Finding the volume of a scientific flask is simple; just look on its side for the scale. Finding the volume of an everyday flask sometimes is a little more scientific. If the size of the flask has been scratched off the bottom or the flask is unmarked, then you can conduct an experiment to measure its volume. Correctly measuring a flask's volume can help illuminate the basic property of volume and the mystery of the size inside.
Things You'll Need
 Water
 Measuring cup or graduated cylinder
 Kitchen scale
Measuring Cup Method

Empty the flask of any contents and allow the inside to dry.

Fill the flask to the top with water.

Pour the water into a measuring cup or graduated cylinder and record the volume.
Kitchen Scale Method

Fill the flask with water, and then weigh the flask on the scale. For example, the weight of the flask filled with water equals 3 kilograms, or 3,000 grams.

Pour the water out, and then weigh the empty flask. For this example, the empty flask weighs 2.25 kg, or 2,250 g.

Subtract the weight of the empty flask from the full flask. For this example, 3,000 g minus 2,250 g equals 750 g.

Divide the weight of the water by its density, which is 1 gram per cubic centimeter (g/cm^3), to calculate the volume. For this example, 750 grams divided divided by 1 g/cm^3 equals 750 cm^3.

Convert the cubic centimeters to milliliters. Cubic centimeters and milliliters measure the same amount of volume. For this example, 750 centimeters^3 becomes 750 mL. The volume of the flask is 750 mL.
References
 Spike's Science Projects: Measuring Volume of Liquids
 School for Champions: Characteristics of Fluids
 Vision Learning: Density
 " The Everything Kids' Easy Science Experiments Book: Explore the World of Science Through Quick and Fun Experiments!"; Elizabeth J. Mills; 2010
 "150 Great Science Experiments: Ingenious, EasyToDo Projects Explore and Explain the Wonders of Science and Technology"; Chris Oxlade; 2001