What starting contact lens parameters does this site calculate?
Starting contact lens parameters are ordered sphere, with cylinder and axis for a toric lens, after vertex, optional spherical equivalent, and 0.25 D rounding.
This site calculates three ordered values:
- Rounded ordered sphere, OD and OS
- Reconstructed cylinder and axis when the intended lens is toric
- The vertex distance applied, stated in millimeters (default 12 mm)
It does not calculate three fitting values a glasses Rx never contained:
- Base curve
- Diameter
- Material or a manufacturer SKU
Those geometric and material values come from keratometry, topography, slit-lamp assessment, and a trial lens. Convert this spectacle Rx to starting contact lens parameters on the Contact Lens Conversion Calculator. Calculate starting parameters there.
Which formula converts spectacle power to contact lens power?
Spectacle power converts to contact lens power as Fc = Fs ÷ (1 − d × Fs), with d in meters (0.012 at 12 mm).
Formula
Fc = Fs ÷ (1 − d × Fs)
Fc is corneal-plane (contact lens) power in diopters. Fs is spectacle-plane power in diopters. d is vertex distance in meters. At the 12 mm default, d = 0.012. A contact lens final vertex is 0, so d equals the spectacle vertex you enter.
Vertex distance is the space between the back of the spectacle lens and the cornea. Spectacle lenses commonly sit about 12 to 14 mm from the cornea. A contact lens sits on the cornea, so its vertex is essentially zero. That plane change is why the glasses number is not the contact lens number. Calculate vertex-compensated contact lens power on the Contact Lens Vertex Calculator and Distance Chart. That page holds the 12 mm chart. This page holds the rule.
Clinical takeaway
Calculate Fc at the stated vertex, then apply optional spherical equivalent and 0.25 D rounding. The formula is the method. The chart is the formula in lookup form.
Why is the default vertex distance 12 mm?
This site defaults to 12 mm for any refraction without a measured back vertex distance.
Independent conversion and vertex references use 12 mm as a common clinical stand-in. Manufacturer fitting tools that apply 12 mm across each power meridian use the same default. Phoropter vertex is about 13.75 mm. Trial frames run about 10 to 16 mm by fit and frame style. Enter the measured millimeters when the refraction lists another value. The algebra does not change. This page does not adopt a 14 mm prefill or a 10 mm phoropter example as the site default. The optics of that default live on Vertex Distance of Contact Lenses.
When does vertex compensation change the ordered sphere?
Vertex compensation becomes clinically important once meridional powers are at or above about ±4.00 D.
The vertex change often stays under a 0.25 D manufacturing step below about ±4.00 D and leaves the ordered sphere alone. Around ±4.00 D the difference becomes more than 0.25 D, per Wikipedia, Vertex distance. Apply the formula to each meridian of a toric Rx, not only to the written sphere.
Minus spectacle meridians convert to less minus starting contact lens power. Plus meridians convert to more plus. Three 12 mm pairs already recomputed from Fc = Fs ÷ (1 − 0.012 × Fs), rounded to the nearest 0.25 D:
- −4.00 D spectacle starts at −3.75 D
- −6.00 D spectacle starts at −5.50 D
- +6.00 D spectacle starts at +6.50 D
Those three pairs sit at the edge of the gate and at the usual chairside checks. The full 12 mm table lives on the vertex calculator already named above.
How is spherical equivalent calculated after vertex?
Spherical equivalent equals Sphere + (Cylinder ÷ 2) after vertex compensation.
Formula
SE = Sphere + (Cylinder ÷ 2)
Use that value only when a spherical soft lens is intended over low, regular astigmatism and residual cylinder is accepted. Spherical equivalent is a clinical compromise. It is not a substitute for indicated toric correction. Apply spherical equivalent after vertex, never before.
Arithmetic from that sourced formula: sphere −3.00 D and cylinder −1.00 D give SE = −3.00 + (−1.00 ÷ 2) = −3.50 D. That identity is not a fitting case. Calculate a starting toric power instead of collapsing the cylinder when cylinder remains clinically indicated. The Toric Contact Lens Calculator applies vertex to each meridian and keeps indicated cylinder.
How is starting power rounded to manufacturing steps?
Ordered power rounds to the nearest 0.25 D manufacturing step after vertex and any optional spherical equivalent.
That step size is the agnostic increment used on independent conversion and vertex references. It is not a catalog grid. It is not a manufacturer rule that rounds cylinder down or axis to 10°. The calculators show the rounded starting parameters next to the unrounded vertex result so the 0.25 D jump is visible.
How do we apply vertex to each toric meridian?
Apply vertex to each toric meridian by converting both principal powers, then reconstructing sphere, cylinder, and axis. The following steps convert those meridians at the corneal plane.
- Write the axis meridian as the sphere, and the meridian 90° away as sphere plus cylinder.
- Convert each meridional power with Fc = Fs ÷ (1 − d × Fs) at the stated vertex (default 12 mm).
- Reconstruct sphere from the first compensated meridian and cylinder as the second compensated meridian minus the first.
- Leave axis unchanged during vertex compensation, then round sphere and cylinder to the nearest 0.25 D.
Vertexing the written sphere alone leaves the second meridian at the spectacle plane. The worked −5.00 −2.00 × 180 meridian split lives on the vertex calculator already named above. This page does not map the result to a trial SKU.
