REI Co-op Mountain Merino C10726 chart
No numeric chart for this model could be read; open the maker’s product page for its size chart.
1 model uses this chart:
REI Co-op size chart for hiking socks
Find your shoe size in the column you know; the letter at the left of that row is your REI Co-op sock size. Numeric ranges for this model are not available in this lookup. Open the product page’s Size chart, or choose another model with a published range here; a medium from another brand cannot settle the REI size.