Just as everyone else has mentioned to order an extra amount, but you might want to ask the concrete plant what their "trip charge" is, too = this will help you make that decision to NOT short yourself on cement/concrete mix.

If the "trip charge minimum" is $150, and 1 yard of concrete is $110; you might as well get an extra yard, right from the beginning, BECAUSE if you are short, that last 1 yard could really cost you (trip charge plus concrete cost).

You can either have a place ready, for a little extra = like a 3x3 foot area, boxed out in front of a shed walk-thru door, or just have the last extra 1/2 yard poured into skidsteer bucket and when done and you are sure you don't need anymore concrete mix, then dump it on gravel road or driveway, and backdrag it, until all the small pieces are distributed evenly to dry.