Somewhere I read that 'This container contains highly flammable petrol '. This particular instruction is written in my native as well as in the English language. My confusion arose when I was going to convert the sentence from my native language to English. I thought that it should be extremely rather than highly. I also found that both have the same meaning in my native language.

Can anybody tell me the difference between highly and extremely? When should I use each one? (Please give examples)

  • Please take help in preparing your question with correct capitalization and grammatically correct sentences.
    – Kris
    Feb 5, 2015 at 6:51
  • Both cannot be having the same meaning in any language. Please consult a good English dictionary for the meanings of the two words. Let us know what you found.
    – Kris
    Feb 5, 2015 at 6:52
  • @Kris please suggest good resources ,that help me in improving my writing. Feb 5, 2015 at 9:36
  • Please visit English Language Learners and see the questions and answers so far. They have a wealth of information useful for English language learners, especially non-native speakers of English. Good Luck.
    – Kris
    Feb 5, 2015 at 11:02
  • If you really want a strong warning, mention also that petrol is EXPLOSIVE. That is not a translation, but it conveys the dual danger. Also, such warnings are usually required to have an accompanying graphic, for the illiterate; check local regulations for requirements. Feb 6, 2015 at 9:14

3 Answers 3


There's nothing wrong with "extremely flammable". Both "highly flammable" and "extremely flammable" can be used, as this ngram shows. Depending on the substance, naturally.

One reason to prefer "highly flammable" to "extremely flammable" with petrol is that "flammable" is in itself quite an alarming term, indicating that a substance is quite likely to catch fire or burn under favourable conditions. "Highly flammable" shifts the substance even further on the scale of flammability.

Another and a bit more general reason to prefer "highly" over "extremely" is that "extremely" is a word that tends to be overused. It means outermost on the scale for flammability. Now petrol, to be sure, is quite flammable, but is it extremely so? (classification of flammability)


The two words overlap in meaning with many collocations, both meaning "to a great degree" as in:

  • highly/extremely influential
  • highly/extremely susceptible
  • highly/extremely sensitive

But there also exist many collocations where only one is possible, for example:

  • highly compensated (not extremely)
  • highly educated (not extremely)
  • highly placed (not extremely)

On the other hand we have:

  • extremely sick (not highly)
  • extremely smart (not highly)
  • extremely poor (not highly)

My sense is that extremely can be replaced by very, while highly can only sometimes be replaced by very. When it can, then highly and extremely are synonymous, as in your example: highly flammable = very flammable = extremely flammable.

  • This answer has a lot of good points. Better formatting would increase their visibility. Feb 5, 2015 at 4:47

In addition to other answers, consider that in respect to something which can be considered "more" or "less," the term "extremely" can refer to both extremes. However, the term "highly" often applies only to the positive extreme.

For example, it is natural to describe some thing or person as "highly sensitive." The same might be described as "extremely sensitive." However, it would seem out of place to describe something as "highly insensitive," while "extremely insensitive" does not carry the same awkwardness.

As a second example: try applying the words "highly" and "extremely" to "rich" and "poor." "Highly rich" might be acceptable, while "highly poor" is certainly not acceptable. "Extremely," by contrast, is applicable to both.

  • 2
    Yes. These words are synonyms, but there are very few synonyms whose distributions (situations where they can be used) are identical. 'It is highly cold' sounds ridiculous. This is probably true when one considers secondary modification of most if not all negative adjectives and adverbs (*highly wicked, *highly boring, *highly painfully ...). Feb 5, 2015 at 11:54
  • 3
    @EdwinAshworth Indeed. The unsuitability of 'highly' with negative properties seems to me to be the main difference between the two. (While I also see some difference in degree.)
    – anemone
    Feb 5, 2015 at 12:07

Not the answer you're looking for? Browse other questions tagged or ask your own question.