Read your glasses prescription without guessing

Keep the signs and units

Read one eye at a time. SPH gives the spherical component; CYL and AXIS describe the amount and orientation of astigmatic correction. The unit D means dioptres, while the degree sign beside AXIS means an angle. ADD, if present, describes additional near power, and prism needs both an amount and base direction. PD is a positioning measurement in millimetres, not another power. A prescription is a set of related fields, so copying only the largest number loses important information.

SPH and CYL are lens powers in dioptres; AXIS specifies cylinder orientation in degrees. ADD records additional near power when prescribed. Copy every sign exactly rather than converting a prescription yourself.

Separate prescription and fitting

PD and fitting height help position lenses in the chosen frame. Ask your dispenser to explain whether PD is recorded for each eye or as a total. A glasses prescription is not a contact lens prescription.

A worked example

Teaching example: SPH −2.00 D, CYL −1.00 D, AXIS 180°. The spherical equivalent is −2.00 + (−1.00 ÷ 2) = −2.50 D. That arithmetic compresses the prescription for some descriptive uses; it does not preserve the cylinder and axis and must not replace the original order. Likewise, 180° is not more severe than 90°. Keep the original notation, because prescriptions can use different cylinder conventions that require a proper conversion to compare.