Glasses vs Contact Lens Prescription: Why the Numbers Differ
−5.00 glasses ≠ −5.00 contacts. Vertex distance explained in plain English: when the difference matters (±4.00 and up), what SPH/BC/DIA mean, and how to order right.
You wear −5.00 glasses, you order −5.00 colored contacts, and the world is… slightly off. Not broken — off. That's not a defective lens; it's the one optical concept nobody explains at checkout: your glasses prescription and your contact lens prescription are not the same number. Here's the difference, when it matters, and how to order the right power the first time.
Vertex distance: the 12 millimeters that change the math
Glasses sit roughly 12mm in front of your eyes; contacts sit directly on them. Moving a lens closer to the eye changes its effective power, so the same eye needs slightly different numbers in each format. The rule of thumb: for myopia, the contact lens power is slightly weaker than the glasses power — a −5.00 glasses wearer typically needs around −4.75 in contacts; at −8.00 glasses, roughly −7.25 in contacts.
When you can ignore all of this
Below ±4.00, the difference rounds to nothing. A −2.50 glasses wearer orders −2.50 contacts and lives happily. This covers most people. The vertex correction only becomes real from about ±4.00 upward, and grows with the prescription — which is why our product pages carry a VERTEX note for higher powers.
When it matters, do it properly
From ±4.00 up, don't DIY the subtraction from a memory of your glasses Rx. Two clean options: use a contact lens prescription if you have one — it already includes the vertex adjustment plus base curve and diameter — or use a standard vertex conversion chart against your current (not expired) glasses prescription. If your last eye exam is more than a year or two old, that's the real blocker: get remeasured first, because a precise conversion of a stale prescription is still the wrong number.
What colored contacts can and can't correct
Sphere (SPH): yes — that's the minus power our lenses come in. Cylinder and axis (CYL/AXIS): no — those correct astigmatism and require toric lenses, which standard colored contacts aren't. Mild astigmatism (under about −0.75 CYL) usually tolerates spherical lenses with minor softness; more than that and no spherical lens will be crisp — the astigmatism section of the blur guide covers the details. ADD/presbyopia: also no; reading correction stays with your reading glasses.
The other two numbers on the vial
BC (base curve) — the lens's curvature, typically 8.5–8.7 on colored lenses; it fits most eyes, and consistent sliding or tightness is the sign yours isn't average (fitting time, not guessing time). DIA (diameter) — the lens's width, which is a look decision as much as a fit one; the complete sizing logic is in the diameter guide.
Ordering on Mojosee, concretely
Pick your style, open the power selector, choose your contact-lens power (0.00 plano if you don't need correction — both eyes same power ships in matched pairs; different powers per eye means ordering the two-power option where available). Most natural styles carry powers; heavy costume patterns and sclera are usually plano-only. Prescription-friendly picks for men are grouped in the men's prescription collection, and single-degree products show a static power label instead of a dropdown — if you don't see a selector, that style comes in one power only.
Shop the lenses in this guide
FAQ
Can I order colored contacts with my glasses prescription?
Under ±4.00: yes, use the same sphere number. Over ±4.00: convert for vertex distance first (chart or eye-care professional) — ordering your raw glasses number will leave you slightly over-corrected.
What does BC 8.6 mean on colored contacts?
Base curve — the curvature radius of the lens back surface in millimeters. 8.5–8.7 fits the large majority of adult eyes. You only need to care if lenses consistently feel tight or slide loose.
Do colored contacts come in astigmatism (toric) versions?
Standard colored contacts are spherical only. Mild astigmatism generally tolerates them with slight softness; moderate-plus needs toric correction — talk to your eye doctor before ordering.
My two eyes have different powers — can I still order?
Yes. Choose styles that offer per-eye power selection, or order two single-power pairs and combine. Never split the difference with one middle power — your dominant eye will complain all day.
Need the actual numbers? Use the glasses-to-contacts power conversion chart.
