Eye Prescription Decoder — What Do Your Numbers Mean?
Eye Prescription Decoder — What Do Your Numbers Mean?
Enter your eye prescription numbers below to get a plain-English explanation of every value — SPH, CYL, AXIS, ADD, PD, OD, and OS. This free decoder also includes a Glasses vs Contacts comparison mode. No signup needed, instant results.
ℹ️ Educational Tool Only. This tool is for informational purposes only. Always consult your optometrist or ophthalmologist for professional advice about your vision and prescription.
Eye Prescription Decoder — What Do Your Numbers Mean?
Understanding how to read your eye prescription can feel like decoding a foreign language — OD, OS, SPH, CYL, AXIS. This free decoder translates every number into plain English, explains what OD and OS mean, and tells you exactly what your vision correction needs are in simple, clear language.
Enter any or all fields — leave blank what you don't have. All fields are optional.
ODRight Eye (Oculus Dexter)
e.g. -3.50 or +1.25
Astigmatism strength
1–180 degrees
Reading addition
OSLeft Eye (Oculus Sinister)
Your Prescription Summary
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Enter your glasses prescription and we'll explain how it may differ for contact lenses.
ODRight Eye
OSLeft Eye
💡 Why contacts differ from glasses: Contact lenses sit directly on your cornea — so for prescriptions over ±4.00 diopters, the effective power differs due to vertex distance. Your optometrist will also specify base curve (BC) and diameter for contacts.
Your Glasses vs Contacts Comparison
⚠️ This comparison is approximate. Always confirm your contact lens prescription with your optometrist — they will also measure your base curve and corneal diameter.
What Does Each Number Mean?
Abbreviation
Full Name
What It Means
OD
Oculus Dexter
Your right eye — Latin for "right eye"
OS
Oculus Sinister
Your left eye — Latin for "left eye"
SPH
Sphere
Overall lens power — negative = nearsighted, positive = farsighted
CYL
Cylinder
Astigmatism correction strength — blank or DS = no astigmatism
AXIS
Axis
The angle (1°–180°) at which astigmatism correction is applied
ADD
Addition
Extra magnification for reading — common after age 40 (presbyopia)
PD
Pupillary Distance
Distance between pupils in mm — needed to centre lenses correctly
DS / Plano
Dioptre Sphere / Plano
"No correction needed" in that field — Plano = 0.00 power
OD stands for Oculus Dexter, Latin for your right eye. OS stands for Oculus Sinister, Latin for your left eye. OU means Oculus Uterque — both eyes. Your right eye (OD) is always listed first. Some modern prescriptions simply say "Right" and "Left."
The sphere (SPH) measures your overall lens power in diopters. A negative SPH (e.g. -2.50) means nearsighted — you see nearby objects clearly but not distant ones. A positive SPH (e.g. +1.75) means farsighted. The higher the absolute number, the stronger the correction needed. Plano or 0.00 means no correction is needed in that direction.
The axis describes the orientation of your astigmatism correction, measured in degrees from 1° to 180°. It only appears when you have a cylinder (CYL) value. An axis of 90° means correction is applied vertically; 180° means horizontally. The axis has no relationship to prescription severity — it is purely directional information for the lens maker.
ADD (Addition) is the extra magnifying power in the lower section of multifocal or bifocal lenses — it helps with close reading. ADD values are always positive (+0.75 to +3.50) and usually the same for both eyes. It relates to presbyopia — the age-related loss of near-focusing ability, typically noticed from your mid-40s onwards. If you have an ADD value, you likely use bifocals, progressive lenses, or reading glasses.
DS (Dioptre Sphere) in the CYL column means no astigmatism. Plano in the SPH column means zero power — no nearsighted or farsighted correction needed in that eye. These are not errors — they are intentional entries meaning "no correction required in this category."
A contact lens prescription is not the same as a glasses prescription. Contact prescriptions also include Base Curve (BC) — the lens curvature to match your cornea — and Diameter (DIA). For prescriptions beyond ±4.00 diopters, the power may be slightly adjusted. Contacts are brand-specific and require a separate fitting by your optometrist. You cannot legally use your glasses prescription to buy contacts.
The difference between glasses and contact prescriptions is vertex distance — the 12–14mm gap between glasses and your eyes. For prescriptions under ±4.00 diopters, the difference is negligible. For stronger prescriptions, a vertex distance correction is applied — e.g. a -6.00 glasses prescription may convert to approximately -5.75 for contacts. Always confirm with your optometrist.
PD (Pupillary Distance) is the distance in mm between your pupils. It ensures the optical centre of each lens aligns with your eyes. A wrong PD causes eye strain, headaches, and blurry vision. Average adult PD is 54–74mm. It may be given as one number (total) or two (one per eye). Your optometrist measures it during your eye exam.
🔔 Disclaimer: This tool is for educational purposes only and does not constitute medical or clinical advice. Always consult a qualified optometrist or ophthalmologist for diagnosis, prescriptions, and professional vision care.
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' + '
Mild (0.25)ModerateVery Strong (6+)
'; } function cylStr(cyl) { var a = Math.abs(cyl); return a < 0.75 ? 'mild astigmatism' : a < 1.75 ? 'moderate astigmatism' : 'significant astigmatism'; } function renderEye(container, side, sph, cyl, axis, add) { var label = side === 'OD' ? 'Right Eye (OD — Oculus Dexter)' : 'Left Eye (OS — Oculus Sinister)'; var html = '
👁️ ' + label + '
'; var any = false; if (sph !== null) { any = true; var s = sphInfo(sph); html += '
You have ' + cylStr(cyl) + ' in your ' + (side === 'OD' ? 'right' : 'left') + ' eye.' + (axis !== null ? ' The correction is applied at ' + axis + '° — the orientation of your cornea\'s irregular curve.' : '') + '
'; } if (add !== null) { any = true; html += '
Addition (ADD) — +' + add.toFixed(2) + '
' + '
You need a reading addition — common in people over 40 (presbyopia). You may use bifocal, progressive, or reading glasses.
