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Background: The use of inappropriately long needles for subcutaneous insulin injection increases the risk of inadvertent intramuscular injection. Aims: To estimate the inadvertent intramuscular injection risk at potential subcutaneous insulin injection sites and to identify the determinants of this risk in adults. Methods: A descriptive cross-sectional study was conducted on adult patients between the ages of 18 and 70 years who were admitted to the National Hospital of Sri Lanka. Skin-to-muscle distance and dermal thickness of potential insulin injection sites were measured using ultrasonography to estimate the intramuscular injection risk. Results: A total of 185 patients were included in the study. The estimated risk of inadvertent intramuscular injections with insulin needles of any length was considerably higher at the mid-arm and mid-thigh compared to the abdomen. The risk of inadvertent intramuscular injection was approximately 60% with 13-mm needles without a skin fold at the arm and thigh. The abovementioned risk was approximately halved when the needle length was 8 mm. We developed formulae to predict the skin-to-muscle distance with or without a raised skin fold based on assigned sex at birth and anthropometric parameters. Conclusions: There is a high inadvertent intramuscular injection risk with currently used needles (8 and 13 mm) with U-100 syringes in our population even with a raised skin fold. To minimize this risk, 13-mm needles should be avoided when injecting insulin subcutaneously especially to the limbs. Sex and anthropometric parameters can be used to predict the distance from skin to muscle surface; hence, the selection of needle length could be personalized based on the derived formulae. [ABSTRACT FROM AUTHOR]