How to use the concrete calculator
- Choose the shape of the pour. Slab or pad for patios and floors; footing or curb for long narrow strips; column or sonotube for round forms and piers; steps for stairs.
- Enter the sizes. Lengths in feet, thickness, depth and diameter in inches. Feet and inches like 10' 6" and fractions like 3 1/2 work too.
- Set how many if you have several identical pads, footings or columns.
- Pick a bag size and add prices to compare bags with ready-mix.
Concrete formulas for each shape
Slab: length × width × (thickness ÷ 12)
Footing: length × (width ÷ 12) × (depth ÷ 12)
Column: π × (diameter ÷ 24)² × (height ÷ 12)
Steps: width × (rise ÷ 12) × (run ÷ 12) × (1 + 2 + … + steps), plus the platform
Cubic yards = cubic feet × (1 + waste) ÷ 27; bags = cubic feet with waste ÷ bag yield
Worked examples
A 12 × 12 ft patio, 4 inches thick
- 12 × 12 × 0.333 = 48.0 cu ft; with 10% = 52.8 cu ft.
- 52.8 ÷ 27 = 1.96 cubic yards, or 52.8 ÷ 0.6 = 88 bags of 80 lb concrete.
- At almost 2 yards, ready-mix is the sensible choice.
A 20 ft footing, 16 in wide and 8 in deep
- 20 × 1.333 × 0.667 = 17.8 cu ft; with 10% = 19.6 cu ft.
- 19.6 ÷ 27 = 0.72 cubic yards, or 33 bags of 80 lb.
Concrete per cubic yard
| One cubic yard pours | Area covered |
|---|---|
| Slab 3 in thick | 108 sq ft |
| Slab 4 in thick | 81 sq ft |
| Slab 5 in thick | 65 sq ft |
| Slab 6 in thick | 54 sq ft |
| 12 in sonotube | 34 ft of tube |
| 8 in sonotube | 77 ft of tube |
Common mistakes to avoid
- Mixing up inches and feet. A 4 in slab is 0.333 ft thick.
- No extra. Forms bow and sub-grades dip; order 5 to 10% more.
- Forgetting the forms. Measure inside the forms, not the outside of the boards.
- Bagging a big pour. Over a yard means 45+ bags and a high risk of cold joints.
- Short ready-mix orders. Running short on a truck pour is costly; round up to the next quarter or half yard.
Frequently asked questions
How do I calculate concrete in cubic yards?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27. A 12 × 12 ft slab 4 in thick is 12 × 12 × 0.333 = 48 cubic feet, or 1.78 cubic yards. Add 5 to 10% for spillage and uneven ground.
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cu ft, so it takes 45 bags; 60 lb bags (0.45 cu ft) take 60, and 40 lb bags (0.3 cu ft) take 90.
How much concrete do I need for a sonotube?
Use the column option. A 12 in sonotube 4 ft long holds π × 0.5² × 4 = 3.14 cu ft, about 6 × 80 lb bags with 10% extra. An 8 in tube 4 ft long holds 1.4 cu ft, about 3 bags.
How do you calculate concrete for steps?
Each step is a block from the ground to its own tread height. With 3 steps of 7 in rise and 11 in run, 4 ft wide, the volume is 4 × 0.583 × 0.917 × (1 + 2 + 3) = 12.8 cu ft, about 24 × 80 lb bags with 10% extra. Add any top platform separately.
Should I use bags or ready-mix?
Bags suit small pours under about a cubic yard. Above that, ready-mix is usually cheaper, stronger and far less work, although small loads often carry a short-load fee. Enter both prices to compare.
How thick should a concrete slab be?
4 in is common for patios, walks and shed bases; 5 to 6 in for driveways. Footings, foundations and structural slabs follow your local code or engineer.