When should you use “number of” and when to use “amount of”?
May 17th, 2011, by Courtney Cummings
Amount: for uncountable objects (liquids, gases, very small solids)
X: “We measured out the proper number of hydrochloric acid.”
O: “We measured out the proper amount of hydrochloric acid.”
Number: for countable objects (most solids, abstract measurements)
X: “The amount of positive results obtained was determined after 24 h of incubation.”
O: “The number of positive results obtained was determined after 24 h of incubation."
When you describe an indefinite number or amount of something, always be sure to use the appropriate noun (amount or number) to remind the reader what you are measuring.
X: “Since the metabolites produced in hepatocytes were below the detection limit…”
O: “Since the amount of metabolites produced in hepatocytes was below the detection limit…”
Without “amount of”, there is confusion that the SIZE of the metabolites is being measured (“the metabolites…were below the detection limit”).
X: “…to determine the amount of copies of DNA…”
O: “…to determine the number of copies of DNA…”
“Copies (of DNA)” are countable so we use “number” here.
Also, remember to check subject-verb agreement and match the appropriate words.
“The number of positive results was low for Group X compared with Group Y.”
“Number” is a singular noun, so we need a singular verb (“was”),
EXTRA: When should you use “No.” and when should you use “number”?
Many authors think “No.” is an abbreviation for “number”, but this is not always true! Although “No.” is an abbreviation for “number”, it cannot always be used in the same situation.
• 号: “No.” is okay!
o “The test item was intravenously administered to Animal No. 12345.”
o “We used Room No. 5 of Japan Laboratories in the present study.”
• 数: “No.” is not okay!
× The no. of animals used in the present study was 20.
O The number of animals used in the present study was 20.
