How do you calculate minimum blank size (MBS)?

• A. MBS = ED + total decentration + 2 mm
• B. MBS = ED + total decentration
• C. MBS = ED + total decentration + 2 mm
• D. MBS = ED + 2 mm

Answer: (C)

Minimum blank size is the stock lens blank diameter needed to produce the finished lens, factoring in how much the lens must be shifted within the blank (decentration) and a small finishing allowance. You add the final lens diameter, the total decentration required to align the lens with the frame, and a 2 mm margin for finishing. That 2 mm handles beveling, trimming, and small manufacturing variations. So the best expression is MBS = ED + total decentration + 2 mm. For example, if the final lens diameter (ED) is 60 mm and the lens must be decentered by 3 mm, the MBS would be 60 + 3 + 2 = 65 mm. Leaving out the decentration or the finishing margin could lead to a blank that’s too small to safely edge and fit.