To express approximations like "about twenty" or "a hundred or so," French uses a specific noun suffix.
The Suffix "-aine"
Turn a number into a noun of approximation by adding the suffix -aine. If the base number ends in an -e, drop it first (for example, trente becomes trentaine). Note a small spelling change for dix (ten): its 'x' changes to a 'z' to become dizaine.
Because these estimators are nouns (literally meaning "a set of about X"), they are all feminine and used with une.
When you state what you are counting, you must connect the estimator to the noun using de or d'.
This pattern works for exact tens up to sixty (une quarantaine, une cinquantaine, une soixantaine), as well as for a hundred (une centaine).
Thousands and Ranges
To say "about a thousand," the suffix pattern is not used. Instead, use the word un millier. Unlike the others, millier is masculine. It still requires de before a noun.
When you need a wider estimate, you can easily pair two of these nouns together.
Finally, even though words like une soixantaine already mean "about sixty," French speakers frequently add the adverb environ (approximately) to emphasize the estimate.
