Saying something is 80 meters long is far easier to process than eight decameters.

No, some measurements just aren’t used, even when they’d be a good fit.

Like lengths. We never use anything above km. Even for things like space, we say “million km” rather than gigametre.

The closest we come to hectometre is hectare, which is used for land area.

We use litres, which is one decimetre cubed. We use hectares, which is one hectometre squared. But the beauty of it is, that you can just convert everything to units that are more widely understood.

• 1 decimetre = 10 centimetres = .1 metres

• 1 hectometre = 100 metres = .1 kilometre

From my experience in Norway, these are typical in context of daily speech:

Weight (gram): tonne (a substitute name for Mg (Mega)), kg, hg, g, mg, μg (mostly in medicine)

Distance (meter): mil (10 km), km, m, dm (kinda rare), cm, mm

Volume (liter): l, dl, cl, ml

In my experience, the deca-predix is very rarely used. Most of the missing prefixes are just substituted for numbers, i.e. saying “a thousand kilometers” is much more common that “a megameter”. Of course, this differs depending on context, as a lot of the prefixes become more common within scientific fields where the sizes are common.

On a separate note, even the numbers can be a bit inconsistent. It has bothered me that it’s often common to say “a thousand milliard” instead of “one billion” (also note that we use the long scale).

These two specifically - I don’t think I’ve ever seen them.

Hectoliters are sometimes used e.g. for measuring beer consumption for an event, decimeters in some informal contexts, some country commonly describe drink sizes in centiliters or deciliters.

Centimeters are common, I’d say more common than millimeters in informal context.

in school in austria we teach dezimeter (tenth of a meter)