Tools / Axial length
Axial Length Estimation
Estimate axial length for each eye using keratometry and refractive error inputs. Built for quick chairside use when optical biometry is unavailable.
Last updated: March 2026
Axial length tool
Axial Length Estimation
Estimate axial length for right and left eyes based on keratometry and refraction values.
OD
Estimated Axial Length
24.62 mm
OS
Estimated Axial Length
24.62 mm
Estimated accuracy ≈ ±0.9 mm vs. optical biometry. Consult your eye care professional for accurate measurement.
How the Axial Length Estimation tool works
This tool provides a quick chairside estimate of ocular axial length (AL) in millimetres (mm) when optical biometry is unavailable. It uses two standard clinical measurements already recorded in routine exams: keratometry and refractive error (spherical equivalent).
It is most useful for initial myopia risk assessment, approximate monitoring of axial elongation, patient education, and screening prior to referral for full biometry. Estimated accuracy is about ±0.9 mm versus optical biometry, which is suitable for risk categorisation but not for precise IOL calculations or research-grade progression tracking.
Inputs (OD and OS)
- Mean keratometry (K) in diopters (D).
- Spherical equivalent refraction (SE) in diopters (D).
- Optional internal adjustment: spectacle-to-corneal-plane vertex correction.
Output
Estimated axial length for each eye (e.g. 24.62 mm), calculated independently for OD and OS.
Core mathematical model (Morgan regression)
AL (mm) = 1 / [(0.22273 / r) + (0.00070 × S) + 0.01368]
- r = mean corneal radius in mm (r = 337.5 / mean K)
- S = corneal-plane spherical equivalent (D)
This model was derived from large datasets of children and young adults and achieves R² ≈ 0.83 in validation studies. Simpler linear approximations exist, but this K-inclusive model is more accurate for chairside estimation.
Step-by-step calculation
- Convert mean K to radius r (mm) using 337.5 / K.
- Calculate SE and apply vertex correction if needed.
- Insert r and S into the Morgan regression formula.
- Display AL to two decimal places and repeat for the fellow eye.
Clinical interpretation & risk categories
Example calculation (typical myopic child)
- Mean K = 43.50 D → r ≈ 7.76 mm
- Spectacle SE = –3.00 D → corneal-plane S ≈ –2.90 D
- AL ≈ 24.8 mm (moderate myopia, monitor closely)
Important clinical notes & limitations
- This is an estimation only. Accuracy is about ±0.9 mm; small changes (less than 0.2 mm) cannot be tracked reliably.
- Best validated in children and myopic populations aged 6–25 years; less accurate in older adults, hyperopes, post-LASIK, or keratoconus eyes.
- Good input data matters: cycloplegic refraction in young patients, averaged K readings, and accurate vertex distance.
- Does not replace optical biometry. Use for screening and education only.
- Not included: lens thickness, anterior chamber depth, posterior corneal power, and ethnicity-specific adjustments.
- Myopia management context: axial length is the gold-standard biomarker; even a rough estimate helps families understand progression beyond refraction alone.