You do not need a scale to measure the weight of a steel tube; you can calculate it as long as you know the density of the steel it is made from. An object's density tells you how much mass it has in a given volume. According to the Engineering toolbox, the density of steel is 7,850 kilograms per cubic meter (7.85 grams per cubic centimetre). You can take some basic measurements to calculate the tube's volume of steel, then deduce its weight using the density value. To calculate the volume, you will need to use a meter stick to measure the inner and outer radii of the tube.

Measure the inner radius of the tube. The inner radius is the distance from the centre of the circle formed by the tube to the inside edge of the steel rim. For example, suppose the inner radius of your tube is 5 centimetres (cm).

Measure the outer radius of the tube. The outer radius is the distance from the centre of the circle formed by the tube to the outer edge of the steel rim. For example, suppose the outer radius of your tube is 6cm.

Measure the length of your tube. Suppose that the length of the tube in our example is 50cm.

Calculate the volume of the entire tube by plugging the outer radius measurement into the formula for the volume of a cylinder. In the example, the volume calculation would look like this:

Volume of tube = pi x radius^2 x length = 3.14 x 6^2 x 50 = 5,652cm^3

Calculate the volume of the hollow centre of the tube by plugging the inner radius measurement into the formula for the volume of a cylinder. In the example, the volume calculation would look like this:

Volume of hollow centre = pi x radius^2 x length = 3.14 x 5^2 x 50 = 3,925cm^3

Calculate the volume of steel in the tube by subtracting the hollow centre from the total volume. In the example, the calculation would look like this:

Volume of steel = 5,652cm^3 - 3,925cm^3 = 1,727cm^3

Multiply the volume of steel in your tube by the density of steel to obtain the mass of steel in the tube. In the example:

Mass of steel in the tube = 1,727cm^3 x 7.85g/cm^3 = 13,557g

Convert mass to weight by dividing your result in grams by 455--the number of grams in a pound (lb).

Weight of steel in the tube = 13,557g / 455g/lb = 13.5kg