How Much Concrete to Order: Slabs, Footings and Waste
Running short on a pour is one of the most expensive mistakes in small construction: the concrete already placed starts to set while you wait for more, leaving a weak cold joint. Over-ordering wastes money but is far less harmful. The goal is a confident number with a sensible margin.
Volume
Concrete volume is length × width × depth. In imperial units, convert inches to feet, multiply to cubic feet, then divide by 27 for cubic yards. In metric, multiply metres to get cubic metres. Our slab calculator and footing calculator do this for you.
How much extra
- 5% for a well-formed slab on a compacted, level base.
- 10% for most DIY slabs and anything with an uneven base.
- 10–15% for hand-dug footings and post holes, which are rarely perfect cylinders.
Bags or ready-mix
An 80 lb bag yields about 0.6 ft³, so a single cubic yard is about 45 bags — around 3,600 lb to lift and mix. As a rough guide, bags make sense under 1 yd³ (0.75 m³), and a truck makes sense above 1.5–2 yd³. Between those, consider a towable mixer from a rental yard.
Placing the order
- Give the volume rounded up to the plant's increment (often 1/4 yd³ or 0.25 m³).
- Specify the strength (for example 3,000–4,000 psi or 25–32 MPa for residential slabs), slump and whether you need air entrainment for freeze-thaw exposure.
- Ask about short-load fees and how much time is included for unloading.
- Have forms, reinforcement, tools and helpers ready before the truck arrives.
Watch
Questions
What strength concrete do I need for a patio?
Many residential flatwork jobs use roughly 3,000–4,000 psi (20–28 MPa) concrete, with air entrainment in freeze-thaw climates. Follow local requirements.