'; } if (!any) { container.classList.add('epd-hidden'); return; } container.innerHTML = html; container.classList.remove('epd-hidden'); } // ── DECODE ── $i('epd-decode-btn').addEventListener('click', function() { var odSph = getVal('epd-od-sph'), odCyl = getVal('epd-od-cyl'), odAxis = getVal('epd-od-axis'), odAdd = getVal('epd-od-add'); var osSph = getVal('epd-os-sph'), osCyl = getVal('epd-os-cyl'), osAxis = getVal('epd-os-axis'), osAdd = getVal('epd-os-add'); var useSingle = $i('epd-pd-single').checked; var pdR = !useSingle ? getVal('epd-pd-r') : null; var pdL = !useSingle ? getVal('epd-pd-l') : null; var pdS = useSingle ? getVal('epd-pd-s') : null; var totalPD = pdS || (pdR !== null && pdL !== null ? pdR + pdL : (pdR || pdL)); var hasAny = [odSph,odCyl,odAdd,osSph,osCyl,osAdd,pdR,pdL,pdS].some(function(v){return v!==null;}); if (!hasAny) { alert('Please enter at least one prescription value to decode.'); return; } renderEye($i('epd-od-out'), 'OD', odSph, odCyl, odAxis, odAdd); renderEye($i('epd-os-out'), 'OS', osSph, osCyl, osAxis, osAdd); // PD if (totalPD !== null) { var pdNote = totalPD < 56 ? '— slightly narrow (verify with optometrist)' : totalPD > 72 ? '— slightly wide (verify with optometrist)' : '— within normal adult range'; $i('epd-pd-out').innerHTML = '
📏 Pupillary Distance (PD)
' + '
PD — ' + totalPD.toFixed(1) + ' mm
' + '
Your pupillary distance is ' + totalPD.toFixed(1) + ' mm ' + pdNote + '.' + (pdR !== null && pdL !== null ? ' R: ' + pdR + ' mm | L: ' + pdL + ' mm.' : '') + ' Your PD is used to align the optical centre of each lens with your pupils.
'; $i('epd-pd-out').classList.remove('epd-hidden'); } else { $i('epd-pd-out').classList.add('epd-hidden'); } // Summary var tags = [], parts = []; var allSph = [odSph, osSph].filter(function(v){return v!==null;}); if (allSph.length) { var avg = allSph.reduce(function(a,b){return a+b;},0)/allSph.length; var si = sphInfo(avg); tags.push('' + (avg<0?'👓 Nearsighted':'👁️ Farsighted') + ''); parts.push(si.str + ' ' + si.dir); } if ([odCyl,osCyl].some(function(v){return v!==null;})) { tags.push('🔄 Astigmatism'); parts.push('astigmatism'); } if ([odAdd,osAdd].some(function(v){return v!==null;})) { tags.push('📖 Presbyopia'); parts.push('reading addition'); } if (!tags.length) tags.push('📋 Prescription entered'); $i('epd-sum-sub').textContent = parts.length ? 'Overall: ' + parts.join(', ') + '.' : 'See individual eye results below.'; $i('epd-sum-tags').innerHTML = tags.join(''); show('epd-results'); $i('epd-results').scrollIntoView({behavior:'smooth',block:'start'}); }); $i('epd-clear-btn').addEventListener('click', function() { ['epd-od-sph','epd-od-cyl','epd-od-axis','epd-od-add', 'epd-os-sph','epd-os-cyl','epd-os-axis','epd-os-add', 'epd-pd-r','epd-pd-l','epd-pd-s'].forEach(function(id){ $i(id).value=''; }); hide('epd-results'); }); // ── CONTACTS COMPARE ── $i('epd-compare-btn').addEventListener('click', function() { var odSph = getVal('epd-c-od-sph'), odCyl = getVal('epd-c-od-cyl'); var osSph = getVal('epd-c-os-sph'), osCyl = getVal('epd-c-os-cyl'); if (odSph === null && osSph === null) { alert('Please enter at least one sphere (SPH) value.'); return; } function convertSph(sph) { if (sph === null) return null; if (Math.abs(sph) > 4.0) { return Math.round((sph / (1 - 0.012 * sph)) * 4) / 4; } return sph; } var html = ''; function eyeRows(side, gSph, gCyl, cSph) { var changed = cSph !== null && gSph !== null && Math.abs(cSph - gSph) >= 0.125; if (gSph !== null) html += '
'; } eyeRows('Right', odSph, odCyl, convertSph(odSph)); eyeRows('Left', osSph, osCyl, convertSph(osSph)); var needsAdj = [odSph,osSph].some(function(v){return v!==null && Math.abs(v)>4.0;}); html += '
' + (needsAdj ? '⚠️ Prescription over ±4.00 — vertex distance adjustment applied. Confirm exact power with your optometrist.' : '✅ Prescription under ±4.00 — contact power is typically the same as glasses for this range.') + '
'; if ([odCyl,osCyl].some(function(v){return v!==null;})) { html += '
📌 You have astigmatism — you will likely need toric contact lenses.
