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Document and Entity Information
 12 Months Ended
Dec. 31, 2020
shares
Document and Entity Information  
Entity Registrant Name GERDAU S.A.
Document Type 20-F
Document Registration Statement false
Document Annual Report true
Document Period End Date Dec. 31, 2020
Document Transition Report false
Document Shell Company Report false
Entity Common Stock, Shares Outstanding 573,627,483
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 Emerging Growth Company false
ICFR Auditor Attestation Flag false
Entity Shell Company false
Entity Central Index Key 0001073404
Current Fiscal Year End Date --12-31
Document Fiscal Year Focus 2020
Document Fiscal Period Focus FY
Amendment Flag false