less/fewer than

Hi Tanit,
prescriptivism is not for me, but I can be prescriptive on demand!

Yes, in theory it should be fewer, but only if you really want to refer to "hectares", which is plural and countable. But as you know (hopefully you do know, lol), quantities followed by plural units of measurement are often considered as uncountable, taking account of the total amount and leaving aside the plurality of the units of measurement. Now, the problem is... I can't give you a good example, because I still have trouble with this too! Ok, I'll try:

About ten liters of water is needed for that chemical reaction. (Let's hope it is fine, LOL)

Another thing is that those two words together (="less than") have become a kind of fixed idiom that is usually used even if it refers to plural countable nouns.
There were less than one hundred people at the meeting.
Using "fewer than" is prescriptively ok, but it doesn't sound great, so you will find "less than" instead, as a kind of idiomatic replacement, even in good writing.
I think I learned this from some style guide, maybe on bartleby.com, but I am not sure.

This is just my opinion though.

Hi K.!

To learn that you can be prescriptive has been a great surprise! I thought you enjoyed learning rules only to come up with a clever way of breaking them!

Kooyeen

But as you know (hopefully you do know, lol), quantities followed by plural units of measurement are often considered as uncountable, taking account of the total amount and leaving aside the plurality of the units of measurement.

Yep, I know, but for some reasons I cannot explain "less than X hectares" didn't seem quite right to my ear.
I wanted to thank you for mentioning bartleby.com. I think I've probably found there the answer to my question (that is: "Which one should I use?"):

Kenneth G. Wilson

Standard English still usually requires this basic pattern: use less with mass nouns and fewer with plural count nouns, as in less employment, fewer jobs. But Common English—and even some Standard—increasingly uses less with plurals, especially after than. Edited English still follows the basic pattern rigorously, however, except in a few idiomatic locutions, as in in ten words or less; in certain phrases involving money, such as less than a thousand dollars; and in some phrases involving plural measures of time and distance or other measures, also with than (less than four days, less than ten miles, less than five cups of coffee). Even in these, Edited English prefers fewer, and for many conservatives, the use of less where fewer is expected remains a strong shibboleth.

Tanit, don't worry! Several months ago I came to a standstill because I instinctively "knew" that is should be less but felt like it was violating the countable rule. It was my friends here to provided a link to the same reference that reminded me that distance, area, and time could be thought of as one until (a journey of less than ten miles, etc.) So every once in a while, natives get flummoxed too!

Hi Tanit

Here is the usage note from the American Heritage Dictionary:

The traditional rule holds that fewer should be used for things that can be counted (fewer than four players), while less should be used with mass terms for things of measurable extent (less paper; less than a gallon of paint). However, less is used in some constructions where fewer would occur if the traditional rule were being followed. Less than can be used before a plural noun that denotes a measure of time, amount, or distance: less than three weeks; less than $400; less than 50 miles. Less is sometimes used with plural nouns in the expressions no less than (as in No less than 30 of his colleagues signed the letter) and or less (as in Give your reasons in 25 words or less).

The sort of usage highlighted above is widely accepted.

I'd say fewer than would tend to be used in such instances if you want to stress or focus on the individual "pieces" that make up the measurement (for whatever special reason).