A 70 kg maintenance patient on 25 kcal/kg and 1.2 g/kg protein is the easy case. Real bedside decisions get harder the moment weight basis, formula density, or permissive underfeeding enter the prescription. This field guide walks through ASPEN/SCCM and ESPEN reference bands, two worked examples (standard maintenance and critical illness permissive underfeeding), the weight-basis decision, six common pitfalls, and the re-feeding syndrome window the calculator does not flag.