Which standards publish contact lens tolerances?
Two: ANSI Z80.20 in the United States and ISO 18369-2 internationally, and they are written to agree.
ANSI Z80.20 is Ophthalmics — Contact Lenses — Standard Terminology, Tolerances, Measurements and Physicochemical Properties, developed by the Accredited Committee Z80 for Ophthalmic Standards with The Vision Council as secretariat. The Optical Training Institute identifies Z80.20-2016 (R2021) as the current Z80 committee standard for contact lenses, soft and rigid.
ISO 18369-2 is Ophthalmic optics — Contact lenses — Part 2: Tolerances, second edition 2012-12-01, prepared by ISO/TC 172 Subcommittee SC 7. It sits in a four-part series: Part 1 vocabulary, classification and labelling; Part 2 tolerances; Part 3 measurement methods; Part 4 physicochemical properties of contact lens materials.
The two agree by design rather than by coincidence. Z80.20’s own foreword states that the revision “was written to include provisions for newer contact lenses and materials now on the public market and to insure that the US standard is in conformance with the International Standard.” That sentence is what licenses reading the ISO tables as the detail behind the ANSI bands, which is how this page is built: the ISO tables below are the published ones, and they match the abbreviated ANSI table the Optical Training Institute publishes.
Both standards apply to lenses worn over the front surface of the eye in contact with the preocular tear film, and both cover rigid corneal and haptic (scleral) lenses as well as soft paralimbal lenses.
Dimensional tolerances for rigid contact lenses
A gas permeable back optic zone radius is toleranced at ±0.05 mm; the same parameter in PMMA is held to ±0.025 mm.
| Parameter | PMMA | Gas permeable | Scleral |
|---|---|---|---|
| Back optic zone radius | ±0.025 mm | ±0.05 mm | ±0.10 mm |
| Back optic zone diameter | ±0.20 mm | ±0.20 mm | ±0.20 mm |
| Back or front peripheral radius, where measurable | ±0.10 mm | ±0.10 mm | ±0.10 mm |
| Back peripheral diameter | ±0.20 mm | ±0.20 mm | ±0.20 mm (preformed) |
| Total diameter | ±0.10 mm | ±0.10 mm | ±0.25 mm |
| Front optic zone diameter | ±0.20 mm | ±0.20 mm | ±0.20 mm |
| Center thickness | ±0.02 mm | ±0.02 mm | ±0.10 mm |
ISO 18369-2:2012, Table 1. The standard notes that the diameter and peripheral-radius tolerances apply to lenses with spherical surfaces and distinct curves, and that blending may make those measurements difficult on a finished lens. Toroidal back optic zone radii carry their own graduated tolerances, widening as the difference between the two principal radii increases.
The Optical Training Institute’s abbreviated ANSI table gives the same numbers in the columns it shares: overall diameter ±0.10 mm for PMMA and rigid gas permeable, base curve radius ±0.025 mm and ±0.05 mm respectively, center thickness ±0.02 mm.
What a base curve radius means as a fitting parameter, rather than as a manufacturing limit, is on Base Curve of Contact Lenses.
Dimensional tolerances for soft contact lenses
A soft back optic zone radius is toleranced at ±0.20 mm — four times the gas permeable band, on a lens that drapes rather than holds its own shape.
| Parameter | Tolerance limit |
|---|---|
| Back optic zone radius | ±0.20 mm |
| Sagitta at specified diameter | ±0.05 mm |
| Total diameter | ±0.20 mm |
| Optic zone diameter | ±0.20 mm |
ISO 18369-2:2012, Table 3. The radius tolerance applies where the manufacturer specifies back surface curvature that way.
One footnote on that table is the row most often quoted without it. The ±0.20 mm radius band applies when the step between successive back optic zone radii in the design is 0.40 mm or greater. For a design with smaller steps, the tolerance is half the design step — a design stepping in 0.30 mm carries a ±0.15 mm tolerance, not ±0.20 mm. A band quoted without that condition can be wider than the standard actually allows.
