I suggest using 1/2″ minus crushed limestone for your fence post installation instead of concrete. Pack it in the hole around the fence post, plumb up the fence post as you insert the crushed stone, then water the top of the stone at completion. The crushed stone sets up like a low grade concrete and allows the fence posts to be re-plumbed if they get out of position over the years.
I avoid concrete because wooden fence posts dry and shrink over time and then wobble or move in the hole. Concrete is only a decent choice with steel fence posts. Over time the concrete fence posts will rise up out of the ground by being heaved from the freezing & expanding soil during cold winters. Crushed stone is a far better choice for wood or steel fence posts.