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PURPOSE To compare the accuracy of the Barrett Universal II (BUII) five-variable formula to previous generation formulae in calculating intraocular lens (IOL) power following paediatric cataract extraction. METHODS Retrospective study of consecutive paediatric patients who underwent uneventful cataract extraction surgery along with in-the-bag IOL implantation between 2012 and 2018 in the Hospital for Sick Children, Toronto, Ontario, Canada. The accuracy of five different IOL formulae, including the BUII, Sanders-Retzlaff-Kraff Theoretical (SRK/T), Holladay I, Hoffer Q and Haigis, was evaluated. Constant optimization was performed for each IOL and for each formula separately. Mean prediction error (PE) and the mean and median absolute PE (APE) were calculated for the five different IOL formulae investigated. RESULTS Sixty-six eyes of 66 children (59% males) with a median age at surgery of 6.2 years (IQR, 3.2-9.2 years) were included in the study. The mean IOL power that was implanted was 23.3 ± 5.1 D (range; 12.0-39.0 D). Overall, the BUII had a comparable median APE to the Hoffer Q, Holladay I, SRK/T and Haigis formulae (BUII: 0.49D versus 0.48D, 0.61D, 0.74D and 0.58D respectively; p = 0.205). The BUII, together with Hoffer Q, produced better predictability within 0.5D from target refraction compared with the SRK/T formula (BUII:51.5%, Hoffer Q:51.5% versus SRK/T:31.8%, p = 0.002 for both). CONCLUSION The BUII formula had comparable accuracy to other tested formulae and outperformed the SRK/T formula, when calculating IOL power within the 0.5D range from target refraction in paediatric eyes undergoing cataract surgery with in-the-bag IOL implantation.