If you wear heels to dance in, you probably have a current preference as to heel height. And suppliers and makers of dance shoes normally tell you what their heel height is, so you can decide which product to buy, if you order online.
There are, however, pretty wide variations to how they measure.
My preferred heel height for tango shoes is, more or less, 7cm - which is about the same as 2.75" if you're American, or you're the kind of British eccentric who measures even small things in the old-fashioned way, instead of only things larger than you can easily carry. At 9cm I'll get joint pain, knee strain, and a choppy, ungainly motion. 8cm is my maximum. Lower is fine by me, right down to socks.
Here are some salsa shoes, made by a company called Oobashoo, who as far as I know are British. They sell these heels as 2.75". I've rested a steel ruler, which starts at zero, in the gully of the seam that goes up the back of each pair of shoes, and taken a picture as near straight-on as I could manage so there's not too much perspective. Then I've continued the ruler's lines at 7cm and 8cm across the picture.
|Oobashoo - 2.75 inch heels (=7cm)|
This next pair are made in Italy for a German brand, Werner Kern. Their range for the tango market is called Nueva Epoca. This style is labelled as a 7cm heel. (The thickness of the heel is completely irrelevant to comfort or function as far as I'm concerned, as long as it's correctly placed, and in many cases I prefer the look of the thicker ones; but a thinner heel is the current fashion for tango.)
|Werner Kern Nueva Epoca - 7cm heel|
Lastly, here are the well-known Argentinian brand, Comme Il Faut (sold in London by Coleccion la Recoleta). Comme Il Faut label them as 7cm heels. I really think this is pushing it. I would label them 8cm. On the upside, if I can dance well in these without hurting myself, then I can probably wear Nueva Epoca's 8cm styles.
|Comme Il Faut - “7”cm heel|
When I buy a new brand I try to have a longish try-on with big steps and pivots, and to wear them first to a nice long gentle practice session, and do some boring technique exercises, just to make sure they're OK, rather than find out they're not when it matters.