IOL Power Calculator
Calculate intraocular lens (IOL) power for cataract surgery using the classic SRK regression formula based on axial length and keratometry.
About this calculator
Intraocular lens (IOL) power selection for cataract surgery has to account for the lens the surgeon is implanting, the eye's axial length, and the cornea's curvature -- get any of those wrong and the patient ends up needing glasses for a refractive error that surgery could have avoided. This calculator implements the classic SRK (Sanders-Retzlaff-Kraff) regression formula, published by ophthalmologists Donald Sanders and Manus Kraff in the Journal of the American Intraocular Implant Society in 1980: Recommended IOL Power equals the lens's manufacturer A-Constant minus 2.5 times Axial Length minus 0.9 times Average Keratometry (K). SRK was derived empirically from a large case series rather than from eye-model optics, which made it fast to calculate by hand in an era before routine computer support -- but that same simplicity is also its known limitation: SRK becomes measurably less accurate at unusually short or unusually long eyes, which is why later formulas were developed, including the theoretical SRK/T formula from the same research group in 1990 (a substantially more complex calculation involving estimated postoperative lens position and a retinal thickness correction, not a variant of this linear equation).
Target Refraction lets you dial in a deliberate postoperative outcome other than full correction (0 D, or emmetropia) -- some patients and surgeons intentionally target slight residual myopia, for example -- and Recommended IOL Power is then rounded to the nearest 0.5 D step, the increment IOLs are actually manufactured in, with Calculated IOL (unrounded) shown alongside so you can see how much that rounding shifted the number and Expected Refraction showing the small refractive consequence of that rounding. This tool is for education and planning discussion only -- real IOL selection depends on the specific formula a surgical practice validates for its biometry equipment and patient population, and should always be performed by the operating surgeon's own calculation workflow, not a general-purpose calculator.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
Recommended IOL Power
20.5 D
Expected Refraction
0 D
Figures current as of 1980. Source: Sanders DR, Kraff MC. Improvement of intraocular lens power calculation using empirical data. J Am Intraocul Implant Soc. 1980;6(3):263-267.
How to Use This Calculator
- Enter the Axial Length in mm as measured by A-scan biometry or IOLMaster (normal range 22–25 mm).
- Enter the Average Keratometry (K) in diopters — use the average of K1 and K2 corneal curvature readings.
- Set your Target Refraction in diopters (0 = emmetropia; negative values for slight myopia post-op).
- Enter the A-Constant for your specific IOL model (provided by the manufacturer, typically 118–119).
- Review the Recommended IOL Power (rounded to nearest 0.5 D) and Expected Refraction with that implant.
How the result changes with A-Constant
| A-Constant | Recommended IOL Power | Expected Refraction |
|---|---|---|
| 103 | 5 D | 0.08 D |
| 109 | 11 D | 0.08 D |
| 116 | 18 D | 0.08 D |
| 123 | 25 D | 0.08 D |
What each input means
- Axial Length
- Axial length of the eye in mm (measured by A-scan or IOLMaster). Normal range: 22–25 mm.
- Average Keratometry (K)
- Average corneal curvature in diopters (average of K1 and K2 readings).
- Target Refraction
- Desired postoperative refraction in diopters (0 = emmetropia, negative = slight myopia).
- A-Constant
- Manufacturer's A-constant for the IOL being used. Varies by lens model (typically 118–119).
How this is calculated
Formula
IOL = A - 2.5 × AL - 0.9 × KWorked example, using the default values
- Identify Input Parameters4 parametersAxial Length = 23.5, Average Keratometry (K) = 43.5, Target Refraction = 0, A-Constant = 118.4 = 4 input(s) provided
- Calculate Recommended IOL Power20.5 = 20.5
- Calculate Expected RefractionExpected Refraction0 = 0
- Calculate Calculated IOL20.5 = 20.5
Figures and sources
- SRK (Sanders-Retzlaff-Kraff) intraocular lens power regression formula (1980) — Sanders DR, Kraff MC. Improvement of intraocular lens power calculation using empirical data. J Am Intraocul Implant Soc. 1980;6(3):263-267.
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Is this the SRK/T formula?
No -- despite the similar name, this calculator uses the original 1980 SRK (Sanders-Retzlaff-Kraff) linear regression formula, not SRK/T. SRK/T is a later (1990), substantially more complex theoretical formula from the same research group that adds terms for estimated postoperative lens position and a retinal thickness correction; it cannot be written as a simple two-term equation the way SRK can, so the two should not be used interchangeably.
Why does Axial Length pull IOL power down so strongly?
Axial Length has the largest coefficient in the SRK formula (2.5 per millimeter), reflecting how sensitive the eye's optical power is to its physical length -- a longer eye (more myopic) needs meaningfully less plus power from the implanted lens to focus light on the retina, and a shorter eye needs more. Because axial length carries the largest weight of the formula's three inputs, small measurement errors in it tend to matter more for the final result than similarly-sized errors in keratometry.
Why is Recommended IOL Power rounded to the nearest 0.5 D?
Intraocular lenses are manufactured in fixed power steps, typically 0.5 diopter increments, so a surgeon can't order an IOL at an arbitrary calculated power like 21.37 D -- the nearest available lens has to be selected. This calculator rounds the raw calculated result to that same 0.5 D step and reports both the rounded Recommended IOL Power and the unrounded Calculated IOL so the size of that rounding adjustment is visible.
Does the A-Constant matter as much as Axial Length or Keratometry?
Yes -- the A-Constant enters the formula as a straight addition with a coefficient of 1, so every diopter of difference in the A-Constant shifts Recommended IOL Power by exactly one diopter. Because A-Constants vary by lens manufacturer and model (commonly in the 118-119 range, but always specific to the implant being used), using the wrong lens's A-Constant is one of the more common sources of calculation error in practice.
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