The math behind your estimate
The inputs, allowance, and rounding stay visible.
- Slab / footing
- Volume = Length × Width × Thickness
- Column
- Volume = π × (Diameter ÷ 2)² × Height
- Steps
- Volume = Width × Run × Rise × (1 + 2 + … + number of steps), solid pour
- Bags
- Bags = Volume × (1 + waste %) ÷ yield per bag, rounded up to a whole bag
Assumptions
- Bag yields use manufacturer published values (e.g. one 80 lb bag yields approx. 0.60 ft³ of mixed concrete).Source: Quikrete Concrete Mix No. 1101 product data
- Wet concrete density assumed at 2,400 kg/m³ (≈150 lb/ft³) for normal-weight mix.
- Bag counts are rounded up to whole bags after applying the waste factor. Real-world usage varies with subgrade preparation and forming accuracy.
Scope note
This tool estimates material quantities only. It does not size structural elements. Slab thickness, footing dimensions, and reinforcement are governed by local building codes and soil conditions — verify with your local building department or an engineer.
Frequently asked questions
- How much concrete do I need for a 10×10 slab?
- A 10 ft × 10 ft slab at 4 inches thick needs about 1.23 cubic yards of concrete before waste. With a 10% waste factor, order about 1.36 cubic yards, or roughly 62 × 80 lb bags of premix.
- How many 80 lb bags of concrete make a yard?
- One 80 lb bag yields about 0.60 cubic feet of mixed concrete, so one cubic yard (27 cubic feet) takes about 45 bags. At that quantity, ready-mix delivery is usually cheaper and far less work.
- Should I order ready-mix or use bags?
- As a rule of thumb, use bagged premix below about 1 cubic yard, and get ready-mix delivery quotes above that. Mixing 45+ bags by hand is slow and risks cold joints between batches.
- What waste factor should I use for concrete?
- 5–10% covers typical spillage and slightly uneven subgrade. Use the higher end for hand-graded ground or thin slabs, and more if your excavation depth varies noticeably.