Volume is calculated in cubic inches by multiplying height x width x depth. Bags that have the same measure in linear inches can vary greatly in volume. For example, a 20 x 15 x 10 inch bag has a volume of 3,000 cubic inches. A 5 x 35 x 5 inch bag has a volume of 875 cubic inches.

Luggage capacity is typically measured in liters (L) and is calculated by multiplying the internal dimensions of the bag: Height x Width x Depth. For a standard rectangular suitcase, the formula is straightforward: if your bag measures 55cm x 40cm x 20cm, you multiply 55×40×20=44,000 cubic centimeters. Since 1,000 cubic centimeters equals 1 liter, this bag has a theoretical capacity of 44 liters. However, real-world capacity is often 10-15% less than the external dimensions suggest due to the thickness of the suitcase walls, the space occupied by the retractable handle system (the "trolley handle"), and the wheels. Many manufacturers use a "bead test" to get an exact measurement, where they fill the bag with small plastic pellets and then pour them into a graduated cylinder to find the true volume. When shopping for a bag, look for the "internal volume" rather than just external dimensions to get a better sense of how much you can actually pack. For reference, a 40-45L bag is the standard maximum for a carry-on, while a large checked bag usually falls between 70L and 110L.