Ovulation tests are the only way to be 100% sure. Technology fails and it doesn't know if your cycle changed for some reason. I bought a pack of 100 OPKS off Ebay for $15 and I use about 5 a month starting 10 days after my period ends until I get a positive.

use ovia! that's what I use and every month I start my period the day it says I'm going to. the more data you enter the more accurate it is

@kdchaos,

several of them lol. but my main ones are period diary and flo

what app are you using?