How is each conversion-chart cell calculated?
Each conversion-chart cell is calculated as Fc = Fs ÷ (1 − d × Fs), with d in meters (0.012 at 12 mm), then rounded to 0.25 D.
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 used for the table.
The following steps convert a written spectacle sphere into the starting contact lens power in the table.
- Write the spectacle sphere as Fs.
- Set vertex distance to 12 mm (d = 0.012 m) unless a measured back vertex distance replaces the default.
- Calculate Fc = Fs ÷ (1 − d × Fs).
- Round Fc to the nearest 0.25 D manufacturing step.
Those four steps are the contact lens conversion chart. The chart is the formula in lookup form. Calculate vertex-compensated contact lens power at a vertex other than 12 mm, or show unrounded Fc, on the Contact Lens Vertex Calculator and Distance Chart.
Clinical takeaway
Look up rounded starting power at 12 mm here. Calculate a non-12 mm vertex or an unrounded Fc on the vertex calculator, then fit on eye.
When does the conversion chart change the ordered sphere?
Vertex compensation becomes clinically meaningful for meridional powers 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, Wikipedia, Vertex distance notes the difference becomes more than 0.25 D; unrounded Δ at the gate is 0.18–0.20 D, and rounding already moves the ordered power. High myopia, high hyperopia, and anisometropia are the three clinical settings where that divergence shows up in acuity or binocular balance. Apply the formula to each meridian of a toric Rx, not only to the written sphere. The conversion chart starts at ±4.00 D for the same reason: that is where compensation begins to move the ordered power. At exactly −4.00 D, unrounded Fc is −3.82 D and the starting step is −3.75 D. At exactly +4.00 D, unrounded Fc is +4.20 D and the starting step is +4.25 D. Glasses SPH is not identical to contact lens SPH at ±4.00 D on this 12 mm chart. Read the 12 mm vertex-distance explanation for why those planes differ.
Convert meridians, OD, and OS on the Contact Lens Conversion Calculator. That tool takes the vertexed sphere into the rest of the conversion path (rounding, optional spherical equivalent when a spherical lens is intended). A single-sphere table does not replace OD/OS inputs.
How does the conversion chart change minus spectacle power?
Minus spectacle lenses convert to less minus starting contact lens power at the corneal plane.
A minus lens gains effective power as it moves onto the cornea, so the ordered contact lens is weaker in minus than the glasses Rx. −4.00 D at 12 mm becomes −3.82 D unrounded, −3.75 D after the 0.25 D step. −6.00 D becomes −5.60 D unrounded and −5.50 D rounded, the usual chairside check for a −6.00 D spectacle Rx. −10.00 D becomes −8.93 D unrounded, −9.00 D rounded. Ordering a −6.00 D contact lens for a −6.00 D spectacle Rx leaves the eye over-minused at the corneal plane.
How does the conversion chart change plus spectacle power?
Plus spectacle lenses convert to more plus starting contact lens power at the corneal plane.
A plus lens loses effective power as it moves onto the cornea, so the ordered contact lens is stronger in plus than the glasses Rx. A +4.00 D spectacle lens at 12 mm becomes +4.20 D before rounding and +4.25 D after. +6.00 D becomes +6.47 D unrounded and +6.50 D rounded. +10.00 D becomes +11.36 D unrounded, +11.25 D at the manufacturing step. Ordering a +6.00 D contact lens for a +6.00 D spectacle Rx leaves the eye under-plussed at the corneal plane.
What can a contact lens conversion chart not convert?
A contact lens conversion chart returns starting sphere only, not a full contact lens prescription.
Spectacle-power conversion does not produce the geometric or material parameters a fitter orders with that power. Pinnacle Eyecare names measurements a table cannot figure out:
- Base curve matched to corneal shape
- Diameter from edge to edge
- Lens brand and material
- OD versus OS meridians, cylinder, and axis as a reconstructed starting Rx
Look up one sphere on this chart. Calculate starting parameters for both eyes, cylinder, and axis on the conversion calculator. Fit remains on eye. Verify power with over-refraction after the trial lens.
Convert this spectacle Rx to starting contact lens parametersHow do you convert toric meridians or both eyes instead of looking up one sphere?
Convert toric meridians and both eyes on a calculator, not on a single-sphere conversion chart. The following steps keep contact lens parameters at the corneal plane when the written Rx is sphero-cylindrical or binocular.
- Enter the spectacle refraction as written, OD and OS.
- Write each principal meridian (sphere, and sphere plus cylinder).
- Convert each meridional power with Fc = Fs ÷ (1 − d × Fs) at the stated vertex (default 12 mm).
- Reconstruct sphere, cylinder, and axis; leave axis unchanged during vertex compensation.
- Round sphere and cylinder to the nearest 0.25 D, then fit the trial lens.
Manufacturer fitting calculators that apply 12 mm across each power meridian use that per-meridian logic. This chart does not map either meridian to a trial SKU. A toric contact lens conversion chart as a separate URL is not this page.
Calculate a toric starting power on the Toric Contact Lens Calculator after both meridians are vertexed.
Can contact lens power convert back to a spectacle chart?
This chart lists spectacle-plane sphere to starting corneal-plane power only. It is not a reverse lookup, and the conversion calculator does not convert contact-lens power back to glasses.
Wikipedia’s Vertex distance page publishes the inverse as Fs = Fc ÷ (1 + d × Fc). This site’s converter does not apply that inverse. Reading a contact-lens power as spectacle power still uses Fc = Fs ÷ (1 − d × Fs) the wrong way. The chart does not restore base curve, diameter, or material, and it is not a glasses prescription to order.
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. This chart is not a mail-order prescription. A licensed fitter still puts a trial lens on and checks the power.
Conversion chart questions
People looking up a contact lens conversion chart ask whether the table is an Rx, why the default is 12 mm, and whether low powers stay identical.
Is a contact lens conversion chart a contact lens prescription?
A contact lens conversion chart is a starting-power lookup, not a contact lens prescription. Base curve, diameter, material, and on-eye confirmation are still required.
Why does this conversion chart use 12 mm?
12 mm is the site default when back vertex distance is unspecified. Override the default on the vertex calculator when you measured another distance.
Does glasses SPH match contact lens SPH at ±4.00 D?
Glasses SPH does not match contact lens SPH at ±4.00 D on this 12 mm chart. −4.00 D starts at −3.75 D; +4.00 D starts at +4.25 D.
Is this contact lens power chart a list of available SKU powers?
This contact lens power chart is a spectacle-to-parameter lookup at 12 mm, not a list of brand SKU powers. Ordered steps still follow on-eye fit.
Does a PDF conversion chart use different numbers?
A PDF conversion chart uses the same 12 mm cells as this table. The on-page chart is that lookup.
Starting contact lens parameters are not a prescription
Not a Rx
Not a prescription / on-eye next step
Starting parameters from this contact lens conversion chart 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 chart do not self-prescribe from the table.
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. Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site.