For retraction of maxillary anteriors by 1 mm, space required is?
For every 1 mm of decrowding, same amount required for alignment. 
For every 1 mm of leveling the curve of spee, 0.5 mm space required in the arch. 
For every 1 mm of incisor retraction, 2 mm space required.
1 mm
2 mm
0.5 mm
1.5 mm