Modified Armitage formula is used to select volume and concentration of local anaesthetic for a paediatric caudal epidural block, according to the desired dermatomal spread:

For example, for a 10 kg child having inguinal surgery:
1 mL/kg × 10 kg = 10 mL of 0.25% bupivacaine.

The modification lowers concentration when a larger volume is required, helping limit the total bupivacaine dose while achieving cephalad spread. Published pediatric caudal-block descriptions also identify the original Armitage approach as volume-based according to surgical level, commonly using 0.5 mL/kg for perineal surgery and 1 mL/kg for inguinal procedures (review of paediatric caudal blocks).

Use only within the local anesthetic maximum-dose limits and local pediatric anesthesia protocol, with dose calculation in mg/kg checked before injection.