Cover Page - shares	3 Months Ended
	Dec. 31, 2020	Feb. 01, 2021
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	1-39093
Entity Registrant Name	BellRing Brands, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	83-4096323
Entity Address, Address Line One	2503 S. Hanley Road
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63144
City Area Code	314
Local Phone Number	644-7600
Title of 12(b) Security	Class A Common Stock, $0.01 par value
Trading Symbol	BRBR
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Ex Transition Period	true
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Central Index Key	0001772016
Current Fiscal Year End Date	--09-30
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Common Class A
Document Information [Line Items]
Entity Common Stock, Shares Outstanding (in shares)		39,509,247
Common Class B
Document Information [Line Items]
Entity Common Stock, Shares Outstanding (in shares)		1