What does this site not calculate?
This site does not calculate base curve, diameter, material, or a manufacturer SKU from a glasses Rx.
Power conversion applies vertex to the spectacle refraction. A full contact lens prescription adds measurements a glasses Rx does not contain. Independent clinic explainers that rank for conversion questions name three missing items a chart cannot figure out:
- Base curve matched to corneal shape
- Diameter from edge to edge
- Lens brand and material
Trial-lens evaluation and over-refraction still decide fit, comfort, and final power. Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site.
Methodology questions
Which formula converts glasses power into contact lens power?
The conversion formula is Fc = Fs ÷ (1 − d × Fs), with d in meters (0.012 at 12 mm).
Vertex each meridian when power is at or above about ±4.00 D, then apply optional spherical equivalent and 0.25 D rounding.
Does a glasses prescription convert into starting contact lens parameters?
A glasses prescription converts into starting contact lens parameters after vertex, optional spherical equivalent, and 0.25 D rounding.
That converted power is not a contact lens prescription to order.
Why does this site default to 12 mm?
This site defaults to 12 mm for any refraction without a measured back vertex distance.
Override it for a known phoropter or trial-frame BVD.
Does vertex compensation change powers under ±4.00 D?
Meridional powers below about ±4.00 D produce a vertex change under 0.25 D at 12 mm.
The ordered sphere often does not move a manufacturing step. The calculators still show the math.
When is spherical equivalent used instead of a toric lens?
Spherical equivalent applies after vertex for an intended spherical lens.
It is not a substitute for indicated toric correction.
Starting contact lens parameters are not a prescription
Starting parameters from these formulas are a chairside number, not an Rx. Independent professional judgment still decides the ordered lens after on-eye fit. Educated patients who searched a conversion formula do not self-prescribe from Fc or SE.
Calculate vertex-compensated contact lens power, convert spectacle prescription to contact lens parameters, and keep those contact lens parameters at the corneal plane until a fitter verifies them. Verify power with over-refraction after the trial lens on the Over-Refraction Calculator for Contact Lens Parameters. The legal bound is on the Medical Disclaimer.
Not a Rx
Not a prescription / on-eye next step
The result is a diagnostic starting point. Fit the trial lens. Verify power with over-refraction after the trial lens.
Patient aside (Grade 8 to 9)
Your glasses sit a little in front of your eye. A contact lens sits on the eye. That's why the two numbers differ. These formulas are not a mail-order prescription. A licensed fitter still puts a trial lens on and checks the power.
What did the clinical reviewer check?
Reviewed by Optom. Deepak Ghimire, B. Optometry, PGDOVS — Consultant Optometrist, Myopia & Contact Lens Specialist. Review covers the formulas, assumptions, and copy on this site. It does not turn calculator output into a prescription, and it is independent of contact lens manufacturers.
Optom. Deepak Ghimire reviewed:
- Vertex identity Fc = Fs ÷ (1 − d × Fs), with d in meters and a 12 mm default
- The clinical gate at or above about ±4.00 D at the spectacle plane, applied by meridian
- Spherical equivalent SE = Sphere + (Cylinder ÷ 2) after vertex, only when a spherical lens is intended
- Cylinder transposition (new sphere = sphere + cylinder; cylinder sign reversed; axis ± 90°)
- RGP keratometry conversion D = 337.5 ÷ r on the RGP guide
- Over-refraction combination and the Moran CORE worked examples used on that calculator
This site converts spectacle power to the corneal plane only. The inverse relationship is Fs = Fc ÷ (1 + d × Fc), published on Wikipedia, Vertex distance. This site does not apply that inverse.
Sources
Clinical claims on this page are attributed to the publications below. Formulas were recomputed independently; manufacturer tools are cited as references, not as affiliations.
- Wikipedia, Vertex distance— around ±4.00 D the difference becomes more than 0.25 D.
- ODReference Contact Lens Conversion Calculator— starting-point workflow; SE = Sphere + (Cylinder ÷ 2) after vertex; ±4.00 D meridional gate; 12–14 mm typical spectacle distance.
- ODReference Vertex Distance Calculator and Power Chart— Fc = Fs ÷ (1 − d × Fs) with d = 0.012 m; 12 mm common default; 0.25 D rounding.
- Omni Calculator Contact Lens Vertex Calculator— equivalent Fc relationship with d in meters. Omni’s ±0.13 D tolerance sentence is not used here.
- Johnson & Johnson Vision ACUVUE Fitting Calculator— 12 mm vertex across each power meridian. Not an ACUVUE SKU picker on this site.
- Art Optical Vertex Adjustments— 12 mm clinical heading.
- Chadwick Optical Effective and Compensated Powers— clinically important at typically ±4.00 D or greater for contact lens fittings.
- OpticianWorks, Vertex Distance— contact lens worn with virtually no vertex distance; compensation above ±4.00 D.
- GPLI conversion charts— spectacle-power column starting at 4.00 D. Vertex millimetres are not disclosed there, so 12 mm is not attributed to GPLI.
- Pinnacle Eyecare on converting glasses prescriptions— conversion is not a full contact lens prescription; base curve, diameter, and material are still missing.