How are eye tests performed?

A comprehensive eye examination typically includes: (1) distance and near visual acuity testing using a Snellen or LogMAR chart; (2) refraction to determine your prescription; (3) slit-lamp examination of the cornea and lens; (4) intraocular pressure measurement to screen for glaucoma; (5) dilated fundus examination to view the optic nerve and retina. The full exam takes 30–60 minutes and is recommended every 1–2 years for adults.

What is considered poor vision?

Common clinical benchmarks: 20/40 is the minimum for unrestricted driving in many US states; 20/200 is the US legal blindness threshold in the better-corrected eye; 20/400 represents severe visual impairment. The WHO defines moderate visual impairment as best-corrected acuity worse than 6/18 (approximately 20/60, decimal 0.33).

What is legal blindness?

In the United States, legal blindness is defined as best-corrected visual acuity of 20/200 or worse in the better eye, OR a visual field diameter of 20 degrees or less (tunnel vision). It is a legal and administrative category that determines eligibility for benefits and services; most legally blind individuals retain some useful vision.

Can vision improve naturally?

Children's refractive errors can change significantly as the eye grows, and may improve without intervention. Adults rarely experience spontaneous improvement in refractive error. Outdoor time in childhood may reduce myopia progression, but does not restore acuity once established. Surgical options such as LASIK, SMILE, and PRK can achieve lasting correction for suitable candidates.

How often should I get an eye exam?

The American Optometric Association recommends: infants at 6–12 months; children at 3 years and before school entry; school-age children annually; adults 18–64 every 1–2 years (annually for contact lens wearers or those at higher risk); adults 65+ every 12 months. Anyone experiencing sudden vision change, flashing lights, new floaters, or eye pain should seek urgent assessment.

Vision Calculator

Convert visual acuity measurements, estimate vision quality, compare eyesight standards, and understand common eye test results instantly.

Visual Acuity Converter

Enter any one format — all others are calculated automatically.

Input Format

Snellen Denominator (20/x)

(20/x)

What is Visual Acuity?

Visual acuity (VA) is a measure of the clarity or sharpness of vision — specifically, the ability of the eye to distinguish fine detail at a standard distance. It is the most commonly measured aspect of vision in clinical settings and is routinely assessed during eye examinations, driving licence tests, and occupational health screenings worldwide.

The gold standard for visual acuity measurement was developed by Dutch ophthalmologist Herman Snellen in 1862. His standardized letter chart — now found in virtually every optometrist's office — forms the basis of the 20/20 notation used across the United States and many other countries.

How Vision Testing Works

The Snellen Chart

The Snellen chart consists of rows of letters that decrease in size from top to bottom. The patient stands 20 feet (or 6 metres) away and reads the smallest line they can see clearly. The result is recorded as a fraction where the numerator is the test distance and the denominator is the distance at which a person with “normal” vision can read the same line.

LogMAR Charts

The ETDRS (Early Treatment Diabetic Retinopathy Study) LogMAR chart is the preferred tool in clinical research because it is more precise and statistically reliable than the Snellen chart. It uses the logarithm of the minimum angle of resolution, providing equal steps between each line and each letter.

Refraction Test

Best-corrected visual acuity (BCVA) measures vision after the optimal prescription has been applied. Uncorrected (unaided) acuity is measured without any lenses. Most medical and legal standards refer to best-corrected acuity since correction removes refractive error as a variable.

Contrast and Colour

Standard Snellen testing measures high-contrast acuity under good lighting. Contrast sensitivity testing and colour vision assessments provide additional dimensions of visual function not captured by the basic acuity score — important for assessing driving fitness, occupational performance, and retinal disease.

6 Ways to Use This Vision Calculator

Convert Between Systems

Instantly translate between Snellen 20/x, metric 6/x, decimal acuity, and LogMAR — useful when comparing results from different countries or clinical reports.

Interpret Your Eye Test

Received a result such as 20/40 or 6/12? Enter it here to understand what it means in plain language — your category, efficiency, and comparison to normal vision.

Understand LogMAR Values

Clinical papers often report acuity in LogMAR. This calculator converts any LogMAR value to familiar Snellen notation in seconds.

Age-Adjusted Assessment

Use the Vision Quality Assessment tab to receive an age-normalised score — the expected norm differs between children, adults, and seniors.

Check Driving Suitability

Most jurisdictions require at least 20/40 (decimal 0.5) in the better eye to drive without corrective lens restrictions. The Assessment tab reports this automatically.

Track Vision Over Time

Compare your last prescription (Snellen notation) with a newer result to see whether your acuity has improved, remained stable, or declined between examinations.

