How does women's swimsuit sizing work?

On the Speedo website, they're sized like dresses: 4, 6, 8. But on Amazon, they're sized 32-40. Does this refer to bust size?

They don't work. Many manufactures of clothing size them according to their own concept of what the number verses sizing should be.

You have to carefully read their measurement details, and do a calculation of different parts of your body in order to come up with "their description in the sizes" available.

You'd be better off getting a suit in person at a store, not online. Tjmaxx usually has a good selection of suits.

Each manufacturer has it's own way of sizing. So you can be a 4 in one and a 10 in another.

It is even the same way for men with pants even though they are SUPPOSED to be measured by inches (32,34,36...) if you actually measure you will find clothes made in the US are actually larger than that.

WHY?

It's a subliminal thing to think you are smaller than what you really are.

Now to answer your question...

Women's swimsuits SHOULD have TWO sizes for a bikini. The size of the suit (which corrosponds to an actual range of sizes) and the bust size which would be the 32-40.