Minimum Blank Size Calculator
This calculator finds the minimum blank size (MBS): the smallest uncut lens diameter that still fills the frame once the optical center is decentered to the patient's PD. Enter the frame's effective diameter (ED), the decentration per eye and a safety margin, which is 2 mm by default.
Minimum Blank Size
The smallest lens blank diameter that will fully fill the frame after decentering — what to order from the lab.
Formula
MBS = ED + 2 · |decentration| + safety margin
Minimum blank size formula
The formula is MBS = ED + 2 × decentration per eye + safety margin. ED is twice the distance from the center of the boxed shape to the farthest point on the lens edge. Decentration per eye = (frame PD − patient PD) ÷ 2, where frame PD = A + DBL.
The decentration is doubled because the blank is centered on the optical center: moving the optical center 4 mm toward the nose means the blank must reach 4 mm farther on the temporal side, which adds 8 mm to its diameter. The 2 mm margin covers edging and small chips. Some textbooks and questions leave it out; set the margin to 0 for that version.
Worked example
Frame: A 53, DBL 16, ED 57. Patient PD: 61. Frame PD = 53 + 16 = 69 mm; total decentration = 69 − 61 = 8 mm, so 4 mm in per eye (the decentration calculator does this step). MBS = 57 + (2 × 4) + 2 = 67 mm. Order a blank of at least 67 mm, for example 70 mm.
Monocular PDs: with OD 30.5 and OS 32, decenter each eye from half the frame PD, 34.5 mm: 4.0 mm OD and 2.5 mm OS. The MBS is 67 mm OD and 64 mm OS.
Common mistakes
Using A instead of ED: 53 instead of 57 gives 63 mm, a blank too small to reach the longest radius of the shape. Entering the total decentration as the per-eye value: 8 instead of 4 gives 75 mm, 8 mm more than needed. Ignoring outward decentration: when the patient's PD is wider than the frame PD, the optical centers move out, and the blank still grows. The calculator ignores the sign of the decentration, so enter 1.5 whether the optical centers move 1.5 mm in or out (-1.5 gives the same blank size: 62 mm with an ED of 57).
When to use it
Check the MBS before ordering uncut lenses, and whenever a large frame meets a narrow PD. If the MBS is larger than the blanks made in that material and design, choose a smaller frame or one whose frame PD is closer to the patient's PD. A closer frame PD also keeps a minus lens thinner at the temporal edge. For multifocals and progressives the lab also allows for the vertical position and inset, so treat the result as a minimum.
Minimum blank size on the ABO exam
MBS is a classic two-step ABO calculation: frame PD and decentration first, then the blank size. Before you calculate, check whether the answer choices include the 2 mm margin. Review Frame Measurements (Boxing System), then test yourself with the free ABO practice test.
Frequently asked questions
What is the minimum blank size formula?
MBS = ED + 2 × decentration per eye + 2 mm. For an ED of 57 mm and 4 mm of decentration per eye, MBS = 57 + 8 + 2 = 67 mm.
Why add 2 mm to the minimum blank size?
It is an allowance for edging and for small chips at the edge of the blank, so the finished lens still fills the frame. Some versions of the formula leave it out, which makes the result 2 mm smaller.
What is effective diameter (ED)?
Twice the distance from the center of the box around the lens shape to the farthest point on its edge. ED is never smaller than the A measurement and is usually larger, because that farthest point normally lies at an angle rather than straight across.
Does decentering out change the blank size?
Yes. The blank size depends on how far the optical center moves, not which way. Decentering 1.5 mm out adds the same 3 mm to the blank as decentering 1.5 mm in.
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