Measure the length and width of the shower – measure the distance from wall to wall at the base of the shower. Then subtract 1/4" from the length and width – this allows the mat to be easily removed for cleaning. Finally, specify if you require the corners to be rounded and by how much. You can always call our customer service team to assist you in measuring your shower. Graphic example: https://www.arbteak.com/pages/custom-shower-mat Customer service team: 1-855-772-8325