What happens if I order less concrete than the calculator suggests for a single pour?

Running short mid-pour is a genuine risk that can force an unplanned cold joint, a weak plane where fresh concrete meets a partially set earlier batch, which can compromise structural performance if it occurs somewhere it should not. It is safer to order a small buffer above the calculator's exact figure for a continuous pour like a slab than to risk stopping partway through. Discuss buffer quantity with your engineer for critical, continuous pours.