How to calculate your due date

So I got 3 positive results on Friday for baby number 3. I was 5 days late for my period. The glow <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> app says I am 4wk and 6 days pregnant. Do u start the count from 1st day of last period or from when u conceived. I know the exact date I had sex (1st march) and my last period started 17th Feb