The formula used for the above calculator is:

If >1 year of age, estimated bladder capacity (ml) = (Age + 2)*30 (Koff, S 1983)
If <1 year of age, estimated bladder capacity (ml) = Weight (kg)*7 (Fairhurst et al. 1991)

This may be useful for cystography and micturating cystourethrography, but the is an estimation of the bladder capacity only and cannot be used alone and nor does it indicate a safe volume to administer. Individual assessment should be made to verify the appropriateness of any values obtained as many factors affect bladder capacity.

If using VURx scoring, regarding the timing of reflux, the cut off between the two categories, early-mid filling, and late filling, is 75% of the predicted bladder capacity (Kirsch et al. 2014). However, early filling reflux at <35% predicted capacity is associated with increased breaktrhough febrile UTI (Alexander et al.)

References

1. Koff, S. A. Estimating bladder capacity in children. Urology 21, 248 (1983).
2. Fairhurst, J. J., Rubin, C. M., Hyde, I., Freeman, N. V. & Williams, J. D. Bladder capacity in infants. J. Pediatr. Surg. 26, 55–57 (1991).
3. Kirsch AJ, Arlen AM, Leong T, Merriman LS, Herrel LA, Scherz HC, Smith EA, Srinivasan AK. Vesicoureteral reflux index (VURx): a novel tool to predict primary reflux improvement and resolution in children less than 2 years of age. J Pediatr Urol. 2014 Dec;10(6):1249-54. doi: 10.1016/j.jpurol.2014.06.019. Epub 2014 Jul 24. PMID: 25511573.
4. Alexander SE, Arlen AM, Storm DW, Kieran K, Cooper CS. Bladder volume at onset of vesicoureteral reflux is an independent risk factor for breakthrough febrile urinary tract infection. J Urol. 2015 Apr;193(4):1342-6. doi: 10.1016/j.juro.2014.10.002. Epub 2014 Oct 8. PMID: 25305355.