How do you calculate your due date?

I have no idea when my due date is (haven’t been to the doctor yet). But I’m trying to at least get an estimate of when it might be. I’m assuming it would be sometime in September but could possibly be October, I have no idea. My last period began on 12/25 and this is from my <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">period tracker</a> (the hearts are the days my husband ...ya know..in me lol.)