Biometry Rules of Thumb: Axial Length, Keratometry and Red Flags
Last updated: October 2026
Short answer: small biometry errors cause big refractive surprises. A 1 mm error in axial length causes roughly 2.5–3 D of refractive error (more in short eyes), and a 1 D error in keratometry causes roughly 1 D of error. Check every result against the fellow eye and against expected ranges before choosing the IOL.
Error rules of thumb
| Measurement error | Approximate refractive effect |
|---|---|
| Axial length 1 mm | About 2.5–3 D (larger in short eyes) |
| Axial length 0.1 mm | About 0.25–0.3 D |
| Keratometry 1 D | About 1 D |
| Wrong eye's data or wrong IOL | Unpredictable and potentially large; prevent it with a theatre check |
Red flags: recheck the biometry if
- Axial length differs between the two eyes by more than about 0.3 mm without an explanation (such as anisometropia).
- Average keratometry differs between eyes by more than about 1 D.
- Average K is very flat or very steep (for example, below about 40 D or above about 47 D).
- Axial length is unusually short or long (for example, below about 22 mm or above about 25 mm).
- The calculated IOL power doesn't fit the patient's refractive history.
- The signal-to-noise ratio or quality indicators on the biometer are poor.
Optical vs ultrasound biometry
- Optical biometry is more accurate and repeatable and is the standard where available. Dense or posterior subcapsular cataracts can block the measurement.
- Contact A-scan compresses the cornea and tends to give a falsely short axial length (a myopic surprise). Immersion A-scan avoids compression and is preferred when ultrasound is needed.
- Silicone oil-filled eyes: sound travels much more slowly in silicone oil (around 980 m/s), so ultrasound readings must be corrected. Optical biometers have a silicone oil mode.
- Staphyloma in high myopia: measure along the visual axis (optical biometry, or B-scan-guided A-scan).
Keratometry pitfalls
- Dry eye and MGD cause inconsistent K readings. Treat the surface first and repeat.
- Contact lens wear changes corneal shape; ask the patient to stop lenses before measurement.
- After corneal refractive surgery, standard K readings are wrong. See post-refractive IOL calculation.
Choosing the formula
- Use modern formulas (for example, Barrett Universal II, Kane, Hill-RBF or similar) rather than older regression formulas, especially in short or long eyes.
- See our IOL formula comparison and IOL calculators.
- Optimise lens constants for your own results whenever you can.
Frequently asked questions
How much refractive error does a 1 mm axial length error cause?
About 2.5–3 D in an average eye, and more in short eyes. Even 0.1 mm causes about a quarter of a dioptre.
When should biometry be repeated?
When there is a large difference between the two eyes, unusual axial length or keratometry, poor quality readings, or a result that doesn't fit the patient's refractive history.
Why does contact A-scan give a myopic surprise?
The probe compresses the cornea, shortening the measured axial length. A shorter axial length leads to a stronger IOL, which leaves the patient myopic.
Related: refractive surprise after cataract surgery and sulcus IOL power adjustment.
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