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Loss Per Share - Summary of Loss Per Share Calculation (Detail) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 28, 2021
Mar. 29, 2020
Loss Per Share [Line Items]    
Net loss attributable to Potbelly Corporation $ (14,472) $ (13,336)
Weighted average common shares outstanding-basic 25,944 23,646
Weighted average common shares outstanding-diluted 25,944 23,646
Loss per share available to common stockholders-basic $ (0.56) $ (0.56)
Loss per share available to common stockholders-diluted $ (0.56) $ (0.56)
Common Share Options [Member]    
Loss Per Share [Line Items]    
Potentially dilutive shares that are considered anti-dilutive 2,544 2,294