The biometry was calculated for one intraocular lens, but at the moment of implantation you reach for a different model. Implant the planned power with a lens that has a different constant and the refractive outcome shifts. This free switcher converts the power so the target stays the same, in one tap, fully offline once installed on your phone or theatre tablet.
A last-minute change of lens is more common than the planning suggests. You will need a conversion when:
Every IOL model carries its own optimised constant, which encodes its effective lens position and optical design. The power your biometry recommended is tied to the constant of the lens it was calculated for. Put a different model in at the same labelled power and it lands at a different refraction, because its constant is different. To hold the target, the power has to move by the difference between the two constants.
You select the lens the biometry was calculated for together with its power, then the lens you are going to implant. The tool returns the equivalent power for the same refractive target using the standard constant-exchange method:
Ptarget = Psource + (Atarget − Asource)
It prefers the optimised SRK/T constant of each lens, shows the result as a large copyable figure, displays the ΔA line with the exact constants used, and warns you when a lens falls back to its nominal A-constant because no optimised value is available.
The rule of 9s falls short because it is stepwise, but the underlying optics are not. As the lens moves forward into the sulcus, the myopic shift grows smoothly and in proportion to the implanted power — there is nothing special about 9, 18 or 27 D. Forcing that smooth relationship into three steps means every lens within a band receives the same correction, and the error is largest at the edges of each band.
This calculator replaces the steps with the curve. It starts from the exact in-the-bag power you planned and applies the power-specific reduction read from an optimised bag-to-sulcus table — values computed across the entire power range from the real change in effective lens position (roughly 5.2 mm in the capsular bag to 4.7 mm in the sulcus). Every power gets its own correction, with no artificial jumps.
On top of that, it is lens-specific. Instead of treating all IOLs at a given power as interchangeable, it uses the optimised constant of the lens you actually select (preferring the SRK/T constant when available), so two different lenses at the same nominal power are corrected differently — as they should be. And because placement geometry changes the maths, you set the optic-capture scenario explicitly rather than assuming full sulcus position.
The result is never a black box: the corrected power is shown as a large, copyable figure, with the delta and the constants used displayed underneath so you can audit the number before you implant.
| Biometry calculated for | Planned power | Switching to | Converted power |
|---|---|---|---|
| Lens A | 21.0 D | Lens B (lower constant) | 20.90 D |
| Lens C | 20.0 D | Lens D (higher constant) | 20.50 D |
The converted power comes straight from the difference between the two constants, with nothing hidden. A small constant difference is a small power change; a large one is not, which is exactly why eyeballing it is risky.
This is a first-order conversion by constant difference, valid for the average eye. It is designed for an intraoperative model swap, not as a replacement for a full biometric recalculation. In short or long eyes, where the relationship between constant and refraction departs from one-to-one, or whenever you have the time, recalculating with the full formula using axial length and keratometry remains the gold standard. Use the switcher to make a safe decision quickly, not to skip the maths when the maths is available.
Tap the button above. Add it to your home screen for full-screen, offline use in theatre.
Select the source lens and its planned power, then the lens you are implanting.
Copy the equivalent power. The ΔA line shows the constants used, so you can audit the conversion before you implant.
Adjust the planned power by the difference between the two lenses’ constants. The switcher does this for you, preferring the optimised SRK/T constant of each model.
Only if both lenses share the same constant. If the constants differ, the same labelled power lands at a different refraction, so the power needs converting.
It is a reliable first-order method for the average eye and for intraoperative decisions. In very short or very long eyes, a full recalculation with axial length and keratometry is preferable when time allows.
The tool falls back to the nominal A-constant and tells you it has done so, so you can judge how much to trust the result.
Optimised IOL constants for most current lenses are maintained in the IOLCon database (the successor to the ULIB user group). Using an optimised SRK/T constant rather than the nominal value gives the most reliable conversion.
