If you can identify the density of a particular stone using various references, you can then use that data to calculate the weight of the stone via the stone’s volume:
Weight = Volume x Density
Note: From a scientific perspective, Density x Volume gives you an object’s Mass. However, since the strength of gravity is held fairly constant (as it is all over Earth), then generally speaking an object’s mass is directly proportional to the object’s weight. So, I will be using “Weight” instead of “Mass.” When you’re talking about huge megaliths, the difference in mass vs. weight of that huge stone over a difference of a few hundred feet of sea level is minutely insignificant. Therefore, Mass = Weight for all intensive purposes.
Abbreviations used on this page:
 Weight:

 oz = ounce
 lb = pound
 ton = US ton
 g = gram
 kg = kilogram
 tonne = Metric tonne

 Length:

 in = inch
 ft = foot/feet
 yd = yard
 mi = mile
 mm = millimeter
 cm = centimeter
 m = meter
 km = kilometer

 Area:

 sqin = square inch
 sqft = square foot
 sqyd = square yard
 sqmm = square millimeter
 sqcm = square centimeter
 sqm = square meter

 Volume:

 cuin = cubic inch
 cuft = cubic foot
 cuyd = cubic yard
 cumm = cubic millmeter
 cucm = cubic centimeter
 cum = cubic meter

Volume
Volume is very straightforward:
Volume = Length x Width x Height
It can be computed and specified in both Metric (cubiccentimeters, cubicmeters, etc.) and US (cubicinches, cubicfeet, etc.).
Example: A stone block that measures 10feet long, 3feet wide, and 4feet tall = 10 x 3 x 4 = 120 cubicfeet
Stone Densities
Density is weight per unit of volume.
The density data was derived from numerous online sources, and generally specify building and sculptinggrade densities (not soft porous variations, such as you find in Limestone).
Stone  US Density (lb/cuft)  Metric Density (g/cucm OR tonnes/cum) 

Alabaster/Calcite =  137  144  2.2  2.3 
Gypsum =  137  175  2.2  2.8 
Sandstone =  137  175  2.2  2.8 
Limestone =  144  181  2.3  2.9 
Marble =  150  169  2.4  2.7 
Rhyolite =  150  169  2.4  2.7 
Porphyry =  156  162  2.5  2.6 
Mica Schist =  156  181  2.5  2.9 
Andesite =  156  181  2.5  2.9 
Dolomite =  156  181  2.5  2.9 
Quartzite =  162  175  2.6  2.8 
Syenite =  162  181  2.6  2.9 
Granite =  162  187  2.6  3.0 
Gneiss =  162  187  2.6  3.0 
Diabase/Dolerite =  162  196  2.6  3.1 
Diorite =  169  187  2.7  3.0 
Gabbro =  169  206  2.7  3.3 
Basalt =  169  206  2.7  3.3 
Therefore, with the above two tables, you can calculate a megalithic stone’s weight simply by multiplying the Volume of the stone by that stone’s Density.
Weight = Volume x Density
 Misc Conversion Tables 
In the event you have a need to convert a unit of measure from Metric to US or vice versa, you can use the conversion tables below . . . .
[By the way, it’s extremely easy to simply search the web for the conversions you need and let it do your work. However, from a personal improvement standpoint it helps your intellectual growth and overall sharpness if you learn to calculate it yourself. There’s no need to dumbdown the civilization any further than it’s already slipped.]
1) First, simply locate the table for the unit of measure you know [Length, Area, Volume, Weight, Density].
2) Choose the table that contains the international standard of measure in the upper left corner that you know [US or Metric].
3) Look down the lefthand column of that table for the specific unit you know.
4) Then look across to see the equivalent units measure.
5) Use the number to multiply with the number of units you know to result in the equivalent converted units.
For example . . . .
If you know you have 200 feet and want to know how many meters:
1) You have “feet” as a measurement so you need the Length conversion tables
2) Choose the US table under length (since the unit of measure you know is feet = US)
3) You know you have some quantity of feet so go down to the line that shows “1 ft.”
4) 1 ft. = 0.3048 meters (m), so . . .
5) We know we have 200 ft, so . . . . 200 x 0.3048 m = 60.96 meters
Length
US  mm  cm  m  km 

