Concrete Calculator
Calculate how much concrete you need for slabs, walls, footings, round columns and stairs. Get results in cubic yards, cubic feet and cubic meters, including waste allowance and bag estimates.
V = L × W × TWhat this calculator does
How much concrete do I need?
The amount of concrete you need is the volume of the space you are filling. Measure length, width and thickness (or diameter and height for round pours), convert every dimension to the same unit, then apply the volume formula for the shape. Concrete is sold in cubic yards for ready-mix and in bags rated by cubic feet, so the calculator reports all three units (yd³, ft³, m³) plus 40 lb, 60 lb and 80 lb bag counts.
How this concrete calculator works
- Select a calculation shape (Rectangular, Round, or Stairs).
- Enter each dimension.
- Choose the unit for each dimension independently.
- Enter a waste percentage (5–10% is a common starting point).
- Review volume, bag counts and the optional rounded order quantity.
- Optionally add a price per cubic yard or per 80 lb bag for a cost estimate.
Formula
Rectangular slabs, walls and footings
V = length × width × thickness — the same rectangular-prism formula is used for slabs, walls, square footings and pads. For slab-specific workflows, see the Concrete Slab Calculator.
Round columns and tube footings
V = π × radius² × height, where radius = diameter ÷ 2.
Concrete stairs
The stair mode approximates the poured volume as a stack of rectangular treads: V ≈ Σ (run × width × i × rise) for i = 1…n. This is an estimate — verify against your specific stair form before ordering.
Conversions and waste
cubic yards = cubic feet ÷ 27 · cubic meters = cubic feet × 0.0283168 · volume with waste = base volume × (1 + waste% ÷ 100) · bags = ⌈volume with waste ÷ bag yield⌉
Variable definitions
length / width / thickness: Rectangular dimensions. Each has its own unit selector; the calculator converts everything to a single internal unit before computing.diameter / height: Round-column or tube-footing dimensions. Radius is diameter ÷ 2.bag yield: Concrete produced by one bag: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb. Actual yield can vary by product.waste %: Extra ordered to allow for spillage and over-pour, added as a percentage.
Step-by-step calculation
- Choose Rectangular, Round, or Stairs.
- Enter dimensions in your preferred units.
- Set the waste percentage.
- Optionally set a supplier rounding increment (0.25 or 0.50 yd³).
- Optionally enter price per cubic yard or per 80 lb bag.
- Order ready-mix by the cubic-yard figure, or bagged mix by the bag count.
Worked example
How to calculate cubic yards of concrete — patio slab, 12 ft × 10 ft × 4 in, 10% waste.
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Base cubic feet: 12 × 10 × 0.333 = 40 ft³
- Cubic yards before waste: 40 ÷ 27 ≈ 1.48 yd³
- With 10% waste: 40 × 1.10 = 44 ft³ ≈ 1.63 yd³ ≈ 1.246 m³
- 80 lb bags: ⌈44 ÷ 0.60⌉ = 74 bags · 60 lb: 98 · 40 lb: 147
How to use this calculator
- Pick the shape tab matching your pour.
- Enter dimensions using whichever unit is easiest per field.
- Adjust the waste percentage for complex forms.
- Turn on supplier rounding if ordering ready-mix in fixed increments.
- Add prices for an optional cost estimate.
Common mistakes
- Mixing units without converting: the calculator handles this, but double-check your inputs match the units you selected.
- Forgetting to convert inches to feet when working by hand.
- Omitting waste: running short mid-pour is far costlier than a small leftover.
- Rounding bag counts downward. Always round up.
- Confusing cubic feet with cubic yards (1 yd³ = 27 ft³).
- Entering diameter as radius in the round mode.
- Treating the estimate as a structural design. This tool sizes concrete quantity; it does not size reinforcement or specify mix strength.
Concrete bag yield table
| Bag size | Yield used |
|---|---|
| 40 lb | 0.30 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
Actual yield can vary by product. Confirm the yield printed on the bag or provided by the manufacturer.
Calculation methodology
- Rectangular volume: length × width × thickness (all dimensions converted to feet).
- Round volume: π × (diameter ÷ 2)² × height.
- Cubic yards = cubic feet ÷ 27. Cubic meters = cubic feet × 0.0283168.
- Waste is applied as a percentage of base volume.
- Bag counts are ⌈volume with waste ÷ bag yield⌉, always rounded up.
- Supplier rounding, when enabled, rounds the order quantity up to the chosen increment.
Frequently asked questions
›How do I calculate how much concrete I need?
Multiply length × width × thickness with all three dimensions in the same unit to get the volume. For a rectangular slab in feet, that is cubic feet; divide by 27 for cubic yards. For a round column, use π × radius² × height. Then add a waste factor so you do not run short.
›How do I calculate cubic yards of concrete?
Convert every dimension to feet, multiply length × width × thickness to get cubic feet, then divide by 27. Example: a 12 ft × 10 ft slab that is 4 in thick is 12 × 10 × (4 ÷ 12) = 40 cubic feet, or about 1.48 cubic yards before waste.
›How many cubic feet are in one cubic yard?
One cubic yard equals 27 cubic feet. Ready-mix concrete is normally ordered in cubic yards, while bagged mix is rated in cubic feet.
›How many bags of concrete do I need?
Divide the total volume in cubic feet (including waste) by the bag yield and round up: an 80 lb bag yields about 0.60 ft³, a 60 lb bag about 0.45 ft³, and a 40 lb bag about 0.30 ft³. The calculator shows all three bag counts.
›Should concrete bag quantities be rounded up?
Yes. Always round bag counts up to the next whole bag. Concrete is unforgiving once you start pouring, and a partial bag will not finish the job.
›How do I calculate concrete for a round column?
Use V = π × radius² × height, where radius = diameter ÷ 2. Select Round in the calculator and enter the diameter and height in your preferred units, and it converts and computes automatically.
›Can this calculator estimate concrete cost?
Yes, optionally. Enter a price per cubic yard for ready-mix or a price per 80 lb bag and the calculator multiplies by the ordered volume or the rounded bag count. Prices are not built in because ready-mix and bag prices vary by region and supplier.
›Does the calculator include waste?
Yes. Enter a waste percentage (5–10% is common for straightforward slab pours) and the tool shows base volume, waste quantity, and volume including waste. Bag counts are computed against the volume including waste.
›Can I use different units for each dimension?
Yes. Each dimension has its own unit selector (feet, inches, yards, meters, centimeters). All inputs are converted to a single internal unit before the volume is calculated, so mixing units is safe.
›What is the difference between calculated volume and rounded order quantity?
Calculated volume is the exact volume from your dimensions plus waste. Ready-mix suppliers typically accept orders in fixed increments (for example 0.25 or 0.50 cubic yard). Use the supplier rounding option to round the order quantity up to the nearest increment, and confirm the actual minimum order and increment with your supplier.
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