v3.26.1
Cover - shares
 6 Months Ended
Jun. 30, 2026
Aug. 03, 2026
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
Document Transition Report false  
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  
Local Phone Number 650  
City Area Code 460-2551  
Title of 12(b) Security Class A common stock, par value  
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 2026  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Common Class A    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   42,644,388
Outstanding shares of Class B common stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   4,542,650