Can you use slang in IELTS Speaking?

Nobody will stop you using slang, but it is not what the marking scale asks for. The scale asks for idiomatic language and a sense of style, which are different things.

What the band descriptors say

Vocabulary is marked under Lexical Resource. The public descriptors mention idiom from Band 7 upwards. At Band 7 they describe “some ability to use less common and idiomatic items and an awareness of style and collocation”, and they add that “inappropriacies occur”. At Band 8 the wording is “skilful use of less common and idiomatic items”, and at Band 9 it is sustained use of accurate and idiomatic language.

Lower down, the same criterion marks the cost of poor choices. Band 6 allows that vocabulary use “may be inappropriate but meaning is clear”, and Band 4 describes frequent inappropriacies and errors in word choice. The word slang does not appear in the Speaking descriptors. The word appropriate does, again and again.

Idiom and slang are not the same

An idiom is an expression that people of all ages and regions understand, even though its meaning is not obvious from the words: “a piece of cake”, “under the weather”, “on the same page”. Slang is informal vocabulary that belongs to a group, a place or a moment, and it often dates quickly.

The descriptors reward the first because it shows range and control. Slang does the opposite job: it depends on the listener sharing your circle, and the examiner is applying the same scale to candidates from many countries.

What IELTS providers say

IDP lists the need for a native accent or slang as a myth and answers that your accent or slang is not part of the evaluation criteria and does not affect your score. In a separate article on the descriptors, IDP says idiomatic language and less common words are important at Band 7 and up, that this does not mean memorised phrases and clichés, and that it takes practice to use them naturally.

Plain advice

Use the vocabulary you would use with a colleague or a teacher you get on well with. Add an idiom when it fits what you are saying, and only if you are sure of its meaning and its grammar. If you are not sure, use a plain word: a correct simple word costs nothing, and a wrong clever one is exactly what the descriptors describe as inappropriate.

To build range, work on collocations and precise everyday words first, then on a small set of idioms you can use without thinking. Paraphrase matters too: from Band 6 the descriptors credit explaining an idea in other words when the exact word will not come.