Does anyone know which line item on the tax form corresponds w/ the earned income amount? I'm trying to determine my IRA contribution limit and had low income last year.

I had low w/ both my gross & net income being less than $5,500 last year.  I am now trying to determine my max amount for a Roth contribution for 2018. I think my limit would be my net profit  from my contracting business. However, I think I could be wrong and it might be taxable income (line 10 on 1040) or possibly something else.  

Can anyone confirm or correct me?  Thank you.