Revision hip arthroplasty is a frequently performed procedure and is projected to increase annually. Removal of a well-fixed acetabular component can involve loss of much needed bone stock. Contemporary instruments allow acetabular removal with minimal morbidity; however, their use requires accurate knowledge of the component size. We describe a technique that allows sizing to be determined accurately, without specialized equipment, in situations where component details are unavailable. Our technique multiplies ratio of head:cup on pre-operative X-ray by the diameter of the index femoral head which is removed intra-operatively to predict index cup size.