easiest way is put it down in water and see how much water it displaces and measure that. but if it is something elseyou can breake it down into known shapes like squares and triangles and add them together and you will get your answer.

1) "Instructions Things You'll Need: Ruler or tape measure Graduated cylinder Overflow can Water Step 1 Measure the length, width and height of the object. This works only for objects that have measurable sides. Step 2 Multiply length by width by height to determine its volume. Step 3 Determine the volume of irregularly shaped solid objects with the use of a graduated cylinder, overflow can and water. A graduated cylinder is a piece of lab equipment specifically designed for accurate liquid volume measurement, ranging in size from 10 milliliters to 2 liters. Overflow cans are used to submerge the solid object, being measured for volume, in water. They also come in an assortment of sizes and feature either angled or straight downspouts. Step 4 Fill the overflow can with water, allowing all excess water to drip out through the downspout. Hold the graduated cylinder below the spout and gently drop the irregularly shaped solid object into the can. Allow all excess water to run into the cylinder. Step 5 Read the amount of water that was displaced from the overflow can into the cylinder. This measurement is equal to the object's volume." Source and further information: http://www.ehow.com/how_4963916_measure-volume-solid-object.html 2) "D = m/V Density is the amount of matter in a given volume. The equation for density is mass divided by volume." Source and further information: http://www.coe.uh.edu/texasipc/units/properties/regular.pdf I you have a solid of know constant density D, you could weight it to get the mass and use this formula to evaluate its volume. Constant density would also mean: no cavities inside! 3) "Surface area and volume measurements provide important information for agriculture and food-processing applications. A machine vision system that uses a nondestructive method to measure volume and surface area of objects with irregular shapes is presented in this paper. The system first takes a series of silhouettes of the object from different directions by rotating the object at a fixed angular interval. The boundary points of each image are then extracted to construct a silhouette. A three-dimensional wire-frame model of the object can be reconstructed by integrating silhouettes obtained from different view angles. Surface area and volume can then be measured by means of surface fitting and approximation on the wire-frame model." Source and further information: http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=OPEGAR000045000002027202000001&idtype=cvips&gifs=yes

you need the FF. graduated cylinder a water and an irregular object put a water inside the graduated cylinder and measure the water b4 u drop a irregular there... and then put an irregular object inside the cylinder and then measure the water level after u drop the object inside the cylinder then minus the lvl of water after putting an object by b4 putting an object and then the differece is the answer.. i hope i can help..

BY SIGHT ONLY == IN NORMAL TERMS YOUR ASKING HOW DO YOU DETERMINE THE QUANITY/WEIGHT OF SOLID ==WITH WHAT-EVER SIZE SHAPE AND YOUR ONLY ANSWER IS BY SIGHT.