How much extra concrete should you order?

Overage by project type

Project condition	Planning overage	Why
Clean rectangular slab	5%-10%	Forms and base are controlled, but depth still varies.
Small slab under 2 yd3	10%	A tiny shortage can ruin finish timing.
Post holes	10%-15%	Hand-dug holes are rarely exact cylinders.
Footing trenches	10%-15%	Crumbled sides and uneven bottoms add volume.
Stairs or complex forms	10%-15%	More edges and leakage points.

Why shortages are expensive

Concrete placement has a time window. If a slab is short by a few cubic feet, the crew cannot always pause cleanly while someone mixes emergency bags. The repair may show as a cold joint, rough patch, weak edge, or visible finish difference.

The smaller the pour, the more painful a small shortage becomes. A missing wheelbarrow on a 1 yard slab is a large percentage of the job. On a larger truckload, the same missing volume is less visible but still disruptive.

Ready-mix versus bags

For ready-mix, ask the supplier how they round partial yards and what minimums apply. A calculated 2.61 yd3 order may become 2.75 or 3 yd3 depending on supplier policy. For bags, round up to whole bags after waste because half-bags are not a purchasing unit.

When not to over-order aggressively

• The forms are precise and the supplier charges steeply for returned concrete.
• The job is indoors or access-limited and extra disposal would be difficult.
• The plan has multiple separate pours where each section can be measured independently.

Example: a 1.24 yd3 slab order

A 10x10 slab at 4 inches calculates to about 1.24 yd3 before waste. With 10% overage, the planning order becomes about 1.36 yd3, which usually rounds to 1.4 yd3 or to the supplier's next allowed increment. That extra tenth of a yard matters because low spots, form bowing, and base variation can consume it quickly.

For bagged concrete, the same logic applies after the bag count is calculated. If the math says 56 bags, buying exactly 56 leaves no margin for spills, slightly deeper edges, or a batch that is mixed too wet and rejected. Round after waste, then keep unopened bags returnable when the store policy allows it.

Decision checklist

• Use 5% only when forms, base, and depth are tightly controlled.
• Use 10% for most small residential slabs and ready-mix orders.
• Use 10% to 15% for rough holes, footings, stairs, and irregular excavation.
• Ask the ready-mix supplier how partial-yard orders are rounded.
• Plan a place for extra concrete before the truck arrives.