Understanding 20/20 and the Acuity Scale

20/20 vision means you can see at 20 feet what a person with normal vision sees at 20 feet. It does not mean “perfect” vision — some people see 20/15 or even 20/10 without correction. The table below describes what the most common Snellen acuity levels actually represent in daily life:

Snellen	Decimal	LogMAR	Clinical Meaning
20/10	2.000	−0.30	Exceptional — far above normal; rare in adults
20/20	1.000	0.00	Normal standard — clinical benchmark for adults
20/40	0.500	0.30	Minimum for most unrestricted driving licences
20/60	0.333	0.48	Difficulty reading standard street signs
20/100	0.200	0.70	Moderate visual limitation; reading glasses needed
20/200	0.100	1.00	US legal blindness threshold in the better eye
20/400	0.050	1.30	Low vision — assistive devices strongly advised

Why Regular Vision Testing Matters

Early Disease Detection

Many serious eye diseases — including glaucoma, age-related macular degeneration (AMD), diabetic retinopathy, and cataracts — produce no symptoms in their early stages. An annual dilated eye exam can detect these conditions before they cause permanent damage.

Refractive Error Correction

Myopia (nearsightedness), hyperopia (farsightedness), and astigmatism are extremely common and easily corrected with glasses, contact lenses, or refractive surgery. Uncorrected refractive error is the leading cause of visual impairment worldwide, according to the WHO.

Children and Learning

Undetected vision problems are a major barrier to learning. The American Optometric Association recommends a comprehensive eye exam before the first grade, and annually thereafter, because children rarely report vision difficulties — they simply assume everyone sees the world the way they do.

Systemic Health Signals

The retina is the only place in the body where blood vessels can be observed directly without surgery. An eye examination can reveal signs of hypertension, diabetes, multiple sclerosis, and cardiovascular disease long before other symptoms emerge.

Common Refractive Conditions Explained

Myopia (Nearsightedness)

The most common refractive error globally, affecting approximately 30% of the world population. The eyeball is too long or the cornea too curved, causing light to focus in front of the retina rather than on it. Distant objects appear blurry; close objects are clear. Prescription lenses carry a minus (−) sign.

Hyperopia (Farsightedness)

The eyeball is shorter than normal, causing light to focus behind the retina. Mild hyperopia is common in children and often corrects itself. Adults with uncorrected hyperopia may experience headaches, eyestrain, and blurred near vision. Prescriptions carry a plus (+) sign.

Astigmatism

An irregularly shaped cornea or lens causes light to focus at multiple points, producing blurred or distorted vision at all distances. Astigmatism frequently coexists with myopia or hyperopia and is corrected with cylindrical lenses or toric contact lenses.

Presbyopia

A natural, age-related reduction in the eye's ability to focus on near objects, typically becoming noticeable in the early-to-mid 40s. The crystalline lens gradually loses its elasticity. Reading glasses or progressive/varifocal lenses are the standard correction.

Core Conversion Formulas

Snellen to Decimal

Decimal = Numerator ÷ Denominator

20/40 → 20 ÷ 40 = 0.500

Decimal to LogMAR

LogMAR = −log₁₀(Decimal)

0.500 → −log₁₀(0.5) = 0.301

LogMAR to Decimal

Decimal = 10^(−LogMAR)

0.301 → 10^(−0.301) = 0.500

Imperial to Metric

6/x → denominator = x × 6/20

20/40 → 6/12 (same decimal)

Visual Efficiency (AMA)

Interpolated from AMA table

LogMAR 0.30 ≈ 84% efficiency

Vision Score

Score = min(100, round(VE × 1.05))

VE 84% → Score ≈ 88/100

Common Misconceptions About Vision

20/20 does not mean perfect vision

20/20 is the average for a healthy adult, not the best possible. Many children and young adults see 20/15 or better. Perfect vision is a myth — acuity varies on a continuous scale.

Glasses don't make your eyes weaker

A common misconception is that wearing glasses causes dependency or weakens the eyes. In reality, prescription lenses correct the optical mismatch; they don't alter the eye's underlying refractive state. Progressive myopia in children is caused by eye growth, not spectacle wear.

Carrots improve vision — but only if you're deficient

Vitamin A (from beta-carotene in carrots) is essential for rhodopsin production in rod cells, and severe deficiency causes night blindness. However, eating extra carrots has no effect on acuity in people with adequate vitamin A levels — a myth popularised during World War II.

Reading in dim light is not harmful

Eye fatigue from poor lighting is real, but reading in dim light does not damage vision permanently. The sensation of strain, headache, and tired eyes is due to the ciliary muscle working harder to maintain focus.

