The Lucidity In Mental Capacity (LIMCap) Assessment Tool was designed to provide untrained doctors with a flexible tool to assess decision making capacity (DMC) in different scenarios and to screen for patients who require further in-depth specialist capacity assessments. Methods: This prospective validation study compares the assessment of DMC between untrained doctors using this tool on various inpatient decisions with trained consultants’ independent assessment and assesses the tool’s user-friendliness.55 tertiary hospital in-patients were selected by their respective consultants and assessed on a single decision and re-interviewed with the tool by registrars. Our exclusion criteria included on-going delirium, depression or psychosis. Their average age was 85 years with 32.73% male. Results: Patients were assessed on decisions regarding discharge planning, resuscitation, financial planning, electing enduring power of attorney(s), medical choices or driving. The LIM Cap was 0.53 sensitive and 0.7 specific, the Cohen’s kappa was 0.3 and the tool took 10.5 minutes (mean) to administer. The feedback demonstrated satisfaction with the instructions (85.73%), method of scoring (85.73%), deciding on DMC (71.43%) and improvement in confidence in assessing DMC (92.86%). However, some registrars (14.29%) struggled with constructing the questions to suit their assessment and assessing for depression and psychosis. Conclusions: The LIM Cap is a versatile tool that was quick and easy to use by untrained doctors to assess DMC in different scenarios. There was fair correlation between the results of the consultants and registrars, and the test detects patients with capacity with good specificity but appears less sensitive in diagnosing patients without capacity.