Concrete Calculator — Cubic Yards, Bags & Cost
Short answer: concrete in cubic yards = length × width × thickness (in feet) ÷ 27. Enter your measurements below and get cubic yards, 40/60/80 lb bag counts, ready-mix vs bagged cost, weight and truckloads — for slabs, walls, footings and columns.
Calculate your concrete
How the calculation works
Rectangular slab, wall or footing: volume (cu ft) = length (ft) × width (ft) × thickness (ft). Thickness in inches ÷ 12 = thickness in feet.
Round column: volume (cu ft) = π × (diameter ÷ 2)² × height, all in feet.
Cubic yards = cubic feet ÷ 27, because one cubic yard is a 3 ft × 3 ft × 3 ft cube = 27 cubic feet.
Bag counts use QUIKRETE published yields: 80 lb bag = 0.60 cu ft, 60 lb bag = 0.45 cu ft, 50 lb bag = 0.375 cu ft, 40 lb bag = 0.30 cu ft. Counts are rounded up — a partial bag still has to be bought.
Worked example: a 10 ft × 12 ft slab at 4 in thick = 10 × 12 × (4 ÷ 12) = 40 cu ft = 1.48 cubic yards. With 10% waste, order 1.63 cubic yards, or 74 × 80 lb bags.
How to measure (so the estimate is right)
- Measure the excavation, not the plan — forms and digging change the real size.
- Take depth readings at several points and use the average — a subgrade that dips half an inch adds real volume.
- Measure twice, in feet and inches, and keep the waste allowance switched on.
- For identical footings, measure one and use the count — do not guess the total.
Common mistakes
- Forgetting waste — order 5–10% extra, or the truck runs short mid-pour.
- Mixing units — feet for length but inches for depth without dividing by 12 (the calculator does this conversion for you).
- Wrong thickness — a 4-inch patio quantity does not cover a 6-inch driveway.
- Under-ordering to save money — a second delivery costs more than the extra half-yard would have.
Common slab sizes at 4 inches thick
| Slab | Volume | 80 lb bags |
|---|---|---|
| 10 × 10 ft | 1.23 cubic yards | 56 |
| 12 × 12 ft | 1.78 cubic yards | 80 |
| 20 × 20 ft | 4.94 cubic yards | 223 |
| 24 × 24 ft | 7.11 cubic yards | 320 |
Volumes are net (before waste). Bag counts use the 0.60 cu ft yield and are rounded up.
Frequently asked questions
How do you calculate concrete in cubic yards?
Multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27. Add 5–10% waste before ordering.
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags. An 80 lb bag yields about 0.60 cubic feet (QUIKRETE published yield), and 27 ÷ 0.60 = 45.
Should I order ready-mix or bags?
Under about 1 cubic yard, bags are usually practical. Over 1 cubic yard, ready-mix delivery is usually cheaper per yard — but check short-load fees, which suppliers charge on small orders, with thresholds varying (some from 3–4 yards, others up to about 8 yards).
How much waste should I add?
5–10% for formed slabs (10% default). Use 15–20% for hand-dug trenches or post holes where sides bulge.
How thick should a concrete slab be?
Typical residential practice: 4 inches for patios, walkways and shed floors; 5–6 inches for driveways and garage floors carrying vehicles. Always check your local building code.
What is the difference between concrete and cement?
Cement is an ingredient — the fine gray powder that binds the mix. Concrete is the finished material: cement plus water, sand and gravel. You pour concrete; you do not pour cement.
Which calculator do I need?
| Calculator | Use it for |
|---|---|
| Concrete Calculator | Any slab, wall, footing or column — yards, bags, cost, weight |
| Cubic Yard Calculator | Any material volume in cubic yards, plus weight |
| Slab Calculator | Patios, driveways, garage floors, shed bases |
| Footing Calculator | Strip footings, walls and round columns, with a count |
| Bags Calculator | Bagged mix only — 40, 50, 60 and 80 lb bags |
| Rebar Calculator | Reinforcement grid: bars, length, pieces, weight |
| Stairs Calculator | Solid concrete steps |
| Column Calculator | Round piers, sonotubes and post holes |
| Block Calculator | Concrete block walls — count, courses, waste |
| Gravel Calculator | Base material: gravel, sand, topsoil, mulch, asphalt |
| Multi-Area Calculator | Several sections added into one order total |
| Mix Ratio Calculator | Mix proportions: cement bags, sand, aggregate and water |
| Weight Calculator | Slab weight in pounds, tons and kilograms |
| Round Slab Calculator | Circular slabs and pads from diameter and thickness |
| Wall Calculator | Poured walls with openings subtracted |
| Curb Calculator | Straight curbs: yards and bags from length and section size |
| Fence Post Calculator | Post count, concrete per hole and total bags for a fence run |
| Deck Footing Calculator | Footing grid count and concrete for round deck footings |
| Sidewalk Calculator | Walkways and paths: yards, bags and control joints |
| Cost Guide | Ready-mix vs bags: which is cheaper for your size |