# I Euler's Formula for planar graphs - 1 or 2 ?

1. May 26, 2016

### DaTario

Hi All,
Regarding the Euler's formula Vertices - Edges + Faces = constant, for planar graphs what is the most frequent number? 1 or 2?

I understand that it is 2 when the outer region is included in the count.

Best wishes,

DaTario

2. May 27, 2016

### micromass

Staff Emeritus
You can use either formula as long as you make it clear whether you include the outer region or not. I have most usually seen $2$ however.

3. May 27, 2016

### DaTario

Thank you, micromass.