Contact Lens Power Calculator — Glasses to Contacts, With Astigmatism
Convert your glasses prescription into a contact lens power, including the spherical equivalent adjustment used when astigmatism-correcting (toric) colored lenses are not available in the design you want.
RIGHT EYE (OD)
LEFT EYE (OS)
Right eye (OD)—
Left eye (OS)—
What this calculator is — and is not.
It estimates a spherical contact lens power. It does not correct astigmatism. Folding half your cylinder into the sphere reduces blur; it does not eliminate it. Vision will stay softer than with glasses, especially for reading and night driving.
This is not a prescription and does not replace an eye exam. In the US, all contact lenses — including plano (0.00) decorative lenses — are medical devices that require a valid prescription. Base curve and diameter must be fitted by an eye care professional. If you have not been fitted for contacts, see an optometrist first.
It estimates a spherical contact lens power. It does not correct astigmatism. Folding half your cylinder into the sphere reduces blur; it does not eliminate it. Vision will stay softer than with glasses, especially for reading and night driving.
This is not a prescription and does not replace an eye exam. In the US, all contact lenses — including plano (0.00) decorative lenses — are medical devices that require a valid prescription. Base curve and diameter must be fitted by an eye care professional. If you have not been fitted for contacts, see an optometrist first.
How the conversion works
Step 1 — Vertex distance (glasses sit ~12mm from the eye, contacts sit on it)
At higher powers this distance changes the effective correction, so the contact lens power is weaker than the glasses power.
| Glasses sphere | Adjustment |
|---|---|
| Weaker than −4.00 | No change |
| −4.00 to −5.75 | Reduce by 0.25 |
| −6.00 to −7.75 | Reduce by 0.50 |
| −8.00 to −9.75 | Reduce by 0.75 |
| −10.00 to −11.75 | Reduce by 1.00 |
Save or share this chart — it covers both the vertex adjustment and the astigmatism step.
Step 2 — Spherical equivalent (astigmatism up to −2.00)
Add half of your cylinder to the adjusted sphere. This is standard optometric practice for estimating the closest spherical power when a toric lens is not available.
Contact power = (Sphere after vertex adjustment) + (Cylinder ÷ 2)
Step 3 — Round to an available power
If the result falls between two stocked powers, choose the weaker one. Over-correcting causes more strain than slightly under-correcting.
Worked examples
| Glasses prescription | Step 1 | Step 2 | Contact power |
|---|---|---|---|
| SPH −4.50, CYL −1.00 | −4.25 | −4.25 + (−0.50) | −4.75 |
| SPH −4.50, CYL −0.75 | −4.25 | −4.25 + (−0.375) | −4.50 (rounded weaker) |
| SPH −5.50, CYL −2.50 | −5.25 | −5.25 + (−1.25) | −6.50 |
| SPH −2.75, CYL −0.50 | −2.75 | −2.75 + (−0.25) | −3.00 |
My two eyes need different powers
An order unit is 1 pair (2 lenses) of the same power — there is no separate left/right selection at checkout.
- Difference of about 0.50 or less — order one pair at the weaker of the two powers. Slight under-correction is much easier to tolerate than over-correction, and the two eyes still work together comfortably.
- Larger difference — order two pairs, one at each power, and wear one lens from each. The remaining lens in each pair stays sealed as a spare.
If you are unsure which side to favour, ask your eye care professional — with a noticeable difference between eyes (anisometropia) the balance matters more than the raw numbers.
Where to use your converted power
Mojosee stocks spherical powers from 0.00 down to −8.00 on prescription lenses. Once you have your number from the calculator above, pick it in the Power dropdown on any of these:
- Prescription colored contacts — coloured lenses available with power
- Prescription cosplay contacts — character lenses with power, for coser who cannot wear glasses in costume
- Men's prescription colored contacts
- Plano 0.00 colored contacts — if you do not need vision correction
Every power-enabled product page carries this same calculator under the Astigmatism & Power tab, and there it only returns powers that lens is actually stocked in.
Frequently asked questions
Can you wear colored contacts if you have astigmatism?
Many people do, using a spherical lens with the power adjusted as above. Vision is softer than with a toric lens or glasses, and the trade-off is usually acceptable for a convention day, a photoshoot or an evening out. It is not a substitute for proper astigmatism correction in daily wear. Ask your optometrist whether it is appropriate for your eyes before you order.
What happens if you wear contacts without astigmatism correction?
Astigmatism blurs light along one axis. A spherical lens cannot fix that axis, so edges and fine detail stay soft while the centre of your vision is usable. People report that walking, talking and photos are fine, but reading a phone or small text is difficult. Higher cylinder means more residual blur — above roughly −2.00 the spherical equivalent stops being practical.
Is it safe to wear them for one day?
The under-correction itself is not damaging, but eye strain, headaches or nausea can occur — especially at higher cylinder. Do not drive at night, take breaks, and remove the lenses if vision or comfort worsens. Safety depends far more on correct base curve, diameter, hygiene and wear time than on the power. Never sleep in them, never share them, and never exceed the replacement schedule.
Why is the contact lens power weaker than my glasses?
Glasses sit about 12mm in front of the cornea; a contact lens sits directly on it. That distance changes the effective power, and the effect grows as the prescription strengthens. Below about −4.00 the difference rounds away; above it, the contact lens power is reduced as shown in the table.
Can I order a different power for each eye?
Enter both eyes above and the calculator returns each one independently — but an order unit is one pair (2 lenses) of the same power, and there is no separate left/right selection at checkout. If the two eyes differ by about 0.50 or less, order a single pair at the weaker power. If the gap is larger, order two pairs, one at each power, and wear one lens from each.
Do you sell toric (astigmatism-correcting) colored contacts?
No. Toric lenses need a cylinder power and a stabilised axis, which requires a thicker lens and a custom order — and when combined with the pigment layer of a colored lens the comfort and oxygen transmission drop noticeably. We stock spherical powers from 0.00 to −8.00 and provide this calculator so you can find the closest usable power. If you need true astigmatism correction for daily wear, see your optometrist about toric lenses.
My calculated power is not available — what should I do?
Choose the next weaker power. If the result is stronger than −8.00 we do not stock it, and stacking two lenses is never an acceptable workaround — it starves the cornea of oxygen and is unsafe.
