Look up the normal axial length for your child's age, sex, and ethnicity — or estimate axial length directly from a spectacle prescription.
Leave blank to use OD only
Enter age, sex, ethnicity, and at least one axial length to see the percentile distribution.
Estimates axial length from spherical equivalent using Bennett's formula: AL = 23.5 + |SE| × 0.40 · accuracy ±0.3 mm
Enter a spectacle prescription to estimate axial length using the Bennett formula.
The Fundamentals
What is axial length, and why is it the key number in myopia?
Axial length (AL) is the distance from the front surface of the cornea to the retina, measured along the visual axis in millimetres. It is the primary structural determinant of refractive error: when this distance is too long, the focal point of incoming light falls in front of the retina rather than on it — producing myopia (nearsightedness).
Unlike a glasses prescription, which can be updated with new lenses, axial length is a permanent physical dimension. The eye does not shrink. This is why clinicians track it over time: each millimetre of elongation is cumulative, and high adult axial lengths are directly associated with blinding complications including myopic maculopathy, retinal detachment, glaucoma, and cataract.
In a typical emmetropic (normal-sighted) adult, axial length is around 23.5 mm. A −3.00 D myopic eye is approximately 24.7 mm; a −6.00 D eye approaches 26.0 mm — the threshold at which lifetime risk of maculopathy reaches 50%.
In myopia, the eyeball is longer than normal, causing light to focus in front of the retina.
Axial Length vs. Diopters
How axial length relates to a glasses prescription
Many parents know their child's prescription in diopters but not their axial length. The two are related — but not equivalent. Every 1 diopter of myopia corresponds to roughly 0.35–0.40 mm of extra axial length (Bennett 1984, Mutti et al. 1998). However, the exact relationship varies between individuals due to differences in corneal curvature and lens power.
This is why direct biometry (measuring axial length with an optical device) is always preferred over prescription-based estimation for clinical decisions. The table below shows approximate correspondences.
Prescription (approx.)
Estimated Axial Length
Adult Risk Level
Context
Plano / emmetropic
~23.5 mm
Low
Population average
−1.00 D
~23.9 mm
Low
Low myopia
−2.00 D
~24.3 mm
Low–Moderate
Mild myopia
−3.00 D
~24.7 mm
Moderate
Moderate myopia
−4.00 D
~25.1 mm
Moderate
Monitoring important
−5.00 D
~25.5 mm
Elevated
Active management needed
−6.00 D
~25.9 mm
High
Near higher-risk range
−8.00 D or more
> 26.5 mm
Very High
Above 26 mm higher-risk range
Biometry Methods
How axial length is measured in clinical practice
Axial length is measured with an optical biometer — a non-contact device that sends a beam of infrared light into the eye and measures the reflected signal with sub-millimetre precision. The process is painless, takes under 30 seconds per eye, and is accurate to within 0.01 mm. It does not require dilation or touching the eye.
Partial Coherence Interferometry
IOLMaster 700
The clinical gold standard. Uses swept-source OCT to measure axial length, corneal curvature, and anterior segment simultaneously. Accuracy: ±0.02 mm. Manufacturer: Carl Zeiss Meditec.
Optical Low Coherence Reflectometry
Lenstar LS 900
Measures axial length plus 8 other ocular parameters in a single exam. Widely used in myopia management clinics. Accuracy: ±0.01 mm. Manufacturer: Haag-Streit.
Ultrasound A-Scan
Ultrasound Biometry
Older contact-based method. Requires anaesthetic eye drops and direct probe contact. Less accurate (±0.10 mm) but available in practices without optical biometers. Use optical biometry where possible.
Reference Tables
Axial length percentile tables by age, sex, and ethnicity
Select a demographic group to view the full normative reference table. Values are linearly interpolated between integer ages in the calculator. Data sourced from Tideman et al. (2016) for European populations and He et al. (2004) / Sanz Diez et al. (2019) for Asian populations.
Green (P50) = median ·
Amber (P75) = above average, monitor ·
Red (P95) = clinically significant, management indicated
Common Questions
Frequently asked questions about axial length
What is axial length in the eye?
Axial length is the front-to-back distance of the eyeball in millimetres, measured from the corneal apex to the retina along the visual axis. It is the primary biometric determinant of refractive error: too long = myopia (nearsightedness), too short = hyperopia (farsightedness). In myopia management, axial length is tracked serially because every millimetre of elongation is permanent and increases the risk of sight-threatening complications in adulthood.
What is a normal axial length for a child?
Normal axial length varies significantly with age, sex, and ethnicity. At age 10, the median (50th percentile) for a European male is approximately 23.18 mm; for an Asian male it is around 24.30 mm. Female values are typically 0.3–0.5 mm shorter. Values above the 75th percentile for the child's demographic group warrant monitoring; above the 95th percentile indicates active myopia management is usually indicated. Use the calculator above to look up exact values for a specific age and demographic.
