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Castelli Thermolite 9 Socks

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Item # CST003H

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Item # CST003H


We're big fans of Castelli's Thermolite 9 socks for a few reasons. First, they're fantastic, durable, comfortable cycling socks. Second, their name tells their whole story. They're thermally insulated socks, but only mildly so, excellent for cool-weather riding conditions, and not so insulating as to be uncomfortably warm when the sun comes out or when the pace cranks up. Thermolite is a medium-density, hollow-core polyester material that promotes breathability and moisture transfer to preserve the comfort of your feet, while still insulating them enough to fend off the chill of fall days. The midfoot is supported by an anatomic band of fabric that prevents cramping and hotspots. The socks have a nine-centimeter cuff, come in the sizes S/M, L/XL, and XXL, and in the colors Black or White.

Tech Specs

Thermolite polyester
9 cm
Recommended Use:
cool weather cycling
Manufacturer Warranty:
30 days

Size chart

Fuji Bicycles


Geometry Chart


Altamira LTD

Seat Tube


Effective Top Tube








Head Tube


Head Tube Angle


Seat Tube Angle


Bottom Bracket Drop


Chainstay Length



44 44.0cm 51.5cm


36.3cm 72.1cm


72.0 deg 75.0 deg 6.9cm 40.5cm 97.1cm
47 47.0cm 53.0cm


37.0cm 74.1cm 13.0cm 72.5 deg 74.5 deg 6.9cm 40.5cm 97.7cm
50 50.0cm 54.5cm 54.0cm 37.0cm 76.0cm 14.0cm 73.0 deg 73.5 deg 6.9cm 40.5cm 97.6cm
53 53.0cm 56.0cm 56.0cm 37.9cm 78.3cm 16.0cm 73.5 deg 73.5 deg 6.9cm 40.5cm 98.6cm
55 55.0cm 57.5cm 58.0cm 38.9cm 80.1cm 18.0cm 73.5 deg 73.5 deg 6.9cm 40.5cm 100.1cm
58 58.0cm 59.0cm 60.1cm 39.7cm 82.1cm 20.0cm 74.0 deg 73.5 deg 6.9cm 40.5cm


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Here's what others have to say...

Avg. ride time: 8h 44m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

Less Bulk Plenty Warm

  • Familiarity:I've used it several times
  • Fit:True to size

Castello makes quality gear. These socks provide enough warmth for the coldest days riding (AZ +/- 35 degrees F) but do not feel so bulky in my shoes. They have worked well for me!

Avg. ride time: 8h 34m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

Fits well and seems to keep my feet warm

  • Familiarity:I've used it several times
  • Fit:True to size

Surprisedly light weight yet they do keep my feet warm


Too big

  • Familiarity:I returned this product before using it
  • Fit:Runs large

I wear a size 42 shoe and got the L/XL based on the size chart. They were way too big. L/XL is probably good for size 44-45 feet.


Tan lines for days

  • Familiarity:I've used it several times
  • Fit:True to size

These are my go to socks for everyday warm weather riding. The 6" cuff is the perfect height to show off another cycling tan line.


Great Socks!!

  • Familiarity:I've used it several times
  • Fit:True to size

These are high quality - long lasting socks.

You won't even know you have them on.

They are so comfortable and wick your feet dry.

My question is with quality gear like this why does some nincomepoop insist on putting the two socks together with a push through plastic T-bar thing that always snags the socks or other garment. I hate it when my clothing has label tags in the neck area, or some other uncomfortable area and if you tear the tag out you tear the garment. In my case here I pulled the two socks apart that were attached with the T-Bar plastic thing that went through both socks when I pulled it out it made a run like in a nylon stocking. Now that sucks!!!!


Best winter sock

  • Familiarity:I've put it through the wringer

Absolutely love these socks. Not overly bulky like wool socks, but keep the feet nice and warm. Also very durable. Had several merino wool socks that have quickly developed holes, these have lasted all winter for me and there's no sign of any holes developing. Nothing bad to say about these socks.


Castelli never disappoints

    What can I say, Castelli clothing never lets me down. These socks fit perfect and feel great.