How Much Axial Length Growth is Normal?
If your child has recently been diagnosed with myopia — or you've been told they're at risk — you're probably facing a flood of unfamiliar terms: axial length, biometry, dioptre targets, growth curves. It can feel overwhelming. This guide explains, in plain language, what these measurements mean, how we use them at For Eyes Optometrists to track your child's eye health, and what success looks like in modern myopia management.
The most important thing to understand from the outset: not all myopia is equal, and active management — started early, before the eye grows too long — is the most powerful thing you can do to protect your child's vision for life.
Key Takeaways
Axial length — the physical length of the eyeball — is the most reliable measure of myopia progression and treatment success in children.
Normal axial elongation is approximately 0.10–0.15 mm per year in school-age children; the refractive equivalent is ≤−0.25 D per year.
Children progressing faster than this are at elevated lifetime risk of glaucoma, retinal detachment, macular degeneration, and cataract.
Children of East Asian descent progress approximately 30% faster than Caucasian children — ethnicity matters when interpreting results.
Annualised axial elongation curves (Graff et al. 2023) are the most clinically informative way to track treatment response when biometry is available.
Success in myopia management is defined across three timeframes: short-term (slow progression), medium-term (cease at the right time), and long-term (reduce lifetime disease risk).
Treatment discontinuation is considered when a child is ≥15–16 years old and has had at least 12 months of minimal progression.
Defining Success in Axial Length and Myopia Management
Success in myopia management has a short-, medium- and long-term dimension (Bullimore & Brennan, 2023). Thinking across all three helps parents understand why we monitor so carefully and why treatment is maintained for many years.
| Timeframe | Goal | What this means in practice |
|---|---|---|
| Short-term | Initiate and minimise progression | Start evidence-based myopia control promptly. Slow axial elongation to physiological rates if possible. Monitor and adjust treatment if targets are not met. |
| Medium-term | Minimise growth and cease treatment appropriately | Continue monitoring through adolescence. Reduce intensity as myopia stabilises. Cease when clinical criteria for stability are met, not simply based on age. |
| Long-term | Reduce lifetime axial length and disease risk | Every millimetre of axial elongation prevented is a permanent reduction in lifetime risk. Goal: reach adulthood with the lowest possible total axial length. |
| THE CARE CONCEPT | ||
| Maximum eye growth reduction with current treatments is approximately 0.4 mm total axial elongation, equivalent to around 1 dioptre of myopia prevented (Brennan et al., 2021, Cumulative Absolute Reduction in Elongation, CARE). While modest, the associated disease risk reductions are clinically very meaningful. | ||
How Axial Length Measurement Works
What axial length means
Axial length is the physical distance from the front surface of the cornea to the retina, measured in millimetres. In a normal adult eye, this is approximately 23.0–24.5 mm. A child's eye at birth is around 16–17 mm, growing through childhood and reaching adult dimensions in the late teens.
In a myopic eye, the axial length is longer than expected for the child's age. As a rough guide, a 1.00 D change in refractive error corresponds to approximately 0.28–0.40 mm of axial length change — but this relationship is not linear, varies with age, and strengthens as children get older (Review of Myopia Management, 2019).
Why axial length outperforms refraction alone
Refractive error (measured in dioptres) can vary by 0.25–0.50 D between visits due to accommodation, time of day, and technique. Modern optical biometers measure axial length to within 0.01–0.02 mm — roughly 5–10 times more precise than refraction. Axial length is also the only reliable measure of myopia control efficacy in orthokeratology patients, where the cornea is intentionally reshaped.
| THE DIOPTRE–AXIAL LENGTH RELATIONSHIP |
| A common simplification is "1 mm = 3.00 D" but the real relationship is more nuanced. In the MiSight 3-year study, 0.1 mm of axial length change corresponded to 0.24 D of myopia. For younger children, 1 dioptre corresponds to approximately 0.28 mm of axial growth; for older children and adults, this rises to 0.35–0.40 mm per dioptre. Always use both measures together. |
How Much Axial Growth is Normal?
The clinical targets
| Measure | Target | Notes |
|---|---|---|
| Annual refractive change | ≤ −0.25 D/year | Equivalent to emmetropic-like progression |
| Annual axial elongation | 0.10–0.15 mm/year | Physiological growth rate for school-age children |
| Natural age-related slowing | ~15% per year | Growth rate naturally declines as children age |
| East Asian vs Caucasian | ~30% faster (East Asian) | Ethnicity must be factored into target-setting |
These thresholds — endorsed by the International Myopia Institute — represent "emmetropic-like" growth. When a child on myopia management achieves this rate, treatment is working as intended. Growth above these thresholds warrants review and likely treatment intensification.
