Glasses Prescription Converter

Transposition steps (plus to minus cylinder, or vice versa):

1. New Sphere = Old Sphere + Old Cylinder
2. New Cylinder = -(Old Cylinder) (flip the sign)
3. New Axis = Old Axis ± 90° (keep the result between 1 and 180)

Optometrists and ophthalmologists sometimes write prescriptions in different cylinder notation formats – plus cylinder or minus cylinder. Both describe the same correction, just expressed differently. This calculator converts between the two formats instantly.

What the prescription values mean:

• Sphere (SPH) – corrects nearsightedness (minus values) or farsightedness (plus values). Measured in diopters.
• Cylinder (CYL) – corrects astigmatism. Can be written as a positive or negative number depending on the notation style.
• Axis – a number from 1 to 180 that describes the angle of your astigmatism correction.

When to use this converter:

• Your optometrist writes in plus cylinder but your online glasses retailer needs minus cylinder (or vice versa).
• You are comparing two prescriptions from different doctors and they look different but may actually be the same.
• You want to verify that a conversion was done correctly.

Practical example: Original prescription: -2.50 / +1.00 × 90

• New sphere: -2.50 + 1.00 = -1.50
• New cylinder: -1.00
• New axis: 90 + 90 = 180 Converted: -1.50 / -1.00 × 180

Tips:

• This conversion does not change your actual prescription – it just rewrites it in the other format.
• Always double-check with your eye care provider if you are unsure about your prescription.
• The axis must always stay between 1 and 180. If adding 90 gives you more than 180, subtract 90 instead.