Free Tool
Cylinder Sign Converter
Convert between positive and negative cylinder prescription formats instantly.
Medical Disclaimer: This tool is for informational purposes only. Always confirm your prescription with your eye care professional before ordering contact lenses or glasses. Contact lenses are medical devices and require a valid prescription.
Original Prescription
Right Eye (OD)
Converted Prescription
Right Eye (OD)
Enter a prescription to see results
Original Prescription
Left Eye (OS)
Converted Prescription
Left Eye (OS)
Enter a prescription to see results
How Cylinder Sign Conversion Works
Contact lens prescriptions are almost always written in minus cylinder form, while some optometrists — particularly in the UK — write glasses prescriptions in plus cylinder form. The two formats describe exactly the same optical correction; only the notation differs.
To convert between the two, three values change:
New SPH = Old SPH + Old CYL
New CYL = Old CYL × −1
New AXIS = Old AXIS ± 90°
The axis shifts by 90° — add 90° if the original axis is 90° or below, subtract 90° if it is above 90°. This keeps the axis within the valid 1°–180° range. The optical result is mathematically identical in both forms.