This is right. The logarithmic nature of dBs means that a huge variation can be described without using prefixes. For example, when measuring sound level, 0 dB is the threshold of hearing, while 130 dB is ‘instant ear damage’. The smallest change in level detectable by the ear is about 1 dB. There is just no need for mdB and I have never come across such a unit.
OK, so if a similar problem were given in metres and kilometres…how is that any harder?
so call it a 1000m or 1500m instead and take the problem of oxygen out of the equation. Seeing as I did read your post carefully I can tell you that oxygen was not mentioned in your orginal problem.
That may be the case, and you seem to be suggesting that it is in part due to neat rounding to appx 6000 ft. Well, as luck would have it a kilometre is already defined as an exactly round 1000m and that would make the vast majority of calculations even easier.
when you use different prefixes for the same type of measure. e.g. 1 cubic centimeter of water is 1 milliliter.
I think it’s more or less clear from Schnitte’s last post, but your example is a little misleading because the bolded prefixes are not attached to the same type of measure: dimension(meter) = L, but dimension(litre) = L³ .
In English-speaking countries, the height of horses is measured in ‘hands’. A ‘hand’ is defined as 4 inches, which happens to be very close to a deci-meter (3.937008 inches) – only a 1.5% difference. (For even the record tallest horse, it’s a very minor difference: 20½ hands vs 21 decimeters).
So back when it was looking like UK/USASA/Australia would switch to metric measurements, the decimeter was suggested as the appropriate measurement for horses’ height. Because it would be so easy for horse people – almost no adjustment at all. But that never happened.
Re: mmHg, obviously you can measure that with a simple manometer made from a U-tube and a ruler. But our Schlenk lines were pulling 40 millitorr. Before we bought digital ones we had some tilty thing with different diameter tubes for more precise, low-pressure measurements. I cannot remember the name.
A good reading is important for vacuum distillations.
And we definitely said “millitorr”.
What, pascals too good for you? 
And what happens if you’re measuring vacuum on the moon or in space? Measurements of millimetres of mercury will be off because the local weights will be different.
Oh, let’s confuse this even more. There’s a metric unit for pressure called a bar, equals 100 kilopascals, and one bar is close to regular atmospheric pressure at sea level – okay, it’s really the pressure at 15 deg C, 111 m elevation, per Wikipedia. So weather forecasts often use millibars, because 1000 mbar (or thereabouts) is easy enough to work with. I’d guess that ‘milli’ is the only prefix that works here. That said, bar is not an ‘official’ metric unit.
You missed the most important one of all: “An area “some multiple” of the size of Rhode Island” WTF is that??? Does anyone outride of RI conceptualize it?
The bar is also used in diving, because, conveniently, ten metres of pressure head of water corresponds to one bar of pressure. So at a depth of 30 metres, pressure is 4 bar (one bar from atmospheric pressure, plus one bar for each ten metres you’re going down). When a watch is labelled as waterproof up to, say, 50 metres, all it means is that it has been tested to withstand pressure up to 6 bar.
No, a cubic centimeter is L³ not L.
It’s (centimeter)³, not centi(meter)³, was my point. Liters of water and kilograms will scale the same way, though: 2 l weigh 2 kg
As for the initial question, I have only ever heard Becquerel (Bq), the SI unit of radioactivity, used with prefixes such as kilo- or mega-. That’s because the Bq is defined as one decay per second, which is very little for radioactive substances, so pretty much all applications (maybe outside basic research) will be orders of magnitude above that. Ironically, the predecessor unit, the Curie (Ci), was suffering from the opposite problem: 1 Ci (equivalent to 37 GBq) was so much that it would often have to be prefixed with centi- or milli- prefixes.
Really? I was going to nominate Liter for the OP, since I can’t recall ever seeing liter with any prefix other than milli- (except perhaps in a general way “in the microliter range”).
Now, 'round here water bills and such will be in cubic feet or gallons (or Million Gallons, for big amounts)
I’ve never seen kilolitre either; around here, that volume is usually called a cubic metre (and billed as such on water bills). The output of German breweries, however, is traditionally measured in hectolitres.
Liter is quite often also used without any prefix at all, like a 2-liter bottle.
I think they just try to avoid fractions or decimals when using units to present to the general public.
I’ve only ever heard calorie paired with kilocalorie. Is there any other common usage?
I was somewhat amused when I looked at a label in the cargo area of a EU-market Ford Transit van which indicated some sort of capacity in “daN” (decaNewtons.) I looked into it a bit more and that was evidently the “lashing” capacity; i.e. the rated force the cargo hold-downs could take. surprised me that daN is a common measure for LC.
I don’t know about industry lingo- but 1 kg weighs roughly 1 daN. That said, if you buy eg a carabiner, it’s always stamped with a rating of however many kN, a daN being way too little…