When is S pronounced as Z at the end of the word

Pronunciation of 's' in Words Ending with Consonant and 's'

For words that end in a consonant and 's', the pronunciation of the 's' is influenced by the preceding consonant and whether it is voiced or unvoiced.

The following table illustrates this rule with examples of animal words:

Word Pronunciation Reason
ducks /dʌks/ 'ck' /k/ is unvoiced, so 's' is also unvoiced /s/
dogs /dɔːgz/ 'g' /g/ is voiced, so 's' is also voiced /z/
cats /kæts/ 't' /t/ is unvoiced, so 's' is also unvoiced /s/
toads /toʊdz/ 'd' /d/ is voiced, so 's' is also voiced /z/

Pronunciation of 's' in Words Ending with Vowels or 'w' or 'y' and 's'

For words that end in vowels or 'w' or 'y' and 's', the 's' is pronounced as a 'z' because all vowels are voiced. Here are some examples:

Word Pronunciation Reason
cows /kaʊz/ 'ow' /aʊ/ is a diphthong (voiced), so 's' is also voiced /z/
boys /bɔɪz/ 'oy' /ɔɪ/ is a diphthong (voiced), so 's' is also voiced /z/
toes /toʊz/ 'oe' /oʊ/ is a diphthong (voiced), so 's' is also voiced /z/
flies /flaɪz/ 'ie' /aɪ/ is a diphthong (voiced), so 's' is also voiced /z/
knees /niːz/ 'ee' /i:/ is a vowel (voiced), so 's' is also voiced /z/

Note that this rule also applies to other 's' endings, such as 'cases' /keɪsɪz/, where the voiced /ɪz/ results in a 'z' sound.