1 in =  25.4  2.54  0.0254  0.00002540 
1 ft =  304.8  30.48  0.3048  0.0003048 
1 yd =  914.4  91.44  0.9144  0.0009144 
1 mi =  1,609,934  160,934  1,609.344  1.609 
Metric  in  ft  yd  mi 

1 mm =  0.03937  0.003281  0.001094  (insignificant) 
1 cm =  0.3937  0.03281  0.01094  (insignificant) 
1 m =  39.37  3.281  1.094  0.0006214 
1 km =  39,370  3280.8  1093.6  0.6214 
Area
US  sqmm  sqcm  sqm 

1 sqin =  645.2  6.452  0.000645 
1 sqft =  92,903.04  929.03  0.0929 
1 sqyd =  836,127.36  8,361.274  0.836 
Metric  sqin  sqft  sqyd 

1 sqmm =  0.00155  (insignificant)  (insignificant) 
1 sqcm =  0.155  0.00108  0.00012 
1 sqm =  1,550.003  10.764  1.196 
Volume
US  cumm  cucm  cum 

1 cuin =  16,387.1  16.387  0.00001639 
1 cuft =  28,316,846.6  28,316.847  0.0283 
1 cuyd =  764,554,858  764,554.858  0.765 
Metric  cuin  cuft  cuyd 

1 cumm =  0.00006102  (insignificant)  (insignificant) 
1 cucm =  0.061  0.00003531  (insignificant) 
1 cum =  61,023.744  35.315  1.308 
Weight
Note: Throughout this website I will not be referring to the “Long Ton” of British origin. I will only use Metric Ton (designated herein as “tonne”) and US Short Ton (designated herein as “ton”).
US  gm  kg  tonne 

1 oz =  28.3495  0.0283495  0.00002835 
1 lb =  453.592  0.453592  0.000454 
1 ton (aka 1 short ton = 2000 lb) =  907,184.74  907.18474  0.907185 
Metric  oz  lb  ton 

1 gm =  0.035274  0.002205  (insignificant) 
1 kg =  35.274  2.20462  0.0011 
1 tonne (aka 1 metric ton = 1000 kg) =  35,273.962  2,204.623  1.10231 
Density
US  g/cucm  g/cum  kg/cum  tonne/cum 

1 lb/cuft =  0.01601846  16,018.4634  16.0184634  0.01601846 
1 lb/cuyd =  0.0005933  593.27642  0.593276  0.0005933 
1 ton/cuft =  32.036927  32,036,926.75  32,036.93  32.036927 
1 ton/cuyd =  1.1865528  1186552.8425  1186.55  1.1865528 
Metric  lb/cuft  lb/cuyd  ton/cuft  ton/cuyd 

1 g/cucm =  62.428  1685.55  0.031214  0.842777 
1 g/cum =  0.000062428  0.00168555  (insignificant)  (insignificant) 
1 kg/cum =  0.062428  1.68555  0.000031214  0.000842777 
1 tonne/cum (1000 kg/cum) =  62.428  1685.55  0.031214  0.842777 
Some online resources:
 Densities of Common Rocks and Minerals (https://www.thoughtco.com/densitiesofcommonrocksandminerals1439119)
 Densities of Rocks Calculated From Their Chemical Analyses (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1076683/pdf/pnas017640027.pdf)
 JustInTools Density Conversion (https://www.justintools.com/unitconversion/density.php?k1=tonnespercubicmeter&k2=poundspercubicfoot)
 National Park Service – Calculating the Weight of Stone (https://www.nps.gov/articles/000/calculatingtheweightofstone.htm)
 Stone Weight Calculator (https://stoneyard.com/calculators/stoneslabweight/)