Systems for determining width/space limits for product mask layouts. A mask writer generates a first pattern on a test mask corresponding to a test mask layout. A lithography tool generates a second pattern on a wafer corresponding to a first pattern on a test mask by a lithography process using a preset exposure dose. A metrology tool measures widths of the first and second pattern. A controller determines a width/space limit for the product mask layout according to the width difference between the first and second pattern.