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Cover - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2025
Feb. 02, 2026
Jun. 30, 2025
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Current Fiscal Year End Date --12-31    
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 Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction false    
Entity Shell Company false    
Entity Public Float     $ 4.8
Documents Incorporated by Reference Portions of the registrant’s definitive Proxy Statement relating to its 2026 Annual Meeting of Stockholders are incorporated by reference in Part III of this Annual Report on Form 10-K where indicated. Such Proxy Statement will be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the fiscal year to which this report relates.    
Amendment Flag false    
Entity Central Index Key 0001835830    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2025    
Series A Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   145,497,436  
Series B Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   159,276,688