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.
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Spectacle Prescription Guide
What's the key thing to understand about 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 is most common in UK/European notation. Some prescriptions also include an AXIS value alongside CYL, which specifies the orientation of the astigmatism correction in degrees.
What should I know about 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.
What's the difference between Glasses and 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. If you wear contact lenses and deal with dryness or irritation between exams, CorneaCare makes eye-care products developed with ophthalmologists specifically for contact lens wearers and dry eye sufferers.
What's the key thing to understand about 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)