Contact Lens Prescription Decoder
Your prescription packs a lot into a few rows of numbers and abbreviations. Type yours in below and get an easy-to-understand translation, what each value means, how strong your correction is, and what kind of lens it points to.
Runs entirely in your browser. Nothing you type is stored or sent anywhere.
| Eye | SPH sphere / PWR | CYL cylinder | AXIS 1 to 180 | ADD near power | Prism Δ diopters | Base direction | BC base curve | DIA diameter | Brand optional |
|---|---|---|---|---|---|---|---|---|---|
| ODright eye | |||||||||
| OSleft eye |
One number, or two split by a slash
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Educational reference only. Confirm every value with your eye care professional before ordering.
How do you read a contact lens prescription?
Left to right, top to bottom. Each row covers one eye, and each column answers one question about the lens that eye needs. Here is a typical example, followed by what every part of it says.
| Eye | SPH | CYL | AXIS | ADD | BC | DIA | Brand |
|---|---|---|---|---|---|---|---|
| OD | -2.75 | -1.25 | 180 | +1.50 | 8.6 | 14.2 | Biofinity Toric |
| OS | -3.25 | DS | +1.50 | 8.6 | 14.2 | Biofinity |
The rows come from Latin. OD is oculus dexter, your right eye, and OS is oculus sinister, your left. When both eyes match, some prescribers collapse the two rows into one labeled OU, both eyes.
SPH is the headline number: the main focusing power, measured in diopters. A minus sign means the lens corrects nearsightedness, a plus sign means farsightedness, and the farther from zero, the stronger the lens. CYL and AXIS travel as a pair and describe astigmatism, the extra correction for a cornea that focuses light unevenly. CYL is the strength and AXIS, a degree from 1 to 180, is the orientation. A blank, DS, or SPH in the cylinder column means no astigmatism correction in that eye.
ADD is a near-focus boost for presbyopia, the age-related change that pushes reading material to arm’s length. BC and DIA describe the physical lens rather than your vision: base curve is the curvature of the back surface in millimeters, and diameter is its width edge to edge. The brand is not a suggestion. United States rules make it part of the prescription itself, so a different brand needs your prescriber’s sign-off.
A quick read on the sample: the right eye is mildly nearsighted with mild astigmatism, the left eye is moderately nearsighted with none, and both get a Medium-band reading boost. Paste the same numbers into the decoder above and it reaches the same conclusion.
What do all the abbreviations stand for?
| Term | Stands for | What it tells you |
|---|---|---|
| OD | Oculus dexter | Your right eye. |
| OS | Oculus sinister | Your left eye. |
| OU | Oculus uterque | Both eyes, used when the two share one set of numbers. |
| SPH / PWR | Sphere or power | The main correction in diopters. Minus for nearsightedness, plus for farsightedness. |
| CYL | Cylinder | The strength of your astigmatism correction. |
| AXIS / AX / X | Axis | Where the astigmatism correction sits, from 1 to 180 degrees. |
| ADD | Addition | Extra near-focus power for presbyopia, always a plus value. |
| BC | Base curve | The curvature of the lens’s back surface in millimeters. A higher number is a flatter lens. |
| DIA | Diameter | The width of the lens in millimeters. |
| D | Diopter | The unit of lens power. |
| PL / Plano | Plano | Zero power in that field. |
| DS | Diopters sphere | Sphere only, no astigmatism correction. Shows up in the CYL column. |
| BAL | Balance | A lens matched to the other eye for comfort and appearance when that eye has little usable vision. |
| Δ / p.d. | Prism diopters | The strength of a prism correction on a glasses prescription. |
| BU / BD / BI / BO | Base direction | Which way a prism points: up, down, in toward the nose, or out. |
| DV / NV | Distance / near vision | Row labels on some glasses prescriptions, splitting far and reading corrections. |
| PD | Pupillary distance | The space between your pupils in millimeters, used to center glasses lenses. |
How are contact lens and glasses prescriptions different?
They are two separate legal documents, and one cannot stand in for the other. A contacts prescription carries three things a glasses Rx never will: a base curve, a diameter, and a named brand, all locked in during a fitting where your prescriber checks how a specific lens sits and moves on your eye. Swap any of those and the same powers can perform differently.
