{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"snellen-vision-conversion"},"result":{"entity_type":"calculator","id":"snellen-vision-conversion","calculator_id":"snellen-vision-conversion","canonical_url":"https://tttkmbb.com/medical/snellen-vision-conversion","name":"Visual Acuity Converter","title":"Visual Acuity Converter – Snellen (20/x, 6/x), Decimal and logMAR with WHO ICD-11 Category","category":"medical","category_name":"Medical & Clinical","tool_name":"convert_visual_acuity","featured_mcp_tool":false,"description":"Converts a visual acuity written as a Snellen fraction in feet or metres, a decimal or a logMAR value into all four notations plus the minimum angle of resolution, the nearest ETDRS/logMAR chart line and the WHO ICD-11 distance-vision-impairment category.","use_when":"You need to translate an acuity between US Snellen (20/40), metric Snellen (6/12), decimal (0.5) and logMAR (0.3) notation, or to place it against the WHO impairment thresholds or the US legal-blindness acuity criterion.","do_not_use_when":"The acuity is recorded as counting fingers, hand motion or light perception (no numeric conversion), or the problem is visual-field loss; results are informational and not medical advice.","inputs":[{"name":"acuity","label":"Visual acuity","type":"string","required":true,"description":"Acuity in any common notation: Snellen feet \"20/40\", Snellen metres \"6/12\", decimal \"0.5\" or \"logMAR 0.3\". A plain number is read as decimal acuity unless format says otherwise (a negative number is read as logMAR).","example":"20/40"},{"name":"format","label":"Input format","type":"enum","required":false,"default":"auto","values":[{"value":"auto","label":"Detect from the text","aliases":["detect"]},{"value":"snellen_ft","label":"Snellen, feet (20/x; a plain number is the denominator x)","aliases":["feet","ft","snellen"]},{"value":"snellen_m","label":"Snellen, metres (6/x; a plain number is the denominator x)","aliases":["metres","meters","m"]},{"value":"decimal","label":"Decimal acuity (1.0 = 20/20)"},{"value":"logmar","label":"logMAR (0 = 20/20)","aliases":["log_mar"]}],"description":"How to read acuity; auto recognises fractions, the word logMAR and plain decimals.","example":"auto"}],"outputs":[{"name":"snellen_feet","label":"Snellen (feet)","type":"string","decimals":4,"description":"20/x notation."},{"name":"snellen_metres","label":"Snellen (metres)","type":"string","decimals":4,"description":"6/x notation."},{"name":"decimal_acuity","label":"Decimal acuity","type":"number","decimals":3,"description":"numerator / denominator (1.0 = 20/20 = 6/6)."},{"name":"logmar","label":"logMAR","type":"number","decimals":2,"description":"log10 of the minimum angle of resolution: 0 = 20/20, higher is worse, each 0.1 is one ETDRS line."},{"name":"mar_arcmin","label":"Minimum angle of resolution","type":"number","unit":"arcmin","decimals":2,"description":"1 / decimal acuity (1 arcmin at 20/20)."},{"name":"category","label":"WHO ICD-11 category","type":"string","decimals":4,"description":"Distance vision impairment category (mild: worse than 6/12 to 6/18; moderate: worse than 6/18 to 6/60; severe: worse than 6/60 to 3/60; blindness: worse than 3/60), applied to this acuity."},{"name":"us_legal_blindness","label":"US legal blindness criterion","type":"string","decimals":4,"description":"Whether the acuity meets the US statutory criterion (20/200 or worse in the better eye with best correction; the alternative visual-field criterion is not assessed)."},{"name":"nearest_standard_line","label":"Nearest chart line","type":"string","decimals":4,"description":"Nearest ETDRS/logMAR chart line (0.1 logMAR steps from 20/10 to 20/800) with its Snellen equivalents."},{"name":"input_interpreted_as","label":"Input read as","type":"string","decimals":4,"description":"How the acuity text was interpreted."}],"input_schema":{"type":"object","properties":{"acuity":{"description":"Acuity in any common notation: Snellen feet \"20/40\", Snellen metres \"6/12\", decimal \"0.5\" or \"logMAR 0.3\". A plain number is read as decimal acuity unless format says otherwise (a negative number is read as logMAR).","type":"string","examples":["20/40"]},"format":{"description":"How to read acuity; auto recognises fractions, the word logMAR and plain decimals.","type":"string","enum":["auto","snellen_ft","snellen_m","decimal","logmar"],"default":"auto","examples":["auto"]}},"additionalProperties":false,"required":["acuity"]},"output_schema":{"type":"object","properties":{"snellen_feet":{"description":"20/x