CN 40 is one of the most searched Chinese shoe labels because the US result changes by gender. Use foot length as the final fit check.
Quick answer: CN 40 is about US women's 9 or US men's 7. It usually points to a 25.0 cm / 9.8 in foot length.
If a listing says CN 40 and also shows 25.0 cm, trust the centimeter value first.
For wide feet or narrow Chinese lasts, CN 40 can still feel tight even when the length is right.
Chinese marketplace shoe labels often behave like EU sizes, but official shoe marking is based on foot length.
Use the main shoe converter for the full CN 34-47 chart.
This page answers one specific size search intent. For the best fit, start with the linked main converter and use the centimeter measurements on the Chinese chart whenever they are available.
Marketplace labels are useful shortcuts, but brand charts, garment type, fabric stretch and body measurements can change the final fit. Treat the lookup as a nearest match, not a guaranteed size.
Chinese size labels are practical lookup shortcuts, not universal brand guarantees. A CN shoe label, clothing size, bra code or Taobao chart row can vary by seller, product category, last shape, fabric stretch and whether the chart shows body measurements or garment measurements.
When possible, use the centimeter value first: foot length for shoes, bust/waist/hip for clothing, under-bust and cup difference for bras, and the specific Taobao chart row for marketplace listings. Then use the converter result as the nearest US, UK or EU match.
For the broader explanation, read Chinese shoe size chart to US complete guide. For why these pages give estimates instead of promises, read Chinese shoe sizes and foot length.