Spectacle Prescription & VA Calculator
Understand your spectacle or contact lens prescription — sphere, cylinder, axis, and add. Calculate spherical equivalent and interpret your vision correction numbers.
Spectacle Prescription Guide
Reading Your Prescription
SPH (sphere): the main correction power in dioptres. Negative = short-sighted (myopia) — distant objects blurry. Positive = long-sighted (hyperopia) — close objects blurry (and sometimes distant too). +1.00 to +3.00: mild hyperopia. +3.00 to +6.00: moderate. Above +6.00: high. −1.00 to −3.00: mild myopia. −3.00 to −6.00: moderate. Below −6.00: high myopia. CYL (cylinder): corrects astigmatism — the cornea or lens is not perfectly spherical. Negative CYL most common in UK/European notation. Some
Spherical Equivalent
Spherical equivalent = SPH + CYL/2. Used for contact lens prescriptions as a single power (toric contacts for astigmatism correction are a separate matter). Example: SPH −2.50, CYL −0.75, AXIS 90. Spherical equivalent = −2.50 + (−0.75/2) = −2.50 − 0.375 = −2.875 D. A −3.00 contact lens would be prescribed (rounded to nearest 0.25). For astigmatism above approximately −1.00 CYL, toric contact lenses are usually recommended instead of the spherical equivalent approximation.
Glasses vs Contact Lens Prescription
A spectacle prescription CANNOT be directly used for contact lenses — they sit on the eye at a different vertex distance (distance from lens to cornea). For powers above ±4.00D, the contact lens power differs from the spectacle prescription by a small but clinically significant amount. Vertex distance correction: contact lens power = spectacle power / (1 − vertex distance × spectacle power). Vertex distance typically 12-14mm. At −8.00D glasses, the contact lens power = −8/(1−0.012×(−8)) = −8/(1.
Astigmatism
Astigmatism occurs when the cornea or lens is slightly oval rather than perfectly spherical — like the surface of an egg rather than a ball. This causes images to focus on two different planes rather than one, creating a blurry or distorted image at all distances. CYL: the difference in power between the two meridians. Up to −0.75D: mild, may not require correction. −0.75 to −1.50D: moderate astigmatism. Above −1.50D: more significant. AXIS: the angle of the most curved meridian. 90° (vertical)
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