notation.","type":"string"},"snellen_metres":{"description":"6/x notation.","type":"string"},"decimal_acuity":{"description":"numerator / denominator (1.0 = 20/20 = 6/6).","type":"number"},"logmar":{"description":"log10 of the minimum angle of resolution: 0 = 20/20, higher is worse, each 0.1 is one ETDRS line.","type":"number"},"mar_arcmin":{"description":"1 / decimal acuity (1 arcmin at 20/20). Unit: arcmin.","type":"number","x-unit":"arcmin"},"category":{"description":"Distance vision impairment category (mild: worse than 6/12 to 6/18; moderate: worse than 6/18 to 6/60; severe: worse than 6/60 to 3/60; blindness: worse than 3/60), applied to this acuity.","type":"string"},"us_legal_blindness":{"description":"Whether the acuity meets the US statutory criterion (20/200 or worse in the better eye with best correction; the alternative visual-field criterion is not assessed).","type":"string"},"nearest_standard_line":{"description":"Nearest ETDRS/logMAR chart line (0.1 logMAR steps from 20/10 to 20/800) with its Snellen equivalents.","type":"string"},"input_interpreted_as":{"description":"How the acuity text was interpreted.","type":"string"}}},"formula":"decimal = numerator / denominator (Snellen) or 10^(−logMAR); logMAR = log10(1 / decimal) = log10(denominator / numerator); mar_arcmin = 1 / decimal; snellen_feet = 20 / (20 / decimal); snellen_metres = 6 / (6 / decimal); nearest line = logMAR rounded to the nearest 0.1","method":"The notations are exact transforms of one another (20/40 = 6/12 = 0.5 = logMAR 0.30); denominators are rounded to whole numbers above 20 and to one decimal below. ICD-11 categories are defined on presenting acuity in the better eye, and the US definition of legal blindness also accepts a visual field of 20° or less. Results are informational only and not a substitute for clinical judgement or medical advice.","sources":[{"name":"WHO – Blindness and vision impairment fact sheet (ICD-11 distance vision impairment categories)","url":"https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment","type":"standard","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Visual acuity (Snellen, decimal and logMAR notation, US legal blindness)","url":"https://en.wikipedia.org/wiki/Visual_acuity","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – LogMAR chart (ETDRS lines and Snellen equivalents)","url":"https://en.wikipedia.org/wiki/LogMAR_chart","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"20/40","inputs":{"acuity":"20/40"},"expected":{"snellen_feet":"20/40","snellen_metres":"6/12","decimal_acuity":0.5,"logmar":0.3,"mar_arcmin":2,"nearest_standard_line":"20/40 (6/12), logMAR 0.3"},"url":"https://tttkmbb.com/api/v1/calculate/snellen-vision-conversion?acuity=20%2F40"},{"name":"6/6","inputs":{"acuity":"6/6","format":"auto"},"expected":{"snellen_feet":"20/20","snellen_metres":"6/6","decimal_acuity":1,"logmar":0,"mar_arcmin":1,"category":"Normal vision (6/6 = 20/20 or better)"},"url":"https://tttkmbb.com/api/v1/calculate/snellen-vision-conversion?acuity=6%2F6&format=auto"}],"faq":[{"q":"How do I read logMAR values?","a":"0.0 is 20/20, each +0.1 is one line worse (0.3 = 20/40, 1.0 = 20/200) and negative values are better than 20/20 (−0.1 = 20/16). It is the scale used in research and ETDRS charts."},{"q":"Why does 20/30 convert to 6/9 rather than the 6/9.5 on my chart?","a":"The conversion is exact (20/30 = 0.667 = 6/9); ETDRS charts step in 0.1 logMAR, so their 6/9.5 line is 20/32 (logMAR 0.2), which nearest_standard_line reports."},{"q":"What is legal blindness?","a":"In the US, best-corrected acuity of 20/200 or worse in the better eye, or a visual field of 20° or less; other countries use different thresholds (the UK certifies severe sight impairment around 3/60)."}],"tags":["snellen to logmar","visual acuity conversion","20/20 vision","6/6 vision","decimal acuity","logmar"],"related":[{"calculator_id":"angle","reason":"Convert the minimum angle of resolution between arcminutes and degrees."},{"calculator_id":"glasgow-coma-scale","reason":"Another bedside neurological assessment converted to a standard scale."}],"links":{"html":"https://tttkmbb.com/medical/snellen-vision-conversion","markdown":"https://tttkmbb.com/medical/snellen-vision-conversion.md","json":"https://tttkmbb.com/medical/snellen-vision-conversion.json","api":"https://tttkmbb.com/api/v1/calculate/snellen-vision-conversion","schema":"https://tttkmbb.com/api/v1/calculators/snellen-vision-conversion","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:49:05Z"}