Free Eye Prescription Calculator
Convert glasses to contacts, find your reading glasses (ADD) strength, and transpose your prescription between plus- and minus-cylinder form — all in one instant, pharmacist-reviewed tool.
Reviewed for accuracy: July 2026 · Formulas based on clinical vertex-distance optics and American Optometric Association guidanceHow to Use This Eyeglasses Prescription Calculator
Whether you need an eye prescription calculator for contact lenses or a straightforward eyeglasses prescription calculator to double-check a new frame order, this tool handles both directions. Enter the sphere (SPH), cylinder (CYL), and axis from your written prescription for each eye, then let the calculator apply the standard vertex-distance formula used in clinical optics to estimate the equivalent contact lens power. This is the same underlying method behind most “glasses to contact lens conversion” and “contact lens to glasses conversion” tools you’ll find online — except every step is shown, not hidden behind a single button.
Why it matters: a spectacle lens sits about 12mm in front of your eye, while a contact lens sits directly on the cornea. For low prescriptions this gap barely matters — but once your sphere power passes ±4.00D, the effective strength you actually need starts to drift, sometimes by a quarter diopter or more.
The Vertex Distance Formula
Contact lens power is calculated from spectacle power using: Fc = Fs ÷ (1 − d × Fs), where Fc is the contact lens power, Fs is the spectacle lens power, and d is the vertex distance in meters (12mm = 0.012m). The calculator applies this separately to your sphere meridian and your sphere-plus-cylinder meridian, then reconstructs the cylinder from the difference — this keeps the astigmatism correction accurate rather than just shifting the sphere alone.
| Spectacle Power (Sphere) | Typical Adjustment |
|---|---|
| Plano to ±4.00D | No meaningful change |
| ±4.25D to ±6.00D | ≈ ±0.25D adjustment |
| ±6.25D to ±8.00D | ≈ ±0.50D adjustment |
| Beyond ±8.00D | Use the full vertex formula (this calculator) |
Common Mistakes When Converting Glasses to Contacts
- Forgetting that the sign flips direction: as spectacle power moves further from zero, contact lens power moves the other way for plus lenses and the same way for minus lenses — the formula handles this automatically.
- Applying vertex correction to low prescriptions where it isn’t clinically necessary, leading to lenses that feel “too strong.”
- Ignoring cylinder and axis entirely — significant astigmatism (roughly 0.75D or more) usually needs a toric contact lens, not just the sphere-equivalent.
- Using an outdated or expired prescription — contact lens fits also depend on base curve and diameter, which a glasses Rx doesn’t include.
This eye prescription calculator gives you a fast, informed starting point for that conversation — not a replacement for it. Contact lenses also require a valid fitting for base curve, diameter, and material, which only an eye care professional can measure directly on your eye.
Looking for more tools? Explore the full Health Calculators hub or start from the DexoCalc homepage for our complete library of free, interactive calculators.
What If My Eyes Are Uneven?
If one eye needs noticeably more magnification than the other, off-the-shelf readers won’t fit both eyes correctly — that’s a sign to get a comprehensive eye exam rather than sizing up a single pair.
How to Convert Contact Prescription to Glasses Calculator Logic — and Find Reading ADD Power
This reading glasses ADD finder uses the same age-based presbyopia curve that opticians use as a starting point before fine-tuning with a diopter card. Presbyopia is the normal, gradual loss of near-focusing ability that begins for most people in their early-to-mid 40s, as the eye’s natural lens becomes less flexible. The “ADD” power on a bifocal or progressive prescription is simply the extra plus power added on top of your distance prescription to bring near objects back into focus.
Age-Based ADD Power Chart
| Age Range | Typical ADD Power |
|---|---|
| 40–43 | +1.00D |
| 44–45 | +1.25D |
| 46–48 | +1.50D |
| 49–50 | +1.75D |
| 51–53 | +2.00D |
| 54–56 | +2.25D |
| 57–59 | +2.50D |
| 60–64 | +2.75D |
| 65+ | +3.00D |
These figures are a well-established starting point, not a diagnosis — actual ADD power varies by individual, reading distance, lighting, and how well your eyes have compensated so far. Always confirm with a diopter reading card or, better, a comprehensive eye exam before buying stronger readers than +2.50D.
Reading Glasses vs. Prescription Bifocals
Over-the-counter readers correct presbyopia only — they assume your distance vision is otherwise normal. If you already wear glasses or contacts for nearsightedness, farsightedness, or astigmatism, your ADD power needs to be combined with your existing correction, which is exactly what bifocal or progressive lenses do. Our Health Calculators hub has related tools if you’re managing several prescriptions at once.
Plus-Cylinder to Minus-Cylinder Prescription Transposition
Some optometrists write astigmatism corrections in plus-cylinder notation, while most U.S. optical labs and contact lens manufacturers use minus-cylinder. Transposition converts between the two without changing the actual optical correction — it’s the same lens, described differently.
The Transposition Rule
- New sphere = old sphere + old cylinder
- New cylinder = old cylinder × −1 (flip the sign)
- New axis = old axis + 90° (or −90° if that would exceed 180°)
This tool performs all three steps instantly and shows both versions side by side, so you can confirm a lab order, compare two prescriptions written in different formats, or simply understand what your eye doctor’s chart actually means.
Frequently Asked Questions
An eye prescription calculator is a tool that reads the sphere, cylinder, and axis values from an optical prescription and converts, interprets, or transposes them — for example, turning a glasses prescription into an estimated contact lens power, or finding the ADD power needed for reading glasses based on age.
Reverse the vertex distance formula: given contact lens power (vertex distance 0mm), solve for spectacle power at 12mm using Fs = Fc ÷ (1 + d × Fc). In practice, use the Glasses ↔ Contacts tab above and treat your contact power as the target — an eye care professional can confirm the exact glasses equivalent, since contacts don’t specify axis-independent meridians the same way.
The formula is mathematically precise for the sphere and cylinder power itself, but contact lenses also depend on base curve, diameter, and material — none of which appear on a glasses prescription. That’s why a professional contact lens fitting is still required even after a conversion estimate.
Because a spectacle lens sits roughly 12mm in front of the eye while a contact lens sits directly on the cornea, the same optical correction requires a slightly different power at each distance — this difference, called vertex distance, only becomes clinically significant beyond about ±4.00D.
Most people first notice presbyopia symptoms — needing to hold text farther away, eye strain after close work — somewhere between age 40 and 45, with ADD power typically increasing every few years until it stabilizes in the mid-60s.
Age-based charts are a reasonable starting point, but actual near-vision strength varies by individual. Testing with a diopter reading card, or getting a comprehensive eye exam, gives a more accurate number — especially if you need stronger than +2.50D.
ADD refers to the additional plus (magnifying) power added to your distance prescription for near tasks like reading. It’s used in bifocal and progressive lenses, and corresponds directly to reading glasses strength if you have no other distance correction.
Transposition converts a prescription between plus-cylinder and minus-cylinder notation without changing the actual correction. It’s needed because different doctors, labs, and countries default to different notations, and some lens manufacturers only accept one format.
No. Transposition is purely a notation change — the sphere, cylinder, and axis describe the same physical lens before and after conversion. Only the way the numbers are written is different.
The underlying formulas match standard clinical optics used in optometry and ophthalmology practice. Accuracy for your individual case still depends on entering your prescription correctly and on factors like base curve and lens material that a calculator can’t measure.
No. This tool is designed to help you understand and estimate your numbers between visits — it does not replace a comprehensive eye exam, a contact lens fitting, or professional medical advice.