Concrete Slab Calculator (Cubic Yards and Bags)
Find how much concrete a slab, patio, driveway or footing needs. Enter length, width and thickness in any units and get cubic yards, cubic metres, waste allowance and the number of bags.
Leave blank for 1.
Spillage and uneven ground. Blank counts as 0.
In your own currency.
Enter the dimensions to see the volume.
About the concrete slab calculator
The Concrete Slab Calculator tells you how much concrete a flat pour needs: a patio, a garage floor, a shed base, a sidewalk, a driveway or a strip footing. Enter the length, width and thickness, and it returns the volume in cubic yards, cubic feet, cubic metres and litres, plus the number of 40 lb, 60 lb, 80 lb and 25 kg bags if you plan to mix your own. Each dimension can use its own unit, so a plan drawn in feet and a thickness given in inches or centimetres work together without any conversion.
Slabs are the simplest pour to estimate, but they are also where small errors add up fastest. A thickness that is off by half an inch across a large area is a lot of extra concrete, which is why the thickness field and the waste allowance both matter.
How to calculate a slab
Steps
- Choose the Rectangle shape for a standard slab. For an irregular outline, measure the area and use Known area instead.
- Enter the length and width and pick a unit for each.
- Enter the Thickness or height, for example 4 in for a walk or 6 in for a driveway.
- Keep the waste allowance at 10 percent unless the base is very flat or very rough.
- Read the order volume and, if you want it, enter a price to see an estimated cost.
Formulas
Convert the thickness to the same unit as the length and width before multiplying, or let the tool do it. One cubic yard is 3 ft by 3 ft by 3 ft, which is 27 cubic feet.
Choosing a slab thickness
Thickness is the biggest lever on volume, so it is worth choosing deliberately. These are typical values, not rules.
- 4 in is common for patios, walkways, shed floors and garage floors that only carry foot traffic and light loads.
- 6 in is common for driveways and areas that carry vehicles, and is often used where heavier loads are expected.
- Thicker edges under walls or at the perimeter are often formed as a footing, which you can estimate as its own rectangle and add to the slab.
Local codes, soil conditions and loading change what is appropriate, so confirm the thickness and any reinforcement with your supplier, local building department or an engineer for anything