Technically, any denier 40 and higher is considered not see through, while anything less than 30 denier would be more transparent. You can find 80 denier or higher tights that are super thick and opaque
How do you know if leggings will be see-through or not?
If you want to know if your leggings are see-through or not. It's easy. Just try them on and do a squat!
If you can see your underwear lines through the leggings, they are see-through. It will only get worse as you wash and re-wear them over time.
How to choose non see through leggings?
Generally, single colored standard leggings with fabric over 200g/gsm would be a lot less see-through or not see through. Good quality leggings are generally 220/gsm or higher, although they may cost more.
Some cheap leggings with fabric that is 160/gsm or even lower are most likely to have see-through problems.
Pattern leggings and leggings with colorful prints have less see through problems. However they may not be as soft or comfortable as single colored leggings. Adding prints or patterns to a piece of fabric often times makes it less comfortable.
Even with thick leggings or very good quality leggings, choosing the right types of underwear is important too
