Cover - shares	9 Months Ended
	Sep. 30, 2025	Dec. 05, 2025
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2025
Entity File Number	001-42934
Registrant Name	BillionToOne, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-1082020
Entity Address, Address Line One	1035 O’Brien Drive
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94025
City Area Code	460-2551
Local Phone Number	650
Title of 12(b) Security	Class A common stock, par value $0.00001 per share
Trading Symbol	BLLN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Central Index Key	0002070849
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	Q3
Amendment Flag	false
Common Class A
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		41,216,110
Common Class B
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		4,552,650