Note also that soft lenses carry a sagitta tolerance of ±0.05 mm at a specified diameter, tighter than any of their linear dimensions. That is the parameter describing how deep the lens is rather than how curved, and it is on Sagittal Depth of Contact Lenses. Overall size as an ordered parameter is on Diameter of Contact Lenses.
Power, cylinder and axis tolerances
Back vertex power is toleranced in bands that widen with power: ±0.12 D at the low end of a rigid lens, ±1.00 D above 20 D on a soft one.
| Parameter | Range | Rigid | Soft |
|---|---|---|---|
| Back vertex power | 0 to ±5.00 D | ±0.12 D | ±0.25 D (to ±10.00 D) |
| over ±5.00 to ±10.00 D | ±0.18 D | ±0.25 D | |
| over ±10.00 to ±15.00 D | ±0.25 D | ±0.50 D (to ±20.00 D) | |
| over ±15.00 to ±20.00 D | ±0.37 D | ±0.50 D | |
| over ±20.00 D | ±0.50 D | ±1.00 D | |
| Cylinder power | to 2.00 D | ±0.25 D | ±0.25 D |
| over 2.00 to 4.00 D | ±0.37 D | ±0.37 D | |
| over 4.00 D | ±0.50 D | ±0.50 D | |
| Cylinder axis | — | ±5° | ±5° |
| Prismatic error | back vertex power to 6 D | 0.25 cm/m | 0.25 cm/m |
| over 6 D | 0.50 cm/m | 0.50 cm/m | |
| Prescribed prism | — | ±0.25 cm/m | ±0.25 cm/m to 6 D; ±0.50 cm/m above |
ISO 18369-2:2012, Tables 2 and 3. The rigid back vertex power tolerance applies to the weaker meridian; prismatic error is measured at the geometrical centre of the optic zone; soft lenses also carry a prism axis tolerance of ±5°. Rigid Table 2 sets a maximum optical centration error of 0.50 mm for scleral lenses.
The rigid power ladder in the Optical Training Institute’s abbreviated ANSI table matches column for column — 0.12, 0.18, 0.25, 0.37, 0.50 D — as does the soft ladder at 0.25, 0.25, 0.50, 0.50, 1.00 D.
The ±5° cylinder axis tolerance is not a rotation allowance. It describes where the cylinder sits inside the lens as manufactured. How far the lens then turns on the eye is a separate, larger and far more variable quantity, and it is assessed at the slit lamp — see Assessing Toric Contact Lens Rotation at the Slit Lamp. A lens can be within the manufacturing band on both counts and still deliver a misaligned cylinder because it rotated 15 degrees on eye.
How is soft-lens center thickness tolerance calculated?
Soft center thickness has no single band: it is calculated from the nominal thickness itself.
ISO 18369-2:2012 Table 3 gives two branches, where tC is the nominal center thickness:
- tC of 0.10 mm or less → tolerance is ±[0.010 mm + 0.10 tC]
- tC greater than 0.10 mm → tolerance is ±[0.015 mm + 0.05 tC]
The standard publishes four worked examples:
| Nominal center thickness | Tolerance |
|---|---|
| 0.035 mm | ±0.014 mm |
| 0.070 mm | ±0.017 mm |
| 0.150 mm | ±0.023 mm |
| 0.300 mm | ±0.030 mm |
Rigid lenses take the flat ±0.02 mm from Table 1 instead. What center thickness governs on the eye — oxygen transmission, flexure, handling — is on Center Thickness of Contact Lenses.
How do you use a tolerance band at the bench?
In four steps, and the band is only the last of them.
- Record what was ordered — the parameter and its nominal value.
- Measure the delivered lens on the appropriate instrument. The measurements, and what each one can and cannot resolve, are on Bench Checks of Contact Lens Parameters.
- Condition the lens first if it is soft. ISO 18369-2 clause 6 requires hydrogel lenses to be equilibrated in the standard saline of ISO 18369-3, or an equivalent solution for the measurement method used. An unconditioned soft lens gives a reading the tolerance was not written for.
