All tools on PDCalculator.net are for educational and informational purposes only. They do not replace professional eye care advice, diagnosis, or treatment. Always consult a licensed optometrist or ophthalmologist before purchasing eyewear or making any changes to your vision correction. Results are based on standard optical formulas and are provided as a general guide only.
📋 What is Plus/Minus Cylinder Conversion?
When you get an eye exam, your prescription can be written in two ways — plus cylinder (+CYL) or minus cylinder (−CYL). Both mean the exact same thing for your eyes, but look completely different on paper. Ophthalmologists often write in plus cylinder; online glasses stores require minus cylinder.
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Plus Cylinder (+CYL)
Common in ophthalmology offices. The cylinder value is a positive number.
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Minus Cylinder (−CYL)
Required by most online retailers like Zenni, EyeBuyDirect, Warby Parker.
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Same Prescription
Both formats describe identical optical power. Converting does not change lens strength.
📐 The 3-Step Formula
Standard formula used by every optician in the world. Our calculator applies it automatically:
// Starting: SPH / CYL × AXIS — e.g. +2.00 / +1.00 × 090Step 1 New SPH = original SPH + original CYL (+2.00) + (+1.00) = +3.00Step 2 New CYL = same magnitude, opposite sign +1.00 → −1.00Step 3 New AXIS = original axis ± 90° (keep between 1–180) 090 + 90 = 180// Result: +3.00 / −1.00 × 180 ✓
❓ Frequently Asked Questions
Does converting change my prescription strength? +
No. Transposition is purely a notation change. The optical power your lenses provide is identical in both formats. Think of it like converting 1km to 1000m — the distance is the same, only the way it is written changes.
Why do all three values change after converting? +
Because sphere, cylinder AND axis all change together to maintain the same optical power. It's normal for the numbers to look very different. For example, −1.00 / +2.00 × 045 becomes +1.00 / −2.00 × 135. Both are correct.
Which format do I use when ordering glasses online? +
Almost all online retailers (Zenni, EyeBuyDirect, Warby Parker, GlassesUSA) require minus cylinder (−CYL) format. If your doctor wrote a plus cylinder prescription, use this tool to convert it first.
What if my cylinder is already zero? +
If your CYL is 0.00, your prescription is purely spherical and no conversion is needed. The axis also has no meaning in this case.
Do I need a doctor to use this tool? +
No. This applies a standard mathematical formula, not a diagnosis. No medical training is needed. However, always double-check with your eye care provider before ordering lenses.
What does OD and OS mean? +
OD = Oculus Dexter = your Right Eye. OS = Oculus Sinister = your Left Eye. OU means both eyes.