No matter what you're trying to accomplish in engineering or physics, keeping your measurements consistent is important. When you measure how much of an object you have, you should know what exactly you're measuring. If you're looking to calculate the weight of a block of concrete, you can do this with the density of concrete, the volume and the acceleration due to gravity, or with the specific weight and the volume
Concrete Weight Calculator
If you know the density, the mass per unit volume of a material (such as concrete), and the volume of the material, you can multiply the density times volume to determine the mass, and, from there, the weight. While mass measures an object's amount of matter, weight is the force a planet exerts on an object due to gravity.
If you know the mass of a material, you can convert mass to weight using the weight equation w = mg, in which w is the weight in Newtons, m is the mass in kilograms, and g is the constant of gravitational acceleration, 9.8 m/s2.
Density of Concrete
The density of normal concrete is 2400 kg/m3, and for lightweight concrete, 1750 kg/m3. For comparison, the density of steel is 7850 kg/m3. That means you can multiple these densities by volume in m3 to determine the mass of the concrete.
Make sure you use consistent units. If the density is given in kg/m3, you will obtain a mass in kg. These densities depend greatly on the elemental materials that comprise them.
This holds true because the equation is a specific situation of Newton's second law, F = ma, that force equals mass times acceleration for an object that has a force applied to it. You can use the gravitational force between the object and the Earth with this equation to determine its weight. In this case, the force is the weight in pounds, the mass is the mass of the object in kilograms, and the acceleration to the gravity on the Earth is 9.8 m/s2.
Specific Weight or Unit Weight
Similarly, if you know the specific weight (or unit weight) of a material, the weight per unit volume (generally given in N/m3, or "newtons per meter cubed"), you can determine its weight by multiplying it by the volume.
If you know the the density of an object, given by the letter ρ ("rho"), mass per unit volume, you can multiply it by the acceleration due to gravity g, 9.8 m/s2 to find the specific weight of an object, indicated by the symbol γ ("gamma") in weight per unit volume. This gives you the specific weight equation γ = ρ__g to compare the three values to one another.
You can use this formula and other similar ones to calculate the unit weight of steel. Using the geometry of something like a steel bar, you can calculate unit weight as the total weight of the bars divided by the volume of the steel bars. If you 2469 kg of steel bars that were 1000 meters long, 2 meters wide and 3 meters in height, you could calculate the volume as 6000 m3. Then, the unit weight would be 2469 kg / 6000 m3, or about 0.41 kg/m3.
Unit Weight of Concrete
You can use the specific weight equation to convert the density of concrete and steel to specific weight. If you know the density ρ and gravitational acceleration g, you can determine specific weight γ by multiplying them_._ Multiplying the densities 1750 kg/m3, 2400 kg/m3 and 7850 kg/m3 for lightweight concrete, normal concrete, and steel, respectively, by 9.8 m/s2, you can determine specific weights as 17150 N/m3, 23520 N/m3, and 76930 N/m3, respectively.