The Concrete Calculator converts the dimensions of a pour into the volume of concrete it needs, in cubic yards, cubic feet and cubic metres. Nine shapes are covered: square slab, round slab, wall, footer, square column, round column, tube, steps, and curb with gutter. Enter length, width and thickness in any unit, and the calculator returns the volume, the number of 40, 50, 60 or 80 lb bags, the pour weight, and the cost at your supplier's price.
Calculate concrete volume by shape
Concrete volume depends entirely on the form being filled. A slab is length × width × thickness. A round column is the area of its circle multiplied by height. A set of steps is a stack of rectangular blocks that grows one step at a time.
The Concrete Calculator holds nine separate form geometries and switches the input fields when you switch shape.
Pick the shape that matches what you are pouring:
- Square slab: patios, driveways, garage floors, shed pads. Length × width × thickness.
- Round slab: circular patios, tank bases, fire pit pads. Diameter and thickness.
- Wall: foundation walls, stem walls, retaining walls. Length × height × thickness.
- Footer: strip footings under a wall. Length × width × depth.
- Square column: pilasters and square piers. Width × length × height.
- Round column: round piers and posts. Diameter and height.
- Tube: Sonotube forms and hollow columns. Outer diameter, inner diameter, height.
- Steps: poured stairs. Rise, run, width, platform depth and step count.
- Curb and gutter: kerb runs with a gutter flag. Curb height, curb depth, flag thickness, gutter width, length.
Each shape has a quantity field. Pouring fifty deck footings means entering one footing and setting quantity to 50, not doing the multiplication yourself.
Apply the concrete volume formula
Every shape resolves to the same two-step arithmetic: find the volume in cubic feet, then divide by 27 to get cubic yards. There are 27 cubic feet in a cubic yard because a yard is three feet on every side, and 3 × 3 × 3 = 27.
Ready-mix concrete is sold by the cubic yard almost everywhere in the United States.
For a rectangular slab:
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)
Volume (yd³) = Volume (ft³) ÷ 27Thickness is nearly always given in inches, so it has to be converted first. Divide inches by 12. A 4-inch slab is 0.333 ft thick; a 6-inch slab is 0.5 ft.
For round forms, replace length × width with the area of the circle:
Area (ft²) = π × (Diameter ÷ 2)²
Volume (ft³) = Area × HeightAnd for a tube or hollow column, subtract the inner circle from the outer one before multiplying by height. That single subtraction is where most hand calculations go wrong.
Estimate a 20 × 12 patio slab
A patio measuring 20 ft by 12 ft, poured 4 inches thick, needs 2.96 cubic yards of concrete before waste. Here is every step, in order, with the units carried through so the arithmetic can be checked. 1. Convert the thickness to feet. 4 in ÷ 12 = 0.333 ft
2. Find the slab area. 20 ft × 12 ft = 240 ft²
3. Find the volume in cubic feet. 240 ft² × 0.333 ft = 80 ft³
4. Convert to cubic yards. 80 ft³ ÷ 27 = 2.96 yd³
5. Add a waste allowance. 2.96 yd³ × 1.05 = 3.11 yd³
Most suppliers deliver in quarter-yard increments, so the order here is 3.25 cubic yards. That slab weighs roughly 12,000 lb once cured, at about 4,050 lb per cubic yard.
Pouring the same patio 5 inches thick instead of 4 raises the requirement to 3.86 cubic yards before waste, a 25% increase from one extra inch. Thickness moves the number more than anything else, which is why the guide below is worth reading before ordering.
Choose slab thickness by project
Slab thickness is set by the load the slab carries and by local frost depth, not by preference. Four inches handles foot traffic and passenger cars. Six inches is the standard once trucks, heavy equipment or a hot tub are involved. Under-thickening a slab is the most expensive mistake on this list, because the fix is demolition.
| Project | Typical thickness | Notes |
|---|---|---|
| Sidewalk or walkway | 4 in (100 mm) | Often with fibre or light mesh |
| Patio or terrace | 4 in (100 mm) | 4-5 in where a grill or fire feature sits |
| Driveway, passenger cars | 4-5 in (100-125 mm) | 5-6 in for regular truck or SUV parking |
| Garage slab | 4-5 in (100-125 mm) | Thickened edge or turned-down perimeter where specified |
| Hot tub or spa pad | 6 in (150 mm) | Confirm the filled weight with the manufacturer |
| Shed or outbuilding pad | 4 in (100 mm) | Thickened edge or piers for heavier structures |
| Stairs and steps | 3-4 in tread | Volume follows total run × width × riser count |
| Strip footings | 8-12 in deep | Width and depth are code and soil dependent |
Footing dimensions are the one row here that should never be taken from a table. They depend on soil bearing capacity and frost line, both of which are local. Check with the building department before excavating.
Compare bagged concrete and ready mix
The break-even sits close to one cubic yard. Below that, bags are cheaper and there is no delivery minimum to meet. Above it, mixing by hand stops being realistic: one cubic yard is forty-five 80 lb bags, roughly 3,600 lb of material, and a pour that has to stay wet while you mix it.
Bag yields, which the calculator uses directly:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Ready-mix suppliers usually set a minimum load of one cubic yard and charge a short-load fee below three or four yards. That fee can add more than the concrete costs, so the real comparison for a two-yard pour is bags plus a rented mixer against ready-mix plus the short-load charge. Get the fee quoted before deciding.
One practical constraint that does not show up in any cost comparison: a slab poured in sections from a mixer will have cold joints where one batch set before the next arrived. For anything that needs to be monolithic, ready-mix is the answer regardless of volume.
