SRK II, SRK/T, Hoffer Q & Holladay 1 Compared
A quick, honest comparison of the four "classic" IOL power formulas most residents learn first — with a working calculator for the one formula simple enough to compute safely by hand, and clear explanations of why the other three needed more sophisticated machinery.
⚠️ Educational use only. This tool is for teaching purposes — to show how SRK II's inputs affect its output. Never use it for actual surgical planning. For real patients, always use a validated calculator (ASCRS, your biometer, or the formula author's own tool) — see the links below.
Try it: SRK II Calculator
SRK II is a second-generation regression formula — straightforward enough that its exact published equation can be safely reproduced here.
Why SRK II wasn't the end of the story
SRK II works reasonably well for eyes with average axial length (22.00–24.50mm), and its correction factors provide a rough adjustment for shorter or longer eyes. But it's a purely empirical regression formula — it doesn't model the actual optics of the eye, which is precisely why it starts to break down at the axial length extremes despite the correction bands. This is exactly what the three formulas below were built to fix.
SRK/T — theoretical optics + regression optimization
Developed by Retzlaff, Sanders, and Kraff in 1990, SRK/T ("T" for theoretical) models the eye as a two-lens optical system (cornea + IOL) and calculates the predicted post-operative lens position from corneal curvature and a corrected axial length, rather than relying purely on regression. It remains particularly well-suited to long eyes (typically >26mm), where SRK II's correction factor is a comparatively crude fix. Its calculation involves several precise intermediate steps (corrected axial length, computed corneal width, estimated corneal height) — genuinely worth understanding conceptually, but not something to reproduce by hand.
Hoffer Q — built for short eyes
Published by Kenneth Hoffer in 1993, the Q formula predicts effective lens position using a personalized ACD (pACD) constant combined with axial length and the tangent of corneal power. It's specifically recommended for short eyes (typically <22mm), where Hoffer's own comparative data showed it outperforming both SRK/T and Holladay 1. Like SRK/T, its full calculation includes several conditional terms and limiter values that make hand calculation impractical.
Holladay 1 — the "Surgeon Factor" approach
Developed by Jack Holladay, this formula uses a personalized Surgeon Factor (SF) — the surgeon-specific distance from the iris plane to the IOL plane — combined with axial length and corneal curvature to estimate effective lens position. It performs comparably to Hoffer Q across most of the normal axial length range, with some data suggesting an edge for Holladay 1 in eyes between 23.50–25.99mm.
Where to actually calculate SRK/T, Hoffer Q, and Holladay 1
🔧 The ASCRS IOL Calculator and modern optical biometers (IOLMaster, Lenstar) compute all three formulas accurately, alongside newer options like Barrett Universal II — see our IOL Calculators page for the fuller modern formula landscape, since Barrett Universal II, Kane, and Hill-RBF have generally surpassed all four "classic" formulas discussed here in comparative accuracy studies.
Key references: Retzlaff JA, Sanders DR, Kraff MC. Development of the SRK/T formula. J Cataract Refract Surg. 1990. | Hoffer KJ. The Hoffer Q formula. J Cataract Refract Surg. 1993. | Sanders DR, Retzlaff J, Kraff MC. Comparison of the SRK II formula and other second-generation formulas. J Cataract Refract Surg. 1988.
Want to know more about treatment? Read about Corneal Topography at Sadbhaav or book a consultation with Dr. Dhaval Patel.