How to use Concrete Calculator
The concrete calculator estimates the volume of concrete needed for slabs, footings, columns, steps and circular pads. Enter the dimensions and it returns cubic yards and cubic metres, the number of 60 lb or 80 lb bags, and an estimated material cost — so you can order exactly what you need without waste or a costly second delivery.
- Choose the shape: slab, footing, column or circular.
- Enter length, width (or diameter) and depth/thickness.
- Add a waste factor (typically 10%).
- Review volume in cubic yards, number of bags and estimated cost.
Your data never leaves your device — 100% private processing.
How concrete volume is calculated
Volume (cu yd) = Length (ft) × Width (ft) × Depth (ft) ÷ 27. One cubic yard weighs about 4,000 lbs. Coverage from bags: a 60 lb bag yields 0.45 cu ft; an 80 lb bag yields 0.60 cu ft. Always add 10% for waste and spillage. Pre-mixed ready-mix concrete is ordered by the cubic yard and is cost-effective for pours over about 1 cubic yard.
| Bag size | Yield | Bags per cu yd | Approx. cost/bag |
|---|---|---|---|
| 60 lb | 0.45 cu ft | 60 bags | ~$5–7 |
| 80 lb | 0.60 cu ft | 45 bags | ~$6–9 |
| 90 lb (rapid-set) | 0.67 cu ft | 40 bags | ~$8–11 |
Mix ratios and concrete strength
Standard residential concrete is 3,000–4,000 PSI compressive strength, achieved with a water:cement ratio of about 0.45–0.55. The classic mix is 1 part cement : 2 parts sand : 3 parts aggregate (by volume). Adding more water weakens the mix; reducing aggregate increases strength and workability. For driveways and garage slabs, 4,000 PSI minimum is recommended.
Worked examples
10×10 ft slab, 4 in thick
Inputs: 100 sq ft × 0.333 ft = 33.3 cu ft ÷ 27
Result: 1.23 yd³; +10% waste ≈ 1.4 yd³ (~62 × 80 lb bags)
Four 12-in × 3-ft deck footings
Inputs: π × 0.5² × 3 ft = 2.36 cu ft each × 4
Result: 9.4 cu ft = 0.35 yd³ (~16 × 80 lb bags)
20×20 ft garage slab, 4 in
Inputs: 400 sq ft × 0.333 ft = 133 cu ft ÷ 27
Result: 4.93 yd³; +10% ≈ 5.4 yd³ — order ready-mix
Recommended slab thickness by application (planning aid)
| Application | Recommended thickness | Min strength (PSI) |
|---|---|---|
| Walkway / patio | 4 inches | 3,000 |
| Driveway (cars) | 4 inches | 3,500–4,000 |
| Driveway (heavy / RV) | 5–6 inches | 4,000 |
| Garage slab | 4–6 inches | 4,000 |
| Footings | 8–12 inches | 3,000 |
Glossary
- Cubic yard
- 27 cubic feet — the standard unit for ordering ready-mix concrete.
- PSI
- Pounds per square inch — the compressive strength of cured concrete.
- Water-cement ratio
- The weight of water divided by the weight of cement; lower = stronger.
- Slump
- A measure of fresh concrete workability; standard residential slump is 4–5 inches.
Related reading
Frequently Asked Questions
Why use Concrete Calculator?
- Calculate cubic yards of concrete for slabs, footings and columns in one step
- Convert volume instantly to 60 lb or 80 lb bag counts for small pours
- Customisable waste factor prevents costly short deliveries or a second truck
- Estimated ready-mix cost lets you compare supplier quotes before calling
Common use cases
- Order the right amount of ready-mix for a residential driveway pour
- Determine how many 80 lb bags are needed to set deck post footings
- Estimate concrete for a garage slab before requesting contractor bids
- Calculate volume for a poured-concrete retaining wall footing
- Confirm material quantities match a contractor's estimate on a walkway job
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