Learn Swedish: Gruset kostar trettio kronor kilot. - The gravel costs thirty kronor per kilo.

In Swedish, unit prices and rates are usually expressed by putting the unit of measurement in its definite singular form, without any preposition. So "per kilo" or "a kilo" translates to kilot (literally "the kilo").

Does this definite form rule work for things other than weight?

Yes! It works for almost all units of measurement, including volume and time. For example, "twenty kronor a liter" is tjugo kronor litern, and "a hundred kronor an hour" is hundra kronor timmen.

Is it grammatically wrong to say trettio kronor per kilo?

No, using per kilo is completely correct and widely understood in Swedish. However, using the definite form (kilot) is much more idiomatic and is the preferred choice in everyday speech.

Grus is an uncountable noun. Does it always take the definite form (gruset) when talking about prices?

Often, yes. In Swedish, uncountable materials and abstract concepts frequently take the definite form when spoken about in a general, generic sense (like pricing a commodity). While English might just say "Gravel costs thirty kronor a kilo," Swedish leans toward Gruset kostar... even if you aren't pointing at a specific pile of gravel.