Eye Prescription to 20/20: What Can Be Converted?
Understand why lens prescription power cannot predict 20/20 vision, and convert an already measured Snellen fraction into equivalent acuity notation.
You cannot reliably turn a glasses prescription into a 20/20 eye-chart score. Diopters describe lens power; visual acuity is measured performance under test conditions. The converter below changes an already measured Snellen fraction into equivalent 20-foot, 6-metre, decimal and logMAR notation. It does not calculate vision from SPH, CYL, AXIS or ADD.
What to remember
- ✓ Prescription power and measured visual acuity are different kinds of information.
- ✓ A measured 20/40 and 6/12 represent the same fraction, 0.5, under the notation conversion.
- ✓ Corrected and uncorrected results must remain labelled separately.
- ✓ Editorial review is not an examination or an ophthalmologist’s clinical review.
Convert measured visual acuity notation
Enter an acuity result already measured on an eye chart, such as 20/40 or 6/12. Enter numerator and denominator in the same distance unit. This converts notation only and never accepts prescription diopters as a substitute for an eye-chart measurement.
Your result
Click Calculate
Enter your values, then click Calculate result.Why the prescription and the eye-chart result differ
A prescription tells a dispenser about a corrective lens. An eye-chart result records how clearly a person identified chart detail during a particular assessment. A lens power is not a denominator on the chart, and the two numbers cannot be substituted for each other. A person may have a substantial prescription and good corrected acuity, while another person with a different prescription may have a different result. An online table that maps every diopter value to one guaranteed Snellen score conceals information that it has not measured.
Use the prescription for its intended purpose and use a recorded chart result for acuity notation. If the report does not contain an acuity measurement, the converter cannot create one. A calculation that multiplies lens power by a fixed factor would produce a number but would not establish what you can actually see. This page therefore answers the prescription-to-20/20 calculator query by stating the boundary directly, then provides the narrower conversion that can be checked mathematically.
Keep the labels beside the number
Record which eye was tested, whether the result used glasses or contact lenses, whether it was distance or near testing, and the notation used by the examiner. A right-eye value, a left-eye value and a both-eyes value are separate observations. Do not average their denominators and call the result a binocular score. Likewise, corrected and uncorrected acuity should not be combined into one number. The converter does not know those conditions, so it cannot restore labels lost while copying data.
If a report uses extra symbols, qualifiers, a letter score or a test that is not expressed as a Snellen fraction, retain the original record rather than forcing it into these two fields. Ask the examining professional to explain an unfamiliar notation. A decimal-equivalent output is a convenient way to display one measured fraction; it is not proof that a different test would produce exactly the same observation.
The mathematical conversion used here
For a supplied Snellen numerator N and denominator D in the same distance unit, decimal acuity is N divided by D. The equivalent 20-foot denominator is 20 divided by that decimal ratio; the equivalent 6-metre denominator is 6 divided by it. The logMAR equivalent is the negative base-ten logarithm of the same positive ratio. These operations only rewrite the measurement. They do not measure the eye, simulate optical correction or choose a prescription.
For example, 20 divided by 40 is 0.5. Dividing 6 by 0.5 gives 12, so the equivalent metric notation is 6/12. The logMAR expression gives approximately 0.3010. Conversely, a measured 6/12 gives the same decimal fraction and a 20/40 equivalent. Preserve the original chart notation and its test conditions alongside any converted form so someone reviewing a record can see what was measured and what was merely calculated.
How to use the measured-acuity converter
Enter only an already measured Snellen fraction. For 20/40, put 20 in the test-distance numerator field and 40 in the denominator field. For 6/12, enter 6 and 12. The two entries must be positive and use the same distance unit. Do not enter a negative sphere power, cylinder power, an axis angle or reading addition. Those fields describe a lens prescription and are not accepted inputs for this mathematical purpose.
