Answer: 1 inch Explanation: They need to be 1/4 depth of slab. 
Elite Concepts 
Answer: 1 inch Explanation: I know that control joints do not prevent cracks and its important to put your cuts in the right locations to minimize cracking.

COMMON SENSE FLOORING, INC. 
Answer: 1.5 inches Explanation: Control joints should be cut at a 1.5 inch depth or more. 
Treasure Coast Concrete Polishing 

ProMatcher 
Answer: 1 inch Explanation: ACI recommends control joint depth equal to 1/4 of the slab thickness. 
Maven Group LLC 
Answer: 1 inch Explanation: Control joints should be 1/4 of the thickness of the concrete slab. 
QUALITY CONTRACTORS 
Answer: 0.25 inches Explanation: All control joint depths must be a minimum of 1/4 the thickness of the slab. This is nonnegotiable. 
Colorado Concrete 

ProMatcher 
Answer: 1 inch Explanation: Usually one third of the thickness. 
Concrete services 
Answer: Not sure Explanation: Depends on the thickness of the slab. 
BellPro Construction LLC 
Answer: 1 inch Explanation: Contraction joints should be straight and continuous 
Black Dirt Services LLC 

ProMatcher 