If youâre like most people, you spend most of your nights awake, pacing your home, a quivering, sobbing, obscenity-shrieking wreck because you canât stop wondering just how the cargo capacity of automobile trunks are measured. Thatâs why Iâm happy to tell you thereâs an answer, and that answer involves eight very specific pieces of magic luggage, conjured from the very aether by the wizards of the Society of Automotive Engineers. Itâs time to reveal all.

I suppose this hypothetical set of luggage isnât really a secret, since itâs mentioned in an official U.S. government document known as SAE J1100. In that document, in section 2.4 Luggage Capacity, it states:

The luggage capacity will be measured with the use of simulated luggage described in paragraph 8.1 and properly installed, detailed in paragraph 8.2, in a luggage compartment separate from the passenger compartment.

Now, it should be mentioned that this set of simulated luggage only appears to be used for determining the capacity of a separate, enclosed trunk area, like what youâd find on a sedan, coupĂŠ, or some sports cars.

Hatchbacks and station wagons seem to have their cargo capacity determined by overall volume, as determined by measurements and math.

While youâd think the SAE could apply similar formulae to computing the volume of a trunk, they take a different approach, arguably a more practical and real-world focused approach involving the sorts of objects one is actually more likely to cram in a trunk.

Sure, if all you carried in your car were large, soft sacks of grain, then you could likely fill the entire irregularly-shaped volume with cargo, but thatâs not generally what most people pack in trunks.

To measure cargo capacity for trunks, the SAE came up with a set of what you could think of as the Platonic ideal of luggage. A hypothetical, idealized set of baggage that the SAE feels reflects what an average American wants to shove in their cars.

Here is whatâs in that set of luggage:

â˘ Menâs 2-suiter

â˘ Womenâs overnight

â˘ Womenâs pullman

â˘ Womenâs wardrobe

â˘ Womenâs train case

â˘ Menâs overnight

â˘ Golf bag containing:

2 woods, 4 irons, 1 putter, 10-1/2 shoes, 3 golf balls

â˘ H-boxes

The first thing you may notice about this set of luggage is that it seems pretty, um, dated. The gender differentiation between womenâs luggage and menâs luggage isnât really all that relevant todayâmodern suitcases are pretty gender-neutral, really.

Also, the womenâs luggage allotment is twice that of the man, if we assume the golf bag to be non-gendered. That feels like a pretty archaic notion, really. The list of luggage seems to have been decided in the era of ads like these:

Even though all of these pieces of luggage specified have somewhat distinctive shapes, theyâre all just defined as rectangular volumes, with their dimensions given. Theyâre not just featureless boxes, though, since âconventional handlesâ are specified.

The only image of any type given in the SAE doc about the luggage is a diagram of the golf bag:

As you can see, you have the option of using real or simulated golf clubs, as long as that shaded area is nice and rigid. They donât specify where the three golf balls are meant to be stored, though.

In addition to the named train cases and wardrobes and whatever, thereâs also things called âH-boxes,â which are roughly shoebox-sized boxes used to fill up any leftover space after the seven primary luggage items are placed in the trunk.

The SAE even describes how the trunk should be packed:

âPlace, in random order, as many as one standard luggage set of luggage into the luggage compartment, excluding H-boxes. When the best load is obtained using the standard luggage set, H-boxes may be added to arrive at the final load. Pieces from subsequent standard luggage sets may be used when the previous set is placed in the luggage compartment. A piece from the standard luggage set may be removed to place an H-box in the compartment, provided the removed piece is replaced.

The standard equipped spare tire and tools shall be properly installed in the luggage compartment. They may be loosened and moved to the limits of the attaching hardware and then retightened to attain the most advantageous position. Standard parts of the vehicle normally stored in the luggage compartment, such as a convertible top, shall be in the stored position when the usable luggage capacity is determined.

The luggage compartment lid or access door must close and lock freely without forcing or excessive slamming with all of the luggage in place in the compartment.â

Of course, automakers want to get the most luggage and H-boxes in their trunks as possible, so theyâre not going to just cram stuff in at random just because the SAE says so. A whole sub-discipline of effective SAE trunk-packing to maximize total volume has sprung up, with scientific papers being published about packing algorithms, and exciting AI-assisted packing visualizations like this:

Since the trunkâs volume that a carmaker is allowed to advertise is based on the total volume of these official synthetic suitcases and whatever, carmakers are very interested in finding the optimal packing of their trunks so a little volume is wasted as possible.