Square or rectangular area:  length times width (L x W)

Cubic area of square or rectangle:  length times width times height (L x W x H)

Area of a circle: radius (1/2 the diameter) squared, times pi (3.14)

Circumference of a circle:  diameter times pi (d*pi), where pi = 3.14

* When calculating for a round structure, remember that your earthbag walls are going to be about a foot in thickness.  Consequently, a structure with a 15 ft outer diameter will have about a 13 ft inner diameter, give or take for the thickness of your plaster both inside & outside.

Length of arc of circle:  diameter times pi times angle of sector, divided by 360.

Example: Your structure is round & you're trying to figure out your door.
If your proposed door is 3 ft wide and your structure is 10 feet in diameter,
then 3 * 360 = 1080 /3.14 = 343.9 / 10 = 34 degrees.

You can reverse this: if diameter is 10 feet & you know the angle of your sector is 30 degrees, then pi * 10 * 30 = 942. Divide this by 360 and you have 2.6 feet (the length of your arc - or, more practically, the width of your dome's door or window).
 

One cubic yard = 3 ft W x 3 ft L x 3 ft H = 27 cubic feet (or 0.77 cubic meters)

To convert cubic feet to cubic yards: multiply cubic feet x 0.037037

One cubic yard of gravel = approx. 1 to 1.6 tons (2000 to 3200 lbs)
(Note that rock type and density can result in wide variation of weight)

One cubic foot of gravel = approx. 45 lbs (a bit over 1/2 a standard sandbag)

One cubic yard of dirt = approx. 0.70 to 1.5 tons (1400 to 3000 lbs)
 (depending upon moisture, clay content, presence of rocks
  or gravel in dirt, how loose or packed it is, etc.)

One cubic yard of 3/4" scoria = approx. 1/2 ton (1,080 lbs)
(a cubic yard of finer scoria - say, 3/8" - will weigh more)