Population-based axial length percentile curves
Just as a paediatrician plots height on a centile chart, your optometrist can plot your child's axial length against normative data for their age, sex, and ethnicity. Charts showing percentiles from the 2nd to 98th are available — the higher the percentile, the longer the eye relative to peers, and the greater the myopia risk.
| AN IMPORTANT LIMITATION OF PERCENTILE CURVES |
| Standard population-based AL percentile charts do not account for refractive error and can underestimate normal growth in children already diagnosed with myopia. A myopic child may appear to track an acceptable percentile while still growing faster than is physiologically ideal. |
| The aim during myopia control is for a child to shift towards lower percentiles over time. Shifting to higher percentiles signals that treatment needs intensification (Bullimore et al., 2025). |
Five Approaches to Monitoring Progression During Treatment
Research by Bullimore & Brennan (2023) and Bullimore et al. (2025) identifies five evidence-based approaches to assessing myopia control targets.
| 1 |
Target emmetropic-like changes Aim for ≤−0.25 D/year refraction and 0.10–0.15 mm/year axial elongation. Best approach when biometry is not available. |
| 2 |
Target below-average progression for age and ethnicity Compare to normative myopic progression rates for the child's age and ethnic background. |
| 3 |
Target half-average progression for age and ethnicity Aim to achieve at least 50% reduction in expected annual progression. |
| 4 |
Guidance from AL percentile curves Plot absolute axial length against age-specific population norms (sex- and ethnicity-specific). Available via biometry software or published normative datasets. |
| 5 |
Guidance from Annualised Axial Elongation (AAE) curves ★ Preferred approach with biometry Compares the rate of annual axial growth to age-adjusted normal ranges, the most clinically informative approach when biometry is available (Graff et al., 2023). At For Eyes, we use this approach to guide treatment decisions. |
The Annualised Growth Zone Chart (Graff et al. 2023)
Because the expected rate of growth naturally declines with age (by approximately 15% per year), the same absolute elongation rate can be acceptable or excessive depending on the child's age. The Graff framework provides age-adjusted zone boundaries:
| Zone | Growth Rate | Clinical action |
|---|---|---|
| Green — physiological | Below age-adjusted threshold (approximately ≤0.15 mm/yr at age 8; ≤0.10 mm/yr by age 16) | Continue current treatment. Monitor 3–6 monthly. |
| Yellow — moderately excessive | Moderately above threshold (approximately 0.15–0.25 mm/yr, age-adjusted) | Consider increasing treatment efficacy at next review. |
| Red — excessive | Well above threshold (approximately >0.25 mm/yr, age-adjusted) | Increase treatment efficacy. Review within 1–3 months. |
Note: Zone boundaries are age-, sex-, and ethnicity-dependent and are obtained from validated clinical tools such as Myopia Solutions or Ocumetra mEYE Suite, or directly from published data. The chart in the accompanying HTML version illustrates these zones graphically.
When Does Axial Length Become a Risk?
| Adult axial length | Approx. myopia | Relative lifetime disease risk |
|---|---|---|
| 22.5–24.5 mm | Emmetropia to −1.00 D | Baseline — low risk |
| 24.5–26.0 mm | −1.00 D to −3.00 D | Modestly elevated |
| 26.0–27.5 mm | −3.00 D to −6.00 D | Significantly elevated — annual dilated retinal exam advised |
| >27.5 mm | >−6.00 D (high myopia) | Substantially elevated — lifelong specialist surveillance essential |
| >30.0 mm | Very high myopia | >90% cumulative risk of vision impairment by age 75 |
When an axial length exceeds 26 mm, we recommend annual dilated fundus examination regardless of prescription, due to the substantially increased risk of retinal pathology.
Monitoring Axial Length in Clinic
The measurement process
At For Eyes Optometrists, we use a modern optical biometer — an instrument that uses completely non-contact light-based measurement (optical low-coherence interferometry or swept-source OCT biometry).
Your child simply looks at a fixation light for a few seconds while the instrument takes multiple automatic readings. No drops, no touching of the eye, no discomfort. Measurement is precise to within 0.01 mm and takes under two minutes.
The instrument also measures corneal curvature, anterior chamber depth, and other parameters providing a comprehensive picture of eye development.
How often do we measure?
For children actively under myopia management, we measure axial length every 3–6 months. Children progressing rapidly, or who have recently started a new treatment, are typically reviewed every 3 months. We plot every measurement on normative curves and AAE charts so you can see at each visit how your child's trajectory is changing.
| WHAT GOOD CONTROL LOOKS LIKE |
| Axial elongation ≤ 0.10–0.15 mm/year — physiological growth rate achieved. |
| Refractive change ≤ −0.25 D/year — prescription stable or near-stable. |
| AL percentile stable or improving — child is not moving up the normative curves. |
| AAE rate in the green zone — age-adjusted elongation within normal limits. |
| Important: For some children — particularly those with very early onset myopia or strong genetic risk — these targets may not be fully achievable even with optimal treatment. Even partial success is clinically meaningful. |
Myopia Stabilisation and Treatment Cessation
Myopia naturally slows with age — by approximately 15% per year — but the age at which it truly stabilises varies considerably. Data from the COMET Group (2013), analysed by Hughes & Woodman-Pieterse (2024), shows: approximately 50% of myopes have stabilised by age 15; 75% by age 18; 90% by age 21; and 96% by age 24.
