Use packing peanuts, not a bathtub

The reliable way to measure a backpack’s capacity with the label gone is to fill it with something cheap and pourable, then measure that material’s volume.

No, water displacement doesn’t work. Submerging the pack in a bathtub measures the outside volume, not what fits inside, and the fabric itself just messes up the number. Don’t bother.

Grab a bag of packing peanuts, dry rice, or even cheap beans. Fill the pack completely, like you would for a trip. Then pour the filler into a cardboard box with straight sides. Measure the box’s length, width, and the height the filler reaches; multiply those numbers and convert to liters. 1 liter = 1,000 cubic centimeters, so if the box is 30 cm × 30 cm and the filler comes up to 40 cm, that’s 36,000 cm³ = 36 liters.

For smaller packs, use a kitchen measuring cup and count how many scoops it takes. It’s not a lab-grade measurement, but it’ll get you within a couple liters, which is close enough to know what you’re dealing with.