Concrete calculator in cubic yards and bags
Enter the length, width and thickness of a slab, or the diameter and depth of a round column, and how many you need. A 10 × 10 ft slab 4 inches thick takes 1.23 cubic yards; with 10% extra that is 1.36 cubic yards, or about 62 bags of 80 lb mix.
Inputs used by this calculator
Concrete Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Shape | Slab, wall or footing (rectangular) | Slab, wall or footing (rectangular) · Round column or post hole | Supplies the shape quantity used by the published equation or result breakdown. |
| Length (ft or m) | 10 | 0 or greater | Supplies the length (ft or m) quantity used by the published equation or result breakdown. |
| Width (ft or m) | 10 | 0 or greater | Supplies the width (ft or m) quantity used by the published equation or result breakdown. |
| Thickness (in or cm) | 4 | 0 or greater | Supplies the thickness (in or cm) quantity used by the published equation or result breakdown. |
| Diameter (in or cm) | 12 | 0 or greater | Supplies the diameter (in or cm) quantity used by the published equation or result breakdown. |
| Height or depth (ft or m) | 4 | 0 or greater | Supplies the height or depth (ft or m) quantity used by the published equation or result breakdown. |
| How many | 1 | 1 to 10000 | Supplies the how many quantity used by the published equation or result breakdown. |
| Extra allowance (%) | 10 | 0 to 100 | Supplies the extra allowance (%) quantity used by the published equation or result breakdown. |
| Units | US: feet and inches | US: feet and inches · Metric: meters and centimeters | Selects the unit or conversion path used by Concrete Calculator. |
How much concrete a slab needs
Multiply length by width by thickness, with the thickness converted from inches to feet. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet. Divide by 27 to get cubic yards, the unit ready-mix suppliers use: 1.23 cubic yards.
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12cubic yards = cubic feet ÷ 27Round columns and post holes
For a round form, tube or post hole, the volume is π times the radius squared times the height. A 12-inch column 4 feet tall holds about 3.14 cubic feet. Enter how many identical holes or columns you are filling to get the total.
volume = π × (diameter ÷ 2)² × heightBags of premixed concrete
Premixed bags are sold by weight but used by volume. QUIKRETE's data sheet lists an 80 lb bag of Concrete Mix at about 0.60 cubic feet, a 60 lb bag at about 0.45 and a 40 lb bag at about 0.30. That is about 45 bags of 80 lb mix per cubic yard. Other brands and mixes yield differently, so check the bag.
How much extra to order
Uneven ground, form flex and spillage use more concrete than the drawing shows. An allowance of 5 to 10 percent is common for slabs on grade; use more if the subgrade is rough. Running short partway through a pour is far more costly than a little left over.
Bags or ready-mix?
Bags suit small jobs such as post holes, steps and small pads. For larger slabs, mixing dozens of bags by hand is slow, and a ready-mix delivery in cubic yards is usually easier. Ask local suppliers about minimum orders and short-load fees before deciding.
What this estimate does not cover
The calculator gives volume only. Thickness, reinforcement, base preparation, control joints and curing depend on the load, soil and local code; follow the product instructions and your building department's requirements.
4-inch slabs: volume and 80 lb bags
| Slab | Cubic yards | With 10% extra | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 1.78 | 1.96 | 88 |
| 10 × 20 ft | 2.47 | 2.72 | 123 |
| 20 × 20 ft | 4.94 | 5.43 | 245 |
| 24 × 24 ft | 7.11 | 7.82 | 352 |
| 30 × 40 ft | 14.81 | 16.30 | 734 |
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, about 56 bags of 80 lb mix, or about 62 with a 10% allowance.
How many 80 lb bags make a cubic yard?
About 45, at roughly 0.60 cubic feet per bag.
How do I convert cubic feet to cubic yards?
Divide by 27.
How thick should a patio slab be?
Four inches is common for patios and walkways; driveways and structural slabs often need more. Follow local code.
How much extra concrete should I order?
5 to 10 percent is common; more for rough or sloped ground.
Can I use metric units?
Yes. Choose metric to enter meters and centimeters and get cubic meters.
Does this include rebar or gravel base?
No. It estimates the concrete volume only.
What to keep in mind
This is a volume estimate, not structural advice. Bag counts use typical yields (80 lb ≈ 0.60 cu ft); check the yield printed on the bag you buy.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.