 degree temp at the top of the chimney to keep a good draft .. my vertical rise is about 20 feet. Some of this is just guess work after comparing the soapstone models.