IOL Power Calculator — Intraocular Lens Power with SRK/T and SRK II
Calculate intraocular lens (IOL) power for cataract surgery from axial length, keratometry and the lens A-constant with the SRK/T and SRK II formulas — for any target refraction, with predicted refraction per lens.
How it works
SRK/T (Retzlaff 1990): r = 337.5/K; corrected AL Lc = L (≤ 24.2 mm) or −3.446 + 1.716L − 0.0237L²; corneal width Cw = −5.41 + 0.58412Lc + 0.098K; corneal height H = r − √(r² − Cw²/4); ACD = H + (0.62467A − 68.747) − 3.336; optical AL Lo = L + 0.65696 − 0.02029L; IOL = 1000·na·(na·r − nc·Lo − 0.001·R·(V(na·r − nc·Lo) + Lo·r)) / ((Lo − ACD)(na·r − nc·ACD − 0.001·R·(V(na·r − nc·ACD) + ACD·r))) with na = 1.336, nc = 0.333, V = 12 mm, R = target refraction. SRK II: P = A1 − 0.9K − 2.5L, A1 adjusted by axial length (+3, +2, +1, 0, −0.5), minus R × 1.25 (or × 1.0 when P ≤ 14 D).
SRK/T is a third-generation theoretical formula that combines a regression-based estimate of the post-operative lens position with vergence optics; it performs especially well in long eyes. SRK II is an older regression formula shown for comparison. Modern practice uses optical biometry, optimised A-constants and newer formulas (Barrett Universal II, Kane, Hill-RBF, EVO), and short eyes are better served by Hoffer Q or Haigis — so treat this as an educational check, not a surgical plan.
Worked example
Axial length 23.50 mm, K1 43.50 D and K2 44.50 D (average K 44.00 D) with an A-constant of 118.4 and an emmetropic target give an SRK/T power of +20.04 D. The nearest lens is +20.0 D, predicted to leave about +0.03 D. SRK II gives 118.4 − 0.9 × 44 − 2.5 × 23.5 = +20.05 D.
Sources: StatPearls — Intraocular Lens Power Calculation · EyeCalc — SRK/T formula
Frequently asked questions
Which IOL formula is the most accurate?
In large comparisons, newer formulas such as Barrett Universal II, Kane and EVO give the most patients within ±0.50 D. SRK/T remains good in average and long eyes, while Hoffer Q or Haigis are traditionally preferred for short eyes.
What is the A-constant?
A lens-specific number from the manufacturer that encodes where the lens sits in the eye. Surgeons personalise (optimise) it from their own outcomes; ULIB and IOLCon publish optimised values.
Why aim for a slightly myopic target?
A small myopic target (around −0.25 to −0.50 D) protects against an unexpected hyperopic surprise and gives some near vision. Monovision deliberately aims one eye at about −1.25 to −1.75 D.
How much does 1 mm of axial length change the IOL power?
Roughly 2.5–3 D in an average eye — which is why accurate optical biometry matters more than the formula choice.
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For trained clinicians only. Always confirm doses and measurements against the product label, local protocol and the full clinical picture — this calculator does not replace professional judgement.