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Minimum blank size
Minimum blank size = ED + 2 x decentration per lens + allowance. The decentration per lens is (frame PD - patient PD) / 2, where the frame PD is A + DBL. A typical edging allowance is about 2 mm. The calculator below runs the whole chain from frame measurements and shows each step.
Minimum blank size calculator
Minimum blank size: 64.0 mm
- Frame PD = A + DBL = 52 + 18 = 70 mm.
- Decentration per lens = (frame PD - patient PD) / 2 = (70 - 64) / 2 = 3.0 mm (inward).
- Minimum blank = ED + 2 x |decentration| + allowance = 56 + 6.0 + 2 = 64.0 mm.
Practice blank sizing
Unlimited generated problems with exact answers and worked steps.
Go deeper
The formula and its variables also live on the printable formula sheet. For the prism a decentration induces once the lens has power, see Prentice's rule.
Frequently asked questions
How do you calculate minimum blank size?
Minimum blank size = effective diameter (ED) + 2 x decentration per lens + an allowance for edging, typically about 2 mm. Decentration per lens is (frame PD - patient PD) / 2, where frame PD = A + DBL.
What is the effective diameter of a frame?
Twice the longest distance from the boxing center of the lens shape to its edge. It captures how far the shape reaches from center, which is what the blank must cover after decentration.
Why does decentration count twice in the formula?
Moving the optical center off the boxing center by some distance leaves the blank short on the opposite side by that same distance, so the diameter has to grow by twice the decentration to keep the shape covered.
What if my calculated blank is bigger than any available lens?
Choose a smaller frame, a frame PD closer to the patient PD, or a lens design offered in larger blanks. The calculator flags unusually large results so you can revisit the frame choice.