v3.25.1
Cover - shares
 3 Months Ended
Mar. 31, 2025
Apr. 30, 2025
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2025  
Document Transition Report false  
Entity File Number 001-41806  
Entity Registrant Name Klaviyo, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 46-0989964  
Entity Address, Address Line One 125 Summer Street  
Entity Address, Address Line Two 6th Floor  
Entity Address, City or Town Boston  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02110  
City Area Code 617  
Local Phone Number 213-1788  
Title of 12(b) Security Series A common stock, par value $0.001 per share  
Trading Symbol KVYO  
Security Exchange Name NYSE  
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  
Amendment Flag false  
Entity Central Index Key 0001835830  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Series A Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   94,683,729
Series B Common Stock    
Document Information [Line Items]    
Entity Common Stock, Shares Outstanding   181,316,512