Dried concrete has a very specific weight per square inch. Therefore, if you are trying to calculate the weight of an entire block or unit of concrete, you find the total area of said unit and multiply that by the weight per square inch. No need to pull up your concrete or use a scale — math can help direct you toward your goal in just a couple of minutes.

How to Calculate the Unit Weight of Concrete

• Use a standard tape measure to find the length of your section of concrete. Convert this measurement into feet by taking whatever measurement you found in inches and dividing it by the number 12. Write this number down.
• Find the width of your section of concrete using a standard tape measure. Convert this measurement into feet by taking whatever measurement you found in inches and dividing it by the number 12. Write this number down.
• Use a standard tape measure to find the depth of your section of concrete. Convert this measurement into feet by taking whatever measurement you found in inches and dividing it by the number 12. Write this number down.
• Multiply the length by the width by the depth. The number you are left with is the total volume of the concrete, in cubic feet.
• Multiply your cubic feet volume measurement by 150. Penn State University estimates that the type of concrete used for building weighs 150 pounds per cubic foot. By multiplying the total cubic feet by 150, you will find the total unit weight of your concrete.

Taken from : http://www.ehow.com/how_7925144_calculate-unit-weight-concrete.html