How to use the concrete slab calculator
- Choose how you measure the slab. Pick a rectangular slab, a round slab, or area and thickness if you already know the square feet.
- Enter the size and the thickness. Type the length and width, or the diameter, or the area. Then type the thickness, or press a quick thickness button for 4, 5 or 6 inches.
- Read the concrete you need. The result shows the cubic yards to order with waste, the area in square feet, the bags in four sizes and the weight.
What is a concrete slab calculator?
A concrete slab calculator is a tool that finds the volume of concrete in a flat slab. It multiplies the area of the slab by its thickness and gives the answer in cubic yards, the unit ready-mix concrete is sold in, and in bags for small pours.
Use it for any flat pour with one thickness: a patio, a shed pad, a garage floor, a walkway or an equipment pad. For a slab with a thicker edge, calculate the flat part and the edge strip separately and add them.
Features
- Three ways to enter a slab: length and width, diameter for a round slab, or area and thickness
- Quick thickness buttons for 4, 5 and 6 inches
- The area of the slab in square feet
- Cubic yards, cubic feet and cubic metres to order, with an adjustable waste allowance
- Bag counts for 40 lb, 50 lb, 60 lb and 80 lb bags
- Weight of the concrete in pounds, short tons and kilograms
- A live 3D preview and a plan view with your dimensions
- A PDF of the estimate and a WhatsApp share button
What you can use it for
- A patio, a walkway pad or a hot tub pad
- A shed base or a garage floor
- A driveway apron or a parking pad
- A round slab for a fire pit, a grain bin or a planter base
- A slab of known square footage from a plan or a quote
The concrete slab formula
| Slab | Formula |
|---|---|
| Rectangular slab | Length × width × thickness |
| Round slab | π × (diameter ÷ 2)² × thickness |
| Area and thickness | Area × thickness |
Put every dimension in feet before you multiply. Thickness is usually given in inches, so divide it by 12 first. The answer is in cubic feet. Divide by 27 to get cubic yards.
Worked example: a 12 ft by 12 ft slab, 4 inches thick
- Area: 12 ft × 12 ft = 144 square feet.
- Thickness in feet: 4 in ÷ 12 = 0.333 ft.
- Volume: 144 × 0.333 = 48 cubic feet.
- Cubic yards: 48 ÷ 27 = 1.78 cubic yards.
- With 10% waste: 48 × 1.10 = 52.8 cubic feet, which is 1.96 cubic yards.
- In 80 lb bags: 52.8 ÷ 0.60 = 88 bags.
These are the numbers the calculator shows when the page opens.
Concrete needed for common slab sizes
Cubic yards of concrete before waste, from the formula above.
| Slab size | Area | 4 inches thick | 6 inches thick |
|---|---|---|---|
| 10 ft × 10 ft | 100 sq ft | 1.23 yd³ | 1.85 yd³ |
| 10 ft × 12 ft | 120 sq ft | 1.48 yd³ | 2.22 yd³ |
| 12 ft × 12 ft | 144 sq ft | 1.78 yd³ | 2.67 yd³ |
| 12 ft × 20 ft | 240 sq ft | 2.96 yd³ | 4.44 yd³ |
| 20 ft × 20 ft | 400 sq ft | 4.94 yd³ | 7.41 yd³ |
| 24 ft × 24 ft | 576 sq ft | 7.11 yd³ | 10.67 yd³ |
| 30 ft × 40 ft | 1,200 sq ft | 14.81 yd³ | 22.22 yd³ |
Volume grows in step with thickness. A slab that is 6 inches thick needs one and a half times the concrete of the same slab at 4 inches.
Choosing the thickness
The calculator does not choose a thickness for you. The right thickness depends on what the slab carries, the ground under it and your local building code. Take the thickness from your plans, your building department or your contractor, then enter it here. The quick buttons only save typing.
What the calculator includes
- The slab has one thickness. Thickened edges, footings under the slab and slopes are not included.
- Each dimension can be up to 10,000 in its unit, and an area up to 1,000,000. Quantity can be 1 to 1,000 and waste 0 to 50%.
- Bag counts use the yields in the Quikrete Concrete Mix data sheet (product 1101, revised October 2022). Another mix can yield a different amount.
- The calculator does not include reinforcement, a gravel base or formwork.
- With area and thickness, the preview is a symbol, because the calculator does not know the outline of the slab.
Is it private?
Yes. The calculation and the PDF are made in your browser. The numbers you type are not sent to our server and are not saved.
Tips for an accurate estimate
- Check the depth in several places after grading. One low corner can change the order more than the slab size.
- Measure inside the forms, because that is the shape the concrete fills.
- If the slab has a thickened edge, calculate the edge as a separate strip and add it.
- Choose the slab thickness for the load it will carry, then enter it in inches.
- Order ready-mix by the rounded-up yard and confirm the total with your supplier the day before the pour.
Related calculators
Frequently asked questions
- How much concrete do I need for a 12x12 slab?
- A 12 ft by 12 ft slab that is 4 inches thick needs 48 cubic feet, which is 1.78 cubic yards. With 10% waste, order 1.96 cubic yards. At 6 inches thick it needs 2.67 cubic yards before waste.
- How do I calculate concrete for a slab?
- Multiply the length by the width to get the area in square feet. Multiply the area by the thickness in feet, which is the thickness in inches divided by 12. Divide the result by 27 to get cubic yards.
- How much concrete do I need for a 20x20 slab?
- A 20 ft by 20 ft slab is 400 square feet. At 4 inches thick it needs 4.94 cubic yards before waste. At 6 inches thick it needs 7.41 cubic yards.
- How much concrete do I need for a 30x40 slab?
- A 30 ft by 40 ft slab, 4 inches thick, is 1,200 × 0.333 = 400 cubic feet, which is 14.81 cubic yards. With 10% waste, order about 16.3 cubic yards.
- How much cement do I need for a slab?
- For a slab you order concrete, not cement on its own. Work out the cubic yards of concrete with this calculator, or the bags of a pre-mixed concrete such as Quikrete Concrete Mix, which already contains the cement, sand and gravel.
- Can I enter square feet instead of length and width?
- Yes. Choose area and thickness, then enter the area in square feet, square yards or square metres. This is useful for an L-shaped or irregular slab when you already know its area.
- How do I calculate a round concrete slab?
- Choose round slab and enter the diameter and the thickness. The formula is π times the radius squared times the thickness. A round slab 10 ft across and 4 inches thick needs 0.97 cubic yards before waste.
- Does the slab calculator tell me how thick the slab should be?
- No. Thickness depends on the load, the ground and your local building code. Take it from your plans or your building department and enter it in the calculator.
Sources
Every figure this calculator uses comes from the published sources below. The formulas are on this page, so any result can be checked by hand.
- Quikrete, Concrete Mix No. 1101: data sheet and product page. Bag yields: 0.30 ft³ (40 lb), 0.375 ft³ (50 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb); bags per pallet: 80, 56 and 42.
- Federal Highway Administration (FHWA), Report FHWA-HRT-05-057. Normal-weight concrete: 145 lb per cubic foot (1.96 tons per cubic yard).