I have a concrete pad (made with stone aggregates) that is 10 ft x 12
ft x 8 inches. Is there a formula that I can use to estimate it's
weight?

Hello-

     It is very difficult to answer this question because the density
of concrete varies widely based on numerous factors.  The average
density of concrete varies from 15 pounds/cubic foot to about 400
pounds/cubic foot.  With that said your concrete pad realistically
weighs anywhere from 8,000-13,000 pounds(based on a 100-160p/cf
range).


Unit Weight Information (ASTMC138 stanard)
<a href="http://www.tpub.com/content/engineering/14071/css/14071_298.htm">http://www.tpub.com/content/engineering/14071/css/14071_298.htm</a>

Search Strategy
"the average" density "of concrete"
<a href="://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&as_qdr=all&q=%22the+average%22+density+%22of+concrete%22>://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&as_qdr=all&q=%22the+average%22+density+%22of+concrete%22</a>

Hope this helps!
-CTSDataQA

Why try to make things so difficult these days?  Get ahold of some
concrete and weigh it (wet/dry) and then multiply it to whatever
number your little heart desires.

Good quality (precast) concrete masses near enough to 2.5 tonnes per
cubic metre... allowing for change of units and typical slab casting I
would estimate the mass of yours at between 4.5 and 5.0 tonnes.(3m x
3.6m x .2m) = 2.16 cu.m.
        2.16 x 2.5 = 5.4 tonnes at maximum.