Biometry Check Before Cataract Surgery: When to Recheck and Which Formula
Last updated: October 2026
A one-page check before you sign an IOL power. Most refractive surprises come from a measurement that should have been repeated, not from the formula.
Recheck the biometry if any of these apply
| Check | Recheck when |
|---|---|
| Axial length (AL) | Under about 22 mm or over about 25 mm, or more than 0.3 mm different from the fellow eye without anisometropia to explain it |
| Mean keratometry (K) | Under about 40 D or over about 47 D, or more than 1 D different between eyes |
| Corneal cylinder | Different axis or magnitude between the biometer and topography (repeat; treat dry eye first) |
| Calculated IOL power | More than about 3 D different between the two eyes, or not in keeping with the old spectacle prescription |
| Quality | Low signal-to-noise or standard deviation flags, a dense cataract, poor fixation |
| History | Previous LASIK, PRK or RK; previous vitrectomy or silicone oil; contact lens wear; keratoconus |
Error rules of thumb
- 1 mm of axial length error gives about 2.5–3 D of refractive error (more in short eyes); 0.1 mm gives about 0.25–0.3 D.
- 1 D of keratometry error gives about 1 D of refractive error.
- Contact (applanation) A-scan compresses the cornea, gives a falsely short AL and leaves the patient myopic. Prefer optical or immersion biometry.
Choosing the formula by axial length
| Axial length | Good choices | Notes |
|---|---|---|
| Short (under 22 mm) | Barrett Universal II, Kane, Hill-RBF, Haigis; Hoffer Q among the older formulas | Check ACD and lens thickness; consider a nanophthalmos work-up if AL is under about 20.5 mm |
| Average (22–26 mm) | Barrett Universal II, Kane, EVO, Hill-RBF; SRK/T and Holladay 1 are acceptable | Use optimised constants |
| Long (over 26 mm) | Barrett Universal II, Kane, EVO; SRK/T or Holladay 1 with the Wang-Koch AL adjustment | Older formulas tend to leave long eyes hyperopic; aim slightly myopic |
| Post-refractive surgery | Barrett True-K, total-keratometry-based formulas, and the ASCRS calculator | See post-refractive IOL calculation |
Special situations
| Situation | What to do |
|---|---|
| Silicone oil in the eye | Use the optical biometer's silicone oil mode. For ultrasound, sound velocity in oil is about 980 m/s (versus 1532 m/s in vitreous). Adjust the IOL power for the oil, typically adding about +3 to +3.5 D for a posterior-chamber IOL if the oil will stay. |
| Staphyloma or high myopia | Measure along the visual axis with optical biometry, or use B-scan-guided A-scan |
| Sulcus IOL (posterior capsule rupture) | Reduce the power, typically by about 0.5–1.0 D depending on the IOL power (see sulcus power adjustment). Use optic capture when possible. |
| Corneal astigmatism of about 1.0–1.25 D or more | Consider a toric IOL or limbal relaxing incisions, and account for posterior corneal astigmatism (use a toric calculator that includes it, such as Barrett). See toric IOL pearls and the LRI calculator. |
| Children | Aim for undercorrection (hyperopia) depending on age, because the eye will grow. See congenital cataract. |
Theatre safety check (every case)
- Right patient, right eye marked, and biometry printout for that eye.
- IOL model, power and constant match the plan; a backup IOL (for the sulcus and for a different model) is available.
- Target refraction agreed with the patient (distance, monovision, near) and written on the sheet.
- Toric axis marked with the patient sitting upright.
Biggest preventable errors: the wrong eye's biometry, the wrong IOL taken from the shelf, and not repeating a measurement that "looked odd". A two-person check of eye, model and power prevents most of them.
Related: biometry rules of thumb (detailed), IOL formulas compared, IOL calculators, A-constant reference and refractive surprise.
Key sources
- Hoffer KJ, Savini G. Update on intraocular lens power calculation study protocols. Ophthalmology 2021;128:e115–20
- Wang L, Koch DD et al. Optimizing intraocular lens power calculations in eyes with axial lengths above 25.0 mm. J Cataract Refract Surg 2011;37:2018–27
- Royal College of Ophthalmologists. Cataract Surgery Guidelines (biometry standards)
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