When designing my business card, I had an epiphane about when to use filled icons vs when to use hollow icons.

• If the design is content dense, then we should use a filled icon
• If the design is content light, then we should use a hollow icon
• If the content is in between, then either should work but the icons should be consistently filled or hollow
• Hollow icons should use the same stroke width (i.e. be consistent)

I'm uncertain if this is a hard and fast rule. In programming, the answer is almost always "it depends" and there are rarely simple problems/solutions.

But let's look at some examples:

Examples

Dense content

Filled icons (better):

Hollow icons (worse):

Light content

Hollow icons (better):

Filled icons (worse):

Icons consistently filled or hollow

Consistently hollow icons (better):

Inconsistent filled and hollow icons (worse):

Hollow icon stroke width

Same stroke width (better):

Different stroke width (worse):

Attribution

Filled email:

• https://thenounproject.com/term/envelope/10788/
• Created by Nicholas Menghini from the Noun Project

Hollow email:

• https://thenounproject.com/term/envelope/225240/
• Created by Designify.me from the Noun Project

Filled phone:

• https://thenounproject.com/term/phone-call/358146/
• Created by icon 54 from the Noun Project

Hollow phone:

• Derived by Todd Wolfson

BLOKK font (used for dense content):

• http://blokkfont.com/