Bag Volume Calculator

Bag capacity is reported as a volume because it is the only figure that compares meaningfully across shapes, and the estimate here treats the bag as a rectangular prism: length times width times depth. Converting the three internal dimensions to centimetres and dividing the product by 1,000 gives litres, the international convention for packs and luggage. The other units follow fixed factors: one cubic inch is exactly 16.387064 cubic centimetres, so one litre equals about 61.024 cubic inches, and one US gallon is 3.785 litres. A daypack measuring 45.7 by 30.5 by 17.8 cm therefore works out at roughly 24.8 litres. The geometric figure always overstates what a bag holds. Rounded corners, seam allowances, foam padding, laptop sleeves, and organiser panels all consume interior space, and packed contents leave air gaps between them, so usable capacity is commonly 60 to 80 percent of the calculated volume; 70 percent is used here as a working figure. Useful reference bands are under 10 litres for pouches and clutches, 15 to 25 litres for a commuting or school bag, 25 to 35 litres for a full daypack, 35 to 50 litres for carry-on travel, and above 60 litres for expedition packs and duffels. Airline rules complicate this further, since a typical 56 by 36 by 23 cm cabin allowance is measured externally including wheels and handles, leaving roughly 35 to 42 litres of interior space.

The litre is a product of the French Revolution's metric reform. Defined in the 1790s as the volume of a cube one tenth of a metre on each side, it was redefined in 1901 as the volume occupied by one kilogram of pure water at maximum density, a definition that made the litre very slightly larger than a cubic decimetre. The General Conference on Weights and Measures reversed that in 1964, restoring the litre to exactly one cubic decimetre, and that is the value underlying every capacity figure printed on a modern pack. Selling bags by stated volume is much younger. Through the mid twentieth century packs were described by frame size, model name, or intended trip length, and American outdoor manufacturers commonly quoted capacity in cubic inches into the 1980s and 1990s before the industry converged on litres for international comparability. Standardisation was pushed hardest by air travel, where the binding quantity is not volume but linear dimensions: the familiar 22 by 14 by 9 inch cabin allowance and the metric 55 by 40 by 20 centimetre style limits are set carrier by carrier, and an IATA attempt in 2015 to promote a common optional 55 by 35 by 20 centimetre cabin bag was shelved after objections from airlines and consumer groups. Manufacturers today typically publish both a litre capacity and a full external dimension list.