Spherical Equivalent Calculator — Eyeglass Prescription SE & Transposition
Calculate the spherical equivalent (SE) of an eyeglass or contact lens prescription — sphere plus half the cylinder — with the myopia/hyperopia category and the plus/minus cylinder transposition.
How it works
Spherical equivalent SE = sphere + cylinder ÷ 2 (dioptres). Transposition: new sphere = sphere + cylinder; new cylinder = −cylinder; new axis = axis ± 90°. Myopia: SE ≤ −0.50 D; high myopia: SE ≤ −6.00 D (International Myopia Institute).
The spherical equivalent collapses a sphero-cylindrical prescription into the single spherical power that puts the circle of least confusion on the retina. It is used to classify myopia in research and screening, to choose a spherical contact lens when astigmatism is small, and as the target in refractive and cataract surgery. Transposition rewrites the same prescription in plus- or minus-cylinder form; the optical effect is identical.
Worked example
A prescription of −2.00 / −1.00 × 180 has a spherical equivalent of −2.00 + (−1.00 ÷ 2) = −2.50 D — myopia. Transposed to plus-cylinder form it reads −3.00 / +1.00 × 090.
Sources: Flitcroft DI et al. — IMI Defining and Classifying Myopia (IOVS 2019) · American Academy of Ophthalmology — How to read an eyeglass prescription
Frequently asked questions
What does spherical equivalent mean on my prescription?
It is the average power of your lens across all meridians: sphere plus half the cylinder. Two prescriptions with the same SE focus light at the same average distance, even if their astigmatism differs.
Can I use the spherical equivalent for contact lenses?
For low astigmatism (around 0.75 D or less) a spherical contact lens at the SE is often acceptable. Higher astigmatism usually needs a toric lens, and contact lenses above ±4.00 D also need a vertex-distance correction.
What spherical equivalent counts as high myopia?
The International Myopia Institute defines myopia as SE of −0.50 D or less and high myopia as −6.00 D or less; high myopia raises the risk of retinal detachment and myopic maculopathy.
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For information only — eyeglass and contact lens prescriptions must be written and fitted by a qualified eye-care professional.