Snellen measures vision, not prescription

The Snellen fraction is a measure of visual acuity, not the prescription power needed to correct it. A person with −6.00 dioptre myopia who is corrected to 20/20 with lenses has the same Snellen result as a person with −1.00 dioptres corrected to 20/20.

About This Calculator

Formula-Based

Conversions use standard optometric formulas: Snellen fraction, decimal acuity, and log₁₀ transformation for LogMAR.

AMA Visual Efficiency

Visual efficiency estimates use the American Medical Association (AMA) Guides to the Evaluation of Permanent Impairment table.

Educational Use Only

This tool is for educational and informational purposes. It is not a substitute for a clinical eye examination by a qualified optometrist or ophthalmologist.

Related tools: If you are tracking overall health, also see our BMI Calculator, Age Calculator, BMR Calculator, Calorie Calculator, Body Fat Calculator, and Water Intake Calculator — or browse all Health Calculators.

Frequently Asked Questions

20/20 means you can read a standard optotype letter at 20 feet that a person with normal vision also reads at 20 feet. It is the clinical benchmark for adequate distance vision in the United States, equivalent to 6/6 in the metric system used in Europe, Australia, and the UK. 20/20 does not mean perfect vision — many people see 20/15 or better.

Visual acuity is a measure of the spatial resolution of the visual system — specifically the smallest detail the eye can resolve at a given distance. It depends on the optical quality of the eye (cornea, lens, refraction) and the neural processing of the retina. High acuity means you can distinguish fine detail; low acuity means detail appears blurred or merged.

LogMAR stands for the logarithm of the minimum angle of resolution. It is calculated as −log₁₀(decimal acuity), giving values where 0.00 = 20/20, positive values indicate worse vision, and negative values indicate better-than-normal vision. It is preferred in clinical research because it is a linear, mathematically equal interval scale — unlike the Snellen fraction, where the steps between 20/20 and 20/30 are not equal to the steps between 20/80 and 20/100.

The Snellen chart is an eye test chart invented by Herman Snellen in 1862. It consists of rows of letters (optotypes) that decrease in size. Each row corresponds to the distance at which a person with normal vision can read it. The patient stands 20 feet (6 m) away and reads the smallest line possible. The result is expressed as a fraction: distance tested / distance of the smallest readable row.

A comprehensive eye examination typically includes: (1) Distance and near visual acuity testing using a Snellen or LogMAR chart; (2) Refraction to determine your prescription; (3) Slit-lamp examination to inspect the cornea, lens, and anterior segment; (4) Intraocular pressure measurement (tonometry) to screen for glaucoma; (5) Dilated fundus examination to view the optic nerve, retina, and macula. The full exam takes 30–60 minutes and is recommended every 1–2 years for adults.

There is no single universal threshold, but common clinical benchmarks are: 20/40 — minimum for unrestricted driving in many US states; 20/200 — the US legal blindness threshold in the better-corrected eye; 20/400 — severe visual impairment; worse than 20/1000 — light perception only. The WHO defines moderate visual impairment as best-corrected acuity worse than 6/18 (decimal 0.3, approximately 20/60).

In the United States, legal blindness is defined as best-corrected visual acuity of 20/200 or worse in the better eye, OR a visual field diameter of 20 degrees or less (tunnel vision). Legal blindness is a legal and administrative category that determines eligibility for benefits and services; it does not necessarily mean total blindness — most legally blind individuals retain some useful vision.

Children's refractive errors can change significantly as the eye grows, and in some cases improve without intervention. Adults rarely experience spontaneous improvement in refractive error. Lifestyle measures such as outdoor time in childhood may reduce myopia progression, but do not restore acuity. Surgical options (LASIK, SMILE, PRK) can achieve lasting correction for suitable candidates.

The American Optometric Association recommends: infants at 6–12 months; children at 3 years and before school entry; school-age children annually; adults 18–64 every 1–2 years (annually if contact lens wearer or at-risk); adults 65+ every 12 months. Anyone experiencing sudden vision change, flashing lights, new floaters, or eye pain should seek urgent assessment regardless of their last exam date.

The Snellen fraction (e.g., 20/40) is widely recognised and easy to communicate, but it is not a linear scale — equal steps on the chart do not represent equal changes in acuity. LogMAR provides equal, proportional steps and is more statistically reliable for research purposes. In clinical practice, both are used: Snellen in general optometry and patient communication; LogMAR/ETDRS charts in clinical trials, low-vision clinics, and research.