How this works it out
It tries your carton six different ways up, because a box that will not fit lying down often fits standing, and works out how many rows, columns and layers each way allows. Then it takes the best of the six.
That is the same thing a good loader does in their head, and it is much more accurate than dividing the volumes, which quietly assumes your boxes can bend.
Then it checks the weight. Every container has a limit on how much you may put in it, and dense cargo hits that limit long before it fills the space. The tool tells you which of the two stops you first, because that is what decides your next move.
Why the answer is a little pessimistic on purpose
Real loading loses space to dunnage, to lashing, to the shape of the door opening, and to the plain fact that people need room to work. Where a number here says a container is nearly full, treat it as full.
The height trap
The door of a container is shorter than the inside. On a standard container the inside is about 2.39 metres tall but the door opening is about 2.28 metres. Anything taller than the door has to be loaded flat or it does not go in at all, whatever the volume says.
What to do with the answer
If space runs out first, your cargo is light and bulky. Look at the packing before you book a second container: smaller cartons, flat packing, or nesting can move you back under the line, and that saves the whole cost of a second box.
If weight runs out first, packing will not help. Consider splitting across two 20ft containers rather than one 40ft, because two 20ft boxes allow more total weight than one 40ft does. That single fact surprises people every week.
If you are just over the line, say 105 percent of one container, that is the most expensive place to be. It is worth an hour of work to get under it: leave a few units behind, ship the overflow by another route, or repack.
The two numbers to check before you book
The road weight limit at the delivery end. A container can be legal at sea and illegal on the road to your warehouse. That is a fine and a re-handling bill.
Whether a 40ft high cube costs much more than a 40ft. Usually it is a small difference for about nine extra cubic metres, which is the cheapest space you will ever buy if your cargo is bulky.