Click Calculate result to generate the equivalent notation. Editing either entry leaves the last submitted result visible until you click again. Check that the displayed original ratio matches the recorded measurement before copying the outputs. Fractions better than the reference, such as 20/10, are mathematically allowed; the converter does not cap every measurement at 20/20 or translate a result into an eye-health classification.
What this page cannot tell you
The notation conversion cannot identify the cause of reduced acuity, judge whether a prescription is current, determine driving or employment eligibility, or assess a change in vision. Rules requiring a vision assessment can depend on the test procedure and jurisdiction, so a converted fraction should not be treated as an official certificate. This page also does not provide a home eye chart, because a calibrated assessment requires more than an arbitrary screen image and viewing distance.
If a measurement seems inconsistent with your prescription, seek clarification from the eye-care professional who performed the examination. Do not infer that the prescription is wrong merely because a converted notation differs from a value found online. Similarly, a favorable chart score cannot establish that every aspect of an eye is healthy. Interpretation requires the full examination and the purpose for which the result is being used.
Share a clear record instead of an inferred diagnosis
A useful note separates measured data from arithmetic. Write the original acuity fraction, the eye and correction conditions, the examination date, and the equivalent notation calculated here. Label the latter as a conversion. Keep lens prescription entries in a separate part of the record with their signs and units. This makes it easier for a professional to understand the information without mistaking a web-generated denominator for an observed test result.
Inputs are processed by the shared calculator in your browser. They are not submitted to an eye-care service or used to create an examination report. Avoid entering names or unnecessary personal records into numeric fields. If you are using someone else’s documented result, preserve the context and share it only appropriately. General arithmetic tools linked below can explain ratios and decimal notation, but none converts lens power into a clinical outcome.
Equivalent notation for measured fractions
| Measured fraction | Decimal ratio | Equivalent 20-foot form | Approximate logMAR |
|---|---|---|---|
| 20/20 | 1 | 20/20 | 0 |
| 20/40 | 0.5 | 20/40 | 0.3010 |
| 6/12 | 0.5 | 20/40 | 0.3010 |
| 20/10 | 2 | 20/10 | −0.3010 |
These are mathematical equivalents of supplied measurements, not conversions from prescription diopters or predictions of vision.
Worked example: rewriting a measured 6/12
Suppose the examination record explicitly states measured distance acuity 6/12. Enter 6 and 12. The ratio is 0.5, the 20-foot equivalent is 20/40 and the logMAR equivalent is about 0.3010. Keep the original 6/12 record and all examination labels. Nothing in this calculation specifies lens power or determines a treatment.
Frequently asked questions
Can you convert −2.00 diopters into 20/20 notation?
No reliable patient-specific Snellen score follows from that lens power alone. This page requires an actual measured acuity fraction. A negative prescription value is not a valid numerator or denominator and is not translated using a guessed lookup table.
Does 20/20 mean no glasses are needed?
A measured result can be recorded with or without correction. Keep that condition with the result. A notation converter cannot determine whether glasses are required, whether a prescription should change or whether an eye examination is otherwise normal.
Is 6/12 equivalent to 20/40?
They have the same numerical ratio of 0.5, so this converter produces equivalent notation. That mathematical equivalence does not remove differences in examination conditions or guarantee a new measurement on another chart will be identical.
Can I average the results for my two eyes?
The calculator does not combine eyes. A binocular measurement is a separate observation and should not be invented by averaging the denominators of two monocular results. Keep each recorded result and its original labels.
What if my report is not a Snellen fraction?
Do not guess a fraction from another test or an unfamiliar abbreviation. Keep the report and ask the examining professional to explain its notation. This converter has a deliberately narrow input format and does not implement every acuity scoring system.
Limitations and responsible use
Educational notation and record-interpretation guide. Mohammad Qasim provides editorial review, not a clinical eye examination or specialist medical review. It does not diagnose, prescribe lenses, certify eligibility or predict visual acuity from a prescription. Use professional care for personal vision concerns.
Sources and further verification
Sources are provided for verification. A link does not imply endorsement, and official rules may change after our review date.