3-inch pot? 5-inch pot?

hi all,
I'm a beginner here so forgive me if this question sounds stupid to u... I heard ppl often talk about 3-inch pot or 5-inch pot. a they talking about the diameter or the radius of the pot?
thx for answering ...

我係度盆口既直徑
5吋盆,即盆口直徑5吋

ic now.....thx!