Measure the mass of your liquid required to completely fill the tube if you are not working from a physics problem which gives you this value. Prevent your digital balance from measuring the mass of the liquid's holding container by placing a clean, empty beaker onto it and pressing the "Tare" button. Pour your liquid into the tube until it is completely filled, then pour the liquid into the beaker you used to "Tare" your balance. Place it on the balance and record the mass. If you are not in a laboratory setting, record the mass of 24.6 grams propanol to complete an example calculation.
Convert the mass of liquid required to fill the tube into volume using the density equation, D = m/v, where "D" is a liquid's density in grams per milliliter, "m" represents the mass in grams and "v" the volume in milliliters. For the example calculation, whose pipe requires 24.6 g of propanol to fill completely, consult the density table provided in "Resources" to find that propanol's density is .804 g/mL, you can plug these values into the equation as follows: .804 = 24.6/v, or v = 24.6/.804 = 30.6 mL propanol.
Measure the length of your tube in centimeters. If your tube is rigid, you can use a centimeter ruler. Otherwise, use a tape measure. For the example calculation, set the tube's length equal to 10 cm.
Extrapolate the diameter of the tube using the equation for volume of a cylinder or tube, v = π(r^2)*h, where "π" = pi = 3.1415, "r" is the "radius" of the tube -- one-half its diameter -- and "h" is its height or, in this case, its length. Keeping in mind that you measured the tube's volume by filling it with liquid, arrange your equation for the example calculation as follows: v = 30.6/mL = (3.1415)(r^2)(10) = 31.415(r^2), so r^2 = 30.6/31.415 = .974. Perform a "square root" function on both sides of your equation to find that the radius is .987 cm, which means that your diameter -- or twice the radius -- is 1.97 cm.