The powers themselves can disagree too. Glasses sit roughly 12 millimeters in front of your corneas while contacts rest on them, and that gap changes the effective strength a lens delivers. The shift is negligible for low prescriptions and grows meaningful past about plus or minus 4.00 diopters, which is why a strong glasses wearer usually sees a slightly different number on their contacts Rx.
Under the FTC Contact Lens Rule, your prescriber must hand over a copy of your contacts prescription after the fitting, whether you ask for it or not. You can fill it anywhere you like.
Can you convert a glasses prescription to contacts?
Not on your own, and no honest calculator will claim otherwise. What the math can do is estimate. Two adjustments are involved. The first is vertex compensation, which translates power from the glasses position to the surface of the eye; switch the decoder above to Glasses mode, and it will run that calculation for any power of plus or minus 4.00 or more. The second is the spherical equivalent, a small power tweak (sphere plus half the cylinder) that lets some people with low astigmatism wear a regular spherical lens instead of a toric.
Both numbers are starting points a fitter would recognize, not an order you can place. Base curve, diameter, and brand only come from an in-person fitting, and your prescriber may land on different powers once the lens is on your eye.
How long does a contact lens prescription last?
Federal rules set a floor of one year from the issue date. Your state can lengthen that window, and your prescriber can shorten it when there is a documented medical reason, so the expiration date printed on the prescription is the one that counts. Glasses prescriptions follow separate state rules, commonly one to two years.
One year is the federal minimum lifespan of a U.S. contact lens prescription. After it lapses, sellers cannot legally fill it, even for the same lenses you have worn for a decade.
What else do people ask about contact lens prescriptions?
Can I order contacts with my glasses prescription?
No. A contacts prescription includes base curve, diameter, and a specific brand, and the powers can differ from your glasses. U.S. rules treat the two as separate prescriptions, so a contact lens fitting has to happen before you can order.
What does a minus or plus number mean?
Minus corrects nearsightedness, where distance goes blurry. Plus corrects farsightedness, where near tasks blur first. The farther the number sits from zero, the stronger the correction.
My prescription says PL or Plano. What does that mean?
Zero power in that field. You can still have astigmatism (a CYL and AXIS) or a reading ADD on the same line, so a Plano sphere does not always mean a plain lens.
Do I need toric lenses?
Toric lenses correct astigmatism, and most start at -0.75 of cylinder. Below that, many wearers do well in a regular spherical lens, sometimes with a small power tweak called the spherical equivalent. Your prescriber makes the final call.
Is axis 180 the same as axis 0?
Yes. The axis scale runs from 1 to 180, and 180 marks the horizontal meridian, so prescribers write 180 rather than 0.
Why do my glasses and contacts show different numbers?
Glasses sit about 12 millimeters in front of your eyes and contacts sit on them. That gap changes the effective power, and the difference grows once a prescription passes about plus or minus 4.00 diopters. Your doctor adjusts for it during the fitting.
What do Low, Med, and High mean on multifocal boxes?
They map to your ADD power. As a rough guide, +1.25 and under lands in Low, +1.50 to +2.00 in Medium, and +2.25 and up in High, though each brand draws the lines a little differently.
How does this decoder work?
The strength labels follow widely used clinical groupings. Nearsightedness reads mild through -3.00, moderate from there to -6.00, and high beyond it. Farsightedness reads low through +2.00, moderate to +5.00, and high past that. Astigmatism reads mild under 1.00 of cylinder, moderate through 2.75, and high at 3.00 and up. ADD powers map to the Low, Medium, and High bands printed on multifocal packaging.
Three calculations run behind the scenes. The spherical equivalent is sphere plus half the cylinder, rounded to the nearest quarter diopter. Vertex compensation uses the standard 12 millimeter distance and applies it to each meridian separately before recombining. Transposition rewrites a plus-cylinder prescription in the minus-cylinder form that contact lens torics use: add the cylinder to the sphere, flip the cylinder’s sign, and rotate the axis 90 degrees.
Eye care practices, educators, and anyone who explains prescriptions for a living: you are welcome to link to this page or point patients here. If you spot a value the decoder reads wrong, tell us and we will fix it.
