Concrete Calculator
This concrete calculator estimates the volume and weight needed for slabs, footings, columns, tubes, curbs and stairs, then converts it to 60 lb or 80 lb bags.
Slabs, Square Footings & Walls
Holes, Columns & Round Footings
Circular Slab or Tube
Curb & Gutter Barrier
Stairs
How to use the Concrete Calculator
Pick the shape that matches what you are pouring, then enter each dimension and choose its unit. You can mix units freely, so a slab can be 10 feet long and 4 inches thick without converting anything yourself.
Use Quantity for identical pours: four deck footings of the same size means a quantity of 4 rather than four separate calculations. Fractions are accepted, so 5 1/2 and 5.5 both work.
Concrete formula
Every shape reduces to a volume, which is then converted to cubic yards, cubic feet and cubic metres.
| Shape | Volume |
|---|---|
| Slab, square footing or wall | l × w × h × qty |
| Hole, column or round footing | π × (d ÷ 2)² × h × qty |
| Circular slab or tube | π × [(d₁ ÷ 2)² − (d₂ ÷ 2)²] × h × qty |
| Curb and gutter barrier | [(curb depth × curb height) + (flag thickness × (gutter width + curb depth))] × length × qty |
| Stairs | width × [run × rise × n(n−1) ÷ 2 + platform depth × n × rise], where n is the number of risers |
Weight and bag counts assume pre-mixed concrete at 133 lb/ft³ (2,130 kg/m³):
weight = volume × densitybags = total weight ÷ bag weight(60 lb or 80 lb)
Worked example
A patio slab 10 ft × 10 ft at 4 inches thick:
- Convert the thickness: 4 in ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.33 ft³
- Cubic yards: 33.33 ÷ 27 = 1.23 yd³
- Weight: 33.33 × 133 = 4,433 lb
- 80 lb bags: 4,433 ÷ 80 = 56 bags
At that size, ready-mix delivery is usually cheaper and far less work than mixing 56 bags by hand.
What the result means
The headline figure is cubic yards, because that is how ready-mix concrete is ordered and priced in the United States. Suppliers normally sell in quarter-yard increments with a minimum load, so round up to the next quarter yard when you place the order.
The bag counts are for pre-mixed bags from a home centre. They are practical up to roughly half a cubic yard; past that, a mixer truck is almost always the better call.
Common mistakes
- Ordering the exact number. Add 5 to 10 percent for spillage, uneven subgrade and over-excavated footings. Running short mid-pour creates a cold joint.
- Mixing up thickness units. A slab entered as 4 feet thick instead of 4 inches gives twelve times too much concrete. Check the unit dropdown next to each field.
- Forgetting quantity. Eight deck footings need a quantity of 8, not one footing calculated once.
- Ignoring subgrade variation. A slab poured over soft ground settles into low spots and consumes more concrete than the nominal thickness suggests.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags. One cubic yard is 27 cubic feet, and an 80 lb bag yields roughly 0.6 cubic feet of mixed concrete. For 60 lb bags the figure is about 60 bags per cubic yard.
How much concrete do I need for a 10×10 slab?
At the common 4 inch thickness, a 10 ft by 10 ft slab needs 1.23 cubic yards, or about 33.3 cubic feet. At 6 inches thick it rises to 1.85 cubic yards.
How much extra concrete should I order?
Most contractors add 5 to 10 percent. Use the lower end for a formed slab on a well prepared base and the higher end for footings dug in soft or irregular ground, where the excavation is rarely the exact size you planned.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways and anything carrying vehicles are usually 5 to 6 inches, with reinforcement. Always follow your local building code, which takes priority over any rule of thumb.
Does this calculator include waste?
No. It returns the exact geometric volume so you can see the true figure, then add your own allowance on top. That keeps the number honest and lets you choose the margin that suits the job.
What does concrete weigh per cubic foot?
Normal weight concrete runs between 140 and 150 lb per cubic foot once cured. This calculator uses 133 lb per cubic foot, the figure quoted for pre-mixed bagged concrete, so bag counts match what the bag labels claim. Lightweight and heavyweight mixes differ substantially.
Sources and assumptions
Volumes are exact geometry. Weight and bag counts use a pre-mixed density of 133 lb/ft³ (2,130 kg/m³), the value quoted for bagged concrete, and divide total weight by nominal bag weight.
Cured normal weight structural concrete is denser, typically 140 to 150 lb/ft³, so the weight of a placed slab will be higher than shown here. Mix design, aggregate and moisture all move the number.
This is an estimating tool. Confirm quantities with your supplier and follow your local building code for thickness, reinforcement and footing depth.