- Compare the measurement with the band for that parameter, that lens type and that power range.
Repeatability belongs in that judgement. A single soft-lens power reading in air is not a precise number, and one that disagrees with the order by a quarter of a diopter is not on its own evidence of a manufacturing deviation.
What a tolerance does not decide
A tolerance decides whether the laboratory delivered the order. It decides nothing about the eye.
Three limits are worth stating explicitly:
- In specification is not the same as well fitting. Every parameter can sit mid-band and the lens can still decentre, ride tight, or move too much. Those are on-eye findings, read at the slit lamp on Contact Lens Fit Assessment.
- The axis band is not a rotation allowance, as above.
- Material bands are material claims. ISO 18369-2 tolerances Dk at ±20% for all types and designs, refractive index at ±0.005 for soft and ±0.002 for rigid lenses, water content at ±2% absolute, and luminous transmittance at ±5% absolute. Those describe the material, not how the lens will perform on a particular cornea — see Oxygen Transmissibility of Contact Lenses.
The bands also do not describe any one laboratory. A manufacturer may hold tighter internal limits than the standard requires, and a custom lens may be specified in steps finer than a stock design. The standard is a floor for conformity, not a description of a supplier.
An out-of-specification lens goes back to the laboratory
A lens that measures outside its band is returned to the supplying laboratory by the practitioner.
It is not modified in office to bring it into specification, not dispensed with a note, and not worn to see whether it settles. Equally, an out-of-band reading is not by itself a safety event, and this site does not adjudicate one: it is a supply question between a practitioner and a laboratory, resolved by remeasurement and a remake.
Patient aside (Grade 8–9)
Lenses are made to a tolerance, which means the finished lens can differ very slightly from the numbers on your prescription. Standards set how much. If a lens is outside that, your eye doctor sends it back — it is not something to adjust at home or wear to see if it improves.
Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site, and Contact Lens Parameters for what each ordered parameter is. Convert spectacle prescription to contact lens parameters for power. Check the delivered lens against the order and against these bands. Verify power with over-refraction after the trial lens. Contact lens parameters at the corneal plane stay unfinished until that sequence is done.
Sources
Every value on this page comes from a published table. Nothing is estimated, interpolated or averaged.
- ISO 18369-2:2012, Ophthalmic optics — Contact lenses — Part 2: Tolerances(second edition 2012-12-01, ISO/TC 172/SC 7; published preview) — scope and introduction; clause 6 conditioning; clause 7 oxygen permeability tolerance of ±20%; Table 1 dimensional tolerances for rigid contact lenses; Table 2 optical tolerances for rigid contact lenses, including the weaker-meridian power ladder, cylinder bands, ±5° axis, prismatic error and the 0.50 mm scleral centration limit; Table 3 dimensional and optical tolerances for soft contact lenses, including the back optic zone radius design-step footnote and the center thickness formula with its four worked examples; Table 4 tolerance limits of material and physical properties.
- ANSI Z80.20, Ophthalmics — Contact Lenses — Standard Terminology, Tolerances, Measurements and Physicochemical Properties(published preview) — full title and secretariat; Accredited Committee Z80 for Ophthalmic Standards; scope covering rigid corneal and haptic lenses and soft paralimbal lenses; contents listing the tolerance tables for corneal, paralimbal and scleral lenses and for material properties; the foreword statement that the revision was written to insure conformance with the International Standard.
- Optical Training Institute, ANSI Standards for Contact Lenses— the abbreviated ANSI tolerance table across PMMA, rigid gas permeable, flexible and soft hydrogel columns (overall diameter, base curve radius, center thickness, and the five spherical power bands), and the identification of Z80.20-2016 (R2021) as the current Z80 committee standard for contact lenses.
External links are citations, not endorsements. This site is manufacturer-agnostic: advertising is labeled and advertisers do not influence formulas, defaults, or copy. That commitment is set out in the terms of use.
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