No Stupid Questions

They are "technically correct" measurements since they are a valid prefix, and could be used if you wanted. but they are very infrequently used in any industry. Since most of the time measurements are better served by higher precision (just using Meters) or need no precision at all over long distance (switch to kilometers), no need for excess measurement types unless necessary

Valid, but rarely used, as it's usually just as fast to say "two hundred meters" instead of "two hecto meters".

However, those prefixes have other (non-SI) uses. A hectare is common way of referring to a 100x100 meter area. And a decare is 10 ares, i.e. 0.1 hectare.

For distance, no. Day to day we use mm, cm, m and km. But in more specialised settings (e.g. construction) I've seen sometimes decameters.

For weight yes, grams, hectograms, kg, tons. Liquids is usually ml, cl, liters, hectoliters (not sure it's spelt that way).

In labs I've also seen also micro and nano of all three units.

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.

In The Netherlands we actually use “hectometerpaaltjes”, which translates to hectometer-signs. They are numbered signs placed on regional roads and highways every 100 meters, which is a hectometer. Although not a direct use of measurement, the term hectometer still is in active use this way.

In France "hectare" (10 000m²) is used for fields and burning forest. Beside that deca or hectometers are never used

As an American who has gotten very used to metric units in studying engineering, the general rule I picked up is that you typically only change units every three orders of magnitude. So 8 decameters would typically be expressed as 80 meters, maybe 0.08 kilometers. Decameters and hectometers are a thing, but they're not common units. Even centimeters don't see much use compared to millimeters.

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).

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

It depends on the situation, sometimes they are really handy but most of the time we stick to kilo, centi and mili.

Where i live, Hecto (100x) is used, for example to measure distances and areas for big properties. 1 hectometro equalls 100 m, or 1 hectarea (hm^2) equals 10000 m^2.

Also, it is widely use for pressure, cause 1 atm is 1013 hPa

Decameters are used but for special situations, like quantifying natural gas consumption

Yes, they are used, but typically in specialized applications which is why you don't see them every day.

In Italy we use hectograms ("ettogrammi", "etti" for short) in day to day life when buying groceries. You don't ask for 200 grams of ham, you just ask for 2 etti.

In Romania we use them for measuring areas. An "ar" is 100m^2 or a square decameter, and a hectare is a 10000m^2 or a square hectometer.

In America, I've seen nurses and diabetics use deciliters in reference to medication or concentration before.

In Poland: decimeters are sometimes used (I have been ordering cut sheet metal priced by square decimeters) , I have not seen decameters in use. Hectopascales are often used in weather reports. Decagrams are often used when buying food where these amounts make most sense (meat, candy).

The 'more exotic' prefixes are usually only used with some specific SI units and in very specific contexts.

I think hectare, decibel are few of them?

I'm American, but follow mostly Europeans and Canadians online and use metrics in my own head just because it makes more sense.

I gather that the deca-/deka- and hecto- (along with a few other) prefixes are similar to imperial furlongs, leagues, stones, barrels, kegs, and hogsheads: They exist, but no one uses them outside of very specialized circumstances.

Sweden uses decimeters and deciliters

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

In metric units, we primarily use the x1000 or x1/1000 steps, with a few notable exceptions. We use the centimeter (1cm = 1/100m) quite commonly, the centiliter (1cl = 1/100l) is also relatively common with drinks, and, IIRC, the Austrians use the deka, short for decagram (1dg = 10g).

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.

Czech republic here: decagram used to weight ham, bacon 🥓 or salami. Usually as "deka" without the gram.

Decilitres/centilitres are relatively common, but they're the only ones that come to mind.

In Austria, some things like ground beef are ordered in decagrams. My wife used to get confused responses when she tried that in Germany :).

I've seen some obscure uses in technical areas, but in general use no they aren't used. Metric is better skipping all those and using the thousands prefixes. cm is used but mm is much better. Europe uses cl but we in Canada don't, we use ml.