To estimate thresholds for Body Mass Index (BMI) and arm circumference above which a longer needle is needed to ensure intramuscular (IM) delivery of a vaccine in the deltoid muscle at the site recommended by New Zealand (NZ) immunization guidelines. A combined analysis of two studies, including 442 adults, with measurements of arm circumference, BMI and skin to deltoid muscle distance (SDMD) at the NZ immunization guideline-recommended IM injection site. Receiver Operator Characteristic curves identified arm circumference and BMI cut-points that gave 100% sensitivity for SDMD thresholds. These thresholds were: SDMD of 20mm, accounting for a minimal penetration of 5mm into muscle with the standard needle; and 25mm, which is the length of a standard needle for IM injection, representing the depth this can reach. Cut-point values for arm circumference, at which a longer needle would be required, were higher for males than females: 35cm versus 30cm for the 20mm cut-point, and 40cm versus 36.7cm for the 25mm cut-point respectively. The BMI cut-points were also higher for male than females: 24.6kg/m2 versus 23.7kg/m2 for the 20mm cut-point, and 38.2kg/m2 vs 31.6kg/m2 for the 25mm cut-point respectively. Arm circumference and BMI cut-points provide practical measures from which to choose a needle length that increases the chance of successful IM vaccination. Based on our data, an arm circumference of 35cm for men and 30cm for women should prompt selection of a longer needle to ensure intramuscular injection at the deltoid site. Thresholds for the different skin to deltoid sites proposed internationally should be determined to enable successful IM vaccination in clinical practice beyond NZ.