Make a repeatable record
Use a sheet of paper on a firm, flat floor, a pencil, and a ruler. Wear the socks you intend to use with the slippers. Mark the back of the heel and the end of the longest toe while standing normally; having someone help can make this easier. Measure both feet and write down the date, units, and socks used. If a brand gives a different measuring method, follow that method for its chart.
Match the measurement to the right chart
A chart headed foot length is intended to receive a foot measurement. A measurement of a removable insole or outsole describes something else. Do not treat these as interchangeable, and do not add a universal amount of extra space before using every chart. Ask the retailer what its numbers mean when the heading is unclear. Keep the original measurement alongside the recommended size so you can revisit the decision.
Watch paired sizes and size systems
A slipper marked 7-8 covers a range rather than a single half size. Check how that brand advises shoppers at the edge of the range, especially with lining or socks. Confirm whether the chart is US women's, US men's, UK, or EU before comparing labels. A familiar number on a different system is not the same size. Use the chart attached to the exact model when one is provided.
Add the questions a ruler cannot answer
Record the pressure you want to avoid: at the end, beside the toes, or above them. Also record whether the heel stayed in place in your last pair. Length alone will not explain all of that. Send the seller a short question with your intended size and ask for the missing inside measurement. If they cannot supply it, choose whether the return terms make an indoor fit check practical. A measurement narrows the shortlist; it does not replace trying the slippers on.