You can choose you own custom size mount to suit your item. To work out the perimeter you need to add all four sides together, then choose the drop down that shows your perimeter. this will also give the cost Then write the size in personalisation, eg 45x35cm outer 35x25 inner when measuring the inner ensure you come in an extra 1cm so your print doesn't fall through Please message for help. The following link are our custom frames https://www.etsy.com/uk/listing/810146093 All Mounts are Acid-Free with a Bevelled cut. Maximum size; 70cm wide, 100cm in length. All of our products are designed & handmade by photoframesandmore in the UK. Made to order. Please double-check all details of your custom order before completing checkout. Due to our fast turnaround time, we are unable to make amendments once an order has been placed. If you need any help or have questions about your order, please get in touch before placing itwe're happy to help! Made To Measure Mounts( Important- Please Read Description Boxes Before Ordering - The Perimeter Is All 4 Sides Added Together