Which IOL formula is most accurate?

No single IOL formula is most accurate for every eye. Barrett Universal II and Kane perform well across broad cohorts, Hoffer Q has strong evidence in short eyes, SRK/T is widely used for normal and longer eyes, and Haigis can be useful when anterior chamber depth is important.

Which IOL formula should be used for short eyes?

For eyes under about 22 mm axial length, Hoffer Q has a long evidence base and is commonly used. Modern formulas such as Barrett Universal II and Kane are also competitive. Very short or nanophthalmic eyes require extra caution and repeat biometry.

Which formula is best after LASIK or PRK?

Post-LASIK and post-PRK eyes should not be planned with ordinary keratometry alone. Use post-refractive methods such as Barrett True-K, ASCRS post-refractive calculator pathways, Masket, Shammas, or history-based double-K methods where appropriate.

Why compare multiple IOL formulas?

Formula agreement is a quality signal. When SRK/T, Hoffer Q, Holladay 1, and Haigis agree closely, the biometry is usually internally consistent. A large spread should prompt review of axial length, keratometry, prior refractive surgery, ocular surface, and lens constants.

Clinical reference for cataract surgeons

IOL Formula
Calculator Guide.

How SRK/T, Hoffer Q, Holladay 1, Haigis, Barrett Universal II, Kane, Hill-RBF, and EVO differ, when each formula is most useful, and why comparing multiple formulas is often more informative than trusting one number.

Compare formulas in IOLDx Read SRK/T vs Barrett

Quick answer

There is no universally best IOL formula. Formula accuracy depends on axial length, keratometry, anterior chamber depth, prior corneal refractive surgery, lens constants, and measurement quality.

For ordinary eyes, modern formulas often agree closely. In short eyes, long eyes, post-LASIK eyes, or unusual keratometry, formula spread becomes a clinical warning signal.

What IOLDx calculates

SRK/T
Hoffer Q
Holladay 1
Haigis
Formula spread and risk flags
Links to official Barrett, Kane, Hill-RBF, and EVO calculators

Which IOL Formula Should You Use?

Clinical situation	Common formula strategy	Planning note
Average eye	Compare several formulas; Barrett/Kane often used as modern references	Agreement within about 0.5 D is reassuring when measurements are clean.
Short eye, AL below about 22 mm	Hoffer Q, Barrett, Kane; confirm constants	Short eyes magnify small ELP errors. Repeat biometry if formula spread is large.
Long eye, AL above about 26 mm	SRK/T, Haigis, Barrett, Kane	Watch for myopic surprise and check whether axial length adjustment is needed.
Post-myopic LASIK or PRK	Barrett True-K, ASCRS pathways, Masket/Shammas or history-based methods	Standard keratometry can overestimate corneal power and drive hyperopic surprise.
Unusual ACD or lens thickness	Haigis, Barrett, Kane	Formulas that model anterior segment dimensions may be more informative.
Large formula disagreement	Do not average blindly; investigate	Check ocular surface, K readings, AL, fixation, prior surgery, and constants.

Core IOL Formulas

Third generation

SRK/T Formula

SRK/T is a regression-theoretical formula widely used for normal and longer axial lengths. It remains useful as a transparent reference formula and often agrees closely with modern formulas in average eyes.

Best use: normal to long eyes, especially as part of a multi-formula comparison.

Short-eye reference

Hoffer Q Formula

Hoffer Q was designed to improve performance in short eyes through a personalized anterior chamber depth model. It remains one of the most recognized formulas for eyes below about 22 mm axial length.

Best use: short eyes, with careful constant selection and measurement review.

ELP estimate

Holladay 1 Formula

Holladay 1 uses axial length, corneal curvature, and a surgeon factor to estimate effective lens position. It is historically important and still useful as a transparent comparison formula.

Best use: ordinary eyes when WTW measurement quality is reasonable.

Three constants

Haigis Formula

Haigis uses a0, a1, and a2 constants to model effective lens position with measured ACD and axial length. It is especially useful when surgeons track outcomes and optimize constants.

Best use: eyes where ACD/AL relationships matter and practices with good constant optimization.

Modern reference

Barrett Universal II

Barrett Universal II is a widely used modern theoretical formula with strong published performance across broad biometric ranges. The Barrett True-K pathway is important for post-refractive eyes.

Best use: broad routine use, extreme eyes, and post-refractive planning via True-K variants.

Newer methods

Kane, Hill-RBF, EVO 2.0

Kane, Hill-RBF, and EVO use newer modeling approaches and often perform well in comparative studies. They are proprietary, so IOLDx links to the official calculators rather than recreating them.

Best use: modern cross-checks, especially when standard formula spread raises concern.

Official Formula Calculators

Several high-performing formulas are proprietary. Use the official calculators when clinical planning requires them:

Barrett Universal II and Barrett True-K: calc.apacrs.org
Kane Formula: iolformula.com
Hill-RBF: rbfcalculator.com
EVO 2.0: evoiolcalculator.com

Compare Four Open Formulas Side by Side

IOLDx Clinical computes SRK/T, Hoffer Q, Holladay 1, and Haigis simultaneously, displays formula spread, and connects the result to FDA SSED defocus data and lens counseling tools.

Open IOL Planner Open Vision Simulator

Formula Spread: The Practical Safety Check

Formula spread is the difference between predicted IOL powers across formulas. A small spread does not guarantee a perfect outcome, but it suggests the input data and assumptions are internally consistent. A large spread is not just a mathematical nuisance; it is a prompt to look for a reason.

Common causes include unstable keratometry, ocular surface disease, poor fixation, unusual axial length, prior LASIK/PRK/RK, posterior corneal astigmatism, and non-optimized constants.

Frequently Asked Questions

Is Barrett Universal II better than SRK/T?

Barrett Universal II performs very well across many published datasets, but SRK/T remains useful as a transparent reference formula, particularly in ordinary and longer eyes. The clinical value is often in comparing them rather than choosing one blindly.

Why is Hoffer Q used for short eyes?

Short eyes are highly sensitive to effective lens position error. Hoffer Q was designed with short-eye behavior in mind and remains a common reference for axial lengths below about 22 mm.

Can standard formulas be used after LASIK?

Post-LASIK and post-PRK corneas break the assumptions behind standard keratometry. Use correction-aware pathways such as Barrett True-K, ASCRS post-refractive methods, Masket, Shammas, or reliable history-based approaches.

Related IOL Planning Resources

References

Barrett GD. An improved universal theoretical formula for intraocular lens power prediction. J Cataract Refract Surg. 1993;19(6):713-720.
Hoffer KJ. The Hoffer Q formula: A comparison of theoretic and regression formulas. J Cataract Refract Surg. 1993;19(6):700-712.
Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg. 1990;16(3):333-340.
Haigis W, Lege B, Miller N, Schneider B. Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis. Graefes Arch Clin Exp Ophthalmol. 2000;238(9):765-773.
Kane JX, Van Heerden A, Atik A, Petsoglou C. Intraocular lens power formula accuracy: comparison of 7 formulas. J Cataract Refract Surg. 2016;42(10):1490-1500.
Hill WE, Abulafia A, Wang L, Koch DD. Pursuing perfection in intraocular lens calculations: III. Criteria for analyzing outcomes. J Cataract Refract Surg. 2017;43(8):999-1002.

IOL FormulaCalculator Guide.