How to tell how big my overjet is?

[sosp12334]

Hi, I'm wondering if there's a way at home to tell how big my overjet is until I can make it to an orthodontist. (This could be awhile). Also, if it could be fixed by Invisalign products. I "measured" my overjet and it seems to be 5mm, but whenever I compare it to other people's pictures, ranging from 6-12mm, it never seems right, or like I have a huge overjet, even though I'm pretty sure I didn't measure it too badly that I would be off 5-7mm. Are pictures of over jets not as comparable as I think? Thanks.