How is axial length measured?
Axial length is measured with an optical biometer — a non-contact device using partial coherence interferometry (PCI) or optical low coherence reflectometry (OLCR). Common devices include the IOLMaster 700 (Zeiss), Lenstar LS900 (Haag-Streit), and Argos (Movu). The test is painless, takes under a minute per eye, and achieves accuracy of ±0.01–0.02 mm. Some practices still use ultrasound A-scan biometry, which requires direct eye contact and is less accurate (±0.10 mm).
What is the relationship between axial length and glasses prescription?
For myopic eyes, each additional diopter of prescription corresponds to approximately 0.35–0.40 mm of extra axial length (Bennett AG, Ophthalmic Physiol Opt. 1984; Mutti DO et al. Invest Ophthalmol Vis Sci. 1998). This is the basis of Mode B (Rx → AL) in the calculator above. However, this relationship has an individual variability of ±0.3 mm, meaning direct biometry is always preferred for clinical decisions. Two children with identical prescriptions may have meaningfully different axial lengths.
At what axial length does risk increase?
Research by Tideman et al. and related myopia literature supports the clinical importance of axial length as risk rises with longer eyes, especially in high myopia. An adult axial length around or above 26.0 mm is often used as an important risk discussion point, but it should not be interpreted in isolation. Clinical assessment should include age, refraction, retinal findings, family history, and longitudinal growth.
Why does axial length matter more than a glasses prescription?
A glasses prescription corrects the optical effect of myopia — it does not address the underlying structural elongation. Axial length is the permanent record of how much the eye has grown. Complications of high myopia (maculopathy, retinal detachment, glaucoma) are caused by physical stretching of retinal and choroidal tissue, which is determined by axial length — not by what prescription the child is wearing. Tracking axial length reveals whether myopia is truly progressing and whether treatment is working, independent of optical corrections.
Can axial length decrease?
No. Axial elongation is a structural change in the size of the eyeball — it cannot be reversed by any currently available treatment. Myopia management interventions (MiSight lenses, Stellest spectacles, orthokeratology, atropine drops) slow the rate of further elongation. The goal is to limit total lifetime axial length. Very occasionally, a measurement may appear slightly lower at a follow-up visit due to measurement variability (±0.02–0.05 mm between instruments or sessions), but true anatomical reduction does not occur.
Is axial length the same in both eyes?
In most people, the two eyes have similar but not identical axial lengths. A difference of up to 0.3 mm is common and clinically unremarkable. Differences above 0.5 mm may indicate anisometropia (significantly different prescriptions between eyes) and should be monitored. In myopia management, both eyes are measured and tracked independently at each visit, since one eye can progress faster than the other and may require different treatment adjustments.
How often should axial length be measured in a child with myopia?
Most myopia management guidelines recommend axial length measurement every 6 months for children who are actively progressing. Annual measurement is appropriate for stable cases. More frequent monitoring (every 3–4 months) may be warranted for children progressing rapidly (> 0.30 mm/yr), for those who have recently started or changed treatment, or for high-risk demographics (Asian ethnicity, myopia onset before age 8, family history). Single measurements establish a baseline; the trend over time drives clinical decisions.
What is the axial length at which myopia control should start?
There is no single threshold — the decision to start myopia management is based on a combination of current axial length (percentile for age), observed growth rate, age of onset, and projected adult outcome. In general: any child with confirmed myopia, especially under age 12, should be considered for myopia management. Children above the 75th percentile for AL with a growth rate above 0.18 mm/yr are at moderate risk; above the 95th percentile or growth above 0.30 mm/yr should prompt careful clinician review and discussion of active myopia-management options. Use the progression calculator to combine these factors.
Sources
Citations
1
Tideman JWL et al. Association of Axial Length With Risk of Uncorrectable Visual Impairment for Europeans With Myopia. JAMA Ophthalmol. 2016;134(12):1355–1363. PubMed →
2
He M et al. Refractive error and visual impairment in urban children in southern China. Ophthalmology. 2004;111(6):1041–1048.
3
Sanz Diez P et al. Growth curves of myopia-related parameters to clinically monitor the refractive development in children. Ophthalmic Physiol Opt. 2019;39(1):74–84. PubMed →
4
Bennett AG. A method of determining the equivalent powers of the eye and its crystalline lens without resort to phakometry. Ophthalmic Physiol Opt. 1984;4(3):201–228.
5
Mutti DO et al. Peripheral refraction and ocular shape in children. Invest Ophthalmol Vis Sci. 1998;39(2):294–302.
6
IMI Clinical Management Guidelines.Invest Ophthalmol Vis Sci. 2019;60(3):M184–M203.
Track axial length across multiple visits
The full MyopiaTracker dashboard lets you record every biometry visit, plot growth against normative curves, and generate family-ready reports — built for clinician workflow.