Too many hyphens make text look cluttered, so avoid them where you can. See ndash;

After an adverb?

Do not use hyphens to link an adverb to an adjective:

genetically modified food, doubly special relativity

The only common exceptions to this are when “well” and “ill” are used as part of a compound adjective preceding the noun, when a hyphen is needed to avoid ambiguity:

well-behaved patients; ill-considered attack

Compound adjectives

Use a hyphen in phrases such as

Green-belt land, zero-gravity environment, state-of-the-art phones,

The boy is six and a half but a six-and-a-half-year-old boy

Measurements

A form of compound adjective that requires a hyphen to link quantities to their units, as in

12-centimetre discs, 3000-kilometre journey

(but not when the measurement is not adjectival, as in: the discs measure 12 centimetres across; the 4-hour flight took us 3000 kilometres).

The same applies when making compound adjective, such as

a 3-metre-long gun barrel

otherwise dubiety creeps in. But the construction is ugly. Writing around it, as in “a gun barrel 3 metres long”, is more elegant.

Sums of money:

No hyphen:

a $3 million budget

Avoiding ambiguity

Use a hyphen to clarify what would otherwise be an ambiguous phrase, as in

a little-known city

Compass points

north-east, south-west, etc

Prepositional nouns

Nouns formed from a verb plus a preposition are often hyphenated:

ready for take-off; paying for the clean-up

(derived from the verbs “take off” and “clean up”, which are not hyphenated)

Ranges (especially dates)

In the period 1979-82 funding was in short supply

though this often reads better if reworded: from 1979 to 1982, funding was in short supply.

Don’t use “2000-2500 years ago” (or, worse, “between 20-25 ºC”). Write “from 2000 to 2500 years ago”, “between 20 and 25 ºC” and so on.

Hyphens are also needed…

When spelling out numbers such as twenty-one, sixty-six, ninety-nine (although house style will dictate using numerals, except at the start of a sentence).