Cover - shares	3 Months Ended
	Dec. 31, 2020	Jan. 29, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	000-30205
Entity Registrant Name	CMC Materials, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-4324765
Entity Address, Address Line One	870 North Commons Drive
Entity Address, City or Town	Aurora
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60504
City Area Code	630
Local Phone Number	375-6631
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	CCMP
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		29,166,737
Entity Central Index Key	0001102934
Current Fiscal Year End Date	--09-30
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1