| Age | Approx. % stable | Approx. % still progressing |
|---|---|---|
| 15 years | ~50% | ~50% |
| 18 years | ~75% | ~25% |
| 21 years | ~90% | ~10% |
| 24 years | ~96% | ~4% |
Data from The COMET Group, 2013; Hughes & Woodman-Pieterse, 2024.
| WHEN WE CONSIDER CEASING TREATMENT |
| At For Eyes, we consider discontinuing pharmacological myopia control when both of the following criteria are met (Hughes, WAVE 2026; Bullimore & Brennan, 2023): |
| Age ≥ 15–16 years AND at least 12 months of minimal or stable progression. |
| Optical treatments (myopia control spectacle lenses or contact lenses) may be continued even after pharmacological cessation. Close monitoring (3–6 monthly) continues after ceasing treatment, and recommencement is always available if progression resumes. Children pursuing higher education or maintaining a myopigenic lifestyle may benefit from more conservative and prolonged treatment. |
Book a Myopia Assessment at For Eyes Optometrist Fremantle
If your child has been diagnosed with myopia, has a strong family history of short-sightedness, or you have any concerns about their vision, we invite you to book a comprehensive myopia assessment. We offer full biometry (axial length measurement), a personalised myopia risk assessment, and evidence-based management plans tailored to your child at our Fremantle, Perth-based Myopia Clinic.
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Our Clinical Approach at For Eyes Optometrists
Managing childhood myopia is one of the most impactful things we do to protect long-term vision. Our evidence-based approach:
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Frequently Asked Questions
What is a normal axial length for a child?
Axial length varies by age, sex, and ethnicity. As a general guide: at age 6, average axial length is approximately 22.5–23.0 mm, growing to around 23.5–24.5 mm by the early teens, with most growth completing by the late teens. What matters most clinically is the rate of change — which is why we compare measurements against age-specific normative curves and plot annualised growth rate on AAE zone charts.
What is axial length in myopia, and why does it matter?
In myopia, the eyeball has grown longer than normal. This means the eye's optical power focuses light in front of the retina rather than on it, causing blurred distance vision. Axial length is both the structural cause of myopia and the most accurate way to track its progression. Because it is objective, stable, and measurable to 0.01 mm precision, it is more informative than refraction alone.
Can myopia control treatments reduce axial length?
No current treatment reliably reduces axial length — once the eye has grown, that structural change is permanent. What myopia management treatments do is slow the rate of future growth. Some studies have observed very small apparent reductions in axial length with orthokeratology, but this is thought to reflect redistribution of fluid within the eye rather than true structural shortening. The clinical goal is always forward-looking: minimise the total axial length reached by adulthood.
Is axial length measurement painful or uncomfortable?
Not at all. Modern biometry uses a completely non-contact optical method — your child looks at a small fixation light for a few seconds while automatic measurements are taken. No air puff, no contact with the eye, no drops. Children of all ages find the test quick and comfortable.
When does axial length stop growing?
In most children, axial length growth slows significantly in the mid-to-late teenage years and becomes negligible by age 18–20. About 10% of myopes are still progressing at age 21, and approximately 4% at age 24. This is why we monitor for stability over two or more consecutive assessments before considering treatment cessation.
How does minimising axial length growth reduce lifetime disease risk?
Every 1 dioptre of myopia saved reduces the risk of myopic maculopathy by approximately 40% (Bullimore & Brennan, 2019). Even the modest gains achievable with current treatments (~0.4 mm total axial elongation reduction, ~1 dioptre) translate into meaningfully lower lifetime disease risk. Keeping axial length as short as possible throughout childhood permanently reduces the cumulative structural burden on the retina and optic nerve.
References
Hughes R. (2026). Defining success in myopia management. WAVE 2026. © Rohan Hughes. All Rights Reserved.
Graff et al. (2023). Annualised axial elongation curves in myopia control.
Bullimore MA, Brennan NA. (2023). Myopia control: why each dioptre matters. Optom Vis Sci.
Bullimore MA, Brennan NA. (2019). Myopia maculopathy risk per dioptre. Ophthalmology.
Bullimore MA et al. (2025). Percentile curves and axial length in myopia management.
Flitcroft DI et al. / International Myopia Institute. (2025). IMI definitions update.
Brennan NA et al. (2021). Cumulative absolute reduction in elongation (CARE). Ophthal Physiol Opt.
Hughes R, Woodman-Pieterse E. (2024). Myopia stabilisation and cessation.
The COMET Group. (2013). Age of myopia stabilisation. Invest Ophthalmol Vis Sci.
Myopia Profile — Haines C. (2020). Six questions on axial length measurement in myopia management.
Review of Myopia Management — Vera-Diaz FA. (2019). The importance of measuring axial length.
BMJ Open Ophthalmology. (2026). Consensus on evidence gaps in childhood myopia (European Delphi study).
Disclaimer: This document is for educational purposes only and does not constitute medical advice. All myopia management decisions should be made in consultation with a qualified optometrist with access to your child's full clinical history. Clinical data and zone boundaries are based on published research current as of 2025–2026.
