MR biometry · IOL exchange

Koch MR IOLX Formula calculator

Picks the replacement IOL power for a lens exchange by anchoring the calculation to the prediction error the original lens actually produced, rather than to an assumed lens position.

Method after Cheng X, Mendes F, Fortingo K, Jaber JM, Chen AJ, Weikert MP, Wang L, Koch DD. Ophthalmology 2026;133:700–708.

Independent implementation — verify before clinical use. This tool reproduces the published relationship R = (a − b) + c using a Holladay 1 vergence engine written from the original 1988 description. It is not the authors' Excel tool and has not been validated against it. Check every case against your own calculation and biometry, and treat the output as a second opinion, not a decision.
Starts empty so no one else's numbers are left in a field. The example is a typical bag-to-bag exchange.

Pre-exchange biometry

Use the same biometry the original surgery was planned on where possible. Keratometry uses the 1.3375 index. MR biometry ignores the bottom three — they are here only because Barrett Rx asks for them.

Original IOL & current refraction

Manifest refraction through the original IOL, at distance. The paper converted all refractions to their 6 m equivalent.

Replacement lens

Engine settings

Recommendation

Implant this power
—
Predicted refraction
—
Exact power to hit target
—
a — measured SE
— D
b — predicted SE, original IOL
— D
a − b — prediction error
— D

Predicted outcome across the power range

New IOL power c — Holladay 1 prediction R — predicted refraction Miss vs target

Side by side

Koch MR IOLX

Computed here from your inputs.

Power—
Predicted refraction—

Barrett Rx

Proprietary — the algorithm is unpublished, so it cannot be reproduced offline. Run it on the ASCRS calculator and enter the result to compare.

Open Barrett Rx calculator

Read down this list as you fill their form — it follows the same order. Nothing transfers automatically.

Enter a Barrett Rx result to see the difference.