Running short mid-pour is the single most expensive mistake in DIY concrete work — a second truck almost always comes with a short-load fee, and the seam between two pours rarely looks or performs as well as one continuous slab. Here are the five mistakes that cause it.
It's easy to measure the outside of your forms and forget that thickness eats volume fast. A 10x10 ft slab isn't 100 cubic feet of concrete — at 4in thick, it's about 33 cubic feet. Always multiply length × width × thickness, not just the footprint.
Uneven subgrade, formwork gaps, and spillage routinely eat 5–10% of your calculated volume. Ordering the exact math-perfect number is a common way to come up 5% short right as the truck's chute runs dry.
Ready-mix trucks deliver in fractions of a yard, but suppliers set minimums. If your calculation lands at 4.1 yards, order 4.25 or 4.5 — not 4. The cost difference for a quarter yard is small compared to a short-load fee for a second delivery.
An L-shaped patio or a slab with a curved edge isn't one rectangle. Break it into rectangles (or approximate curves as trapezoids), calculate each section separately, and add the volumes together.
If your gravel base isn't fully compacted before the pour, the slab can settle unevenly into soft spots, effectively "eating" concrete in low areas. Compact your base in lifts and check for consistent depth before ordering.
The fix for all five is the same: use a calculator that adds a waste buffer automatically and round up to your supplier's ordering increment. The Concrete Calculator on this site does both by default.