Understand what concrete contains
Concrete is portland cement, water, sand and coarse aggregate. The cement and water form a paste; the paste coats the sand and stone and hardens around them. Strength comes from hydration, a chemical reaction, not from drying, which is why concrete cures underwater and why keeping a fresh slab damp makes it stronger.
The water-to-cement ratio governs almost everything. Less water gives higher strength and lower workability. More water is easier to place and measurably weaker. Adding water on site to make a stiff load easier to screed is the most common way a 4,000 psi mix ends up performing at 3,000.
Residential slabs typically specify 3,000 to 4,000 psi at 28 days. Driveways and exterior flatwork in freeze-thaw climates also need air entrainment, usually 5 to 7 percent, which creates microscopic voids that give freezing water somewhere to expand into. Order the mix by strength and exposure, not by the word "concrete".
Order extra for waste and spillage
Order five to ten percent more than the calculated volume. Subgrade is never perfectly level, forms bow outward under the weight of wet concrete, and some material is always lost to the chute, the wheelbarrow and the edges. Running short mid-pour is far more costly than the surplus.
Five percent covers a well-prepared slab on a compacted, screeded base. Ten percent is appropriate when the subgrade is uneven, when forms are long and lightly braced, or when pouring against ragged excavation. Footings dug by machine into soft ground can overrun by more than ten percent, because the trench walls slump.
The calculator applies 5% by default and the field is editable. It shows both figures, the geometric volume and the ordering volume, so the allowance is never hidden inside a single number you cannot check.
Handle sloped and irregular pours
Two adjustments cover almost every awkward pour. For a slab that changes thickness across its length, use the average of the minimum and maximum thickness. For a shape that is not one of the nine forms, split it into shapes that are, calculate each, and add the results.
For a sloped slab:
Average thickness = (minimum thickness + maximum thickness) ÷ 2A driveway apron running from 4 inches at the garage to 6 inches at the street is calculated at 5 inches throughout. The Concrete Calculator has a sloped-slab toggle that takes both figures and applies this automatically.
For an L-shaped patio, draw a line that divides it into two rectangles, measure each, and run the calculator twice. Add the two results. The same approach handles a rectangular patio with a semicircular bay: rectangle plus half a round slab. Use the Add another area control rather than doing the addition by hand, so the working stays visible.
Curved edges that follow no clean geometry can be approximated by a rectangle drawn to the average width. The error is small relative to the waste allowance you are adding anyway.
Frequently asked questions
How many bags are in one cubic yard of concrete?
One cubic yard of concrete takes 45 bags at 80 lb, 60 bags at 60 lb, 72 bags at 50 lb, or 90 bags at 40 lb. An 80 lb bag yields about 0.60 cubic feet once mixed, and there are 27 cubic feet in a cubic yard. Forty-five 80 lb bags weigh 3,600 lb, which is worth picturing before choosing bags over delivery.
How much does a cubic yard of concrete weigh?
A cubic yard of cured normal-weight concrete weighs about 4,050 lb, or just over two US tons. That is roughly 2,400 kg per cubic metre, or 150 lb per cubic foot. A 4-inch slab therefore adds about 50 lb per square foot to whatever it sits on, a figure that matters when pouring over a deck, a basement floor, or existing structure.
How much extra concrete should I order?
Order 5 to 10 percent extra. Use 5 percent on a level, well-compacted, properly formed slab, and 10 percent where the subgrade is uneven or the forms are long. Footings in soft or hand-dug ground can need more. Running out during a pour leaves a cold joint that permanently weakens the slab, so the surplus is cheap insurance.
How long does concrete take to cure?
Concrete is walkable in 24 to 48 hours, takes light vehicle traffic after 7 days, and reaches its specified design strength at 28 days. It continues gaining strength for years after that. Keep the surface damp for the first 7 days, particularly in heat or wind, because concrete that dries before it hydrates never reaches the strength it was mixed for.
What weather suits a concrete pour?
Pour between 50 °F and 85 °F (10 °C to 30 °C). Below 40 °F hydration slows sharply and fresh concrete can be damaged by freezing. Above 90 °F the surface sets before the body of the slab does, which causes plastic shrinkage cracking. In hot weather, pour early, dampen the subgrade, and cover the slab as soon as it is finished.
How do I calculate concrete for stairs?
Set the calculator to Steps and enter rise, run, width, platform depth and step count. The volume is a stack of blocks that grows by one tread with each step, so the total is width × run × rise × n(n+1)/2, plus the platform. A five-step run at 7 in rise, 11 in run and 36 in wide takes about 0.48 cubic yards including the landing.
Can I pour new concrete over old concrete?
Yes, provided the existing slab is sound, clean and at least 2 inches of new concrete goes on top. Anything thinner delaminates. Use a bonding agent, or leave a deliberate bond break and treat the new layer as an independent slab with its own control joints. Cracks in the old slab will reflect through the new one unless that separation is built in.
Does the calculator work in metric units?
Yes. Every dimension field has its own unit selector covering inches, feet, yards, centimetres and metres, and results are shown in cubic yards, cubic feet and cubic metres at the same time. Mixed units are handled directly, so a slab entered as 6 m long, 4 m wide and 100 mm thick needs no manual conversion.
Summary
The Concrete Calculator turns the dimensions of a slab, wall, footing, column, tube, step set or curb into the volume of concrete required, then converts that volume into bag counts, weight and cost. Volume in cubic feet divided by 27 gives cubic yards, the unit ready-mix is sold in.
Thickness drives the total more than any other dimension, so confirm it against the project guide before ordering: 4 inches for foot traffic and cars, 6 inches for heavy loads. Add 5 to 10 percent for waste, use bags below one cubic yard and ready-mix above it, and split awkward shapes into simple ones rather than estimating.