This article analyses the jus in bello proportionality standard under international humanitarian law to assist judge advocates and practitioners in achieving a measure of clarity as to what constitutes 'excessive' collateral damage when planning or executing an attack on a legitimate military objective when incidental harm to civilians is expected. Applying international humanitarian law, the author analyses existing US practice to evidence the need for states to adopt further institutional mechanisms and methodologies to clarify targeting principles and proportionality assessments. A subjective-objective standard for determining 'excessive' collateral damage is proposed, along with a seven-step targeting methodology that is readily applicable to the US, and all other state and non-state actors engaged in the conduct of hostilities. Keywords: proportionality, excessive, collateral damage, civilian casualties, legitimate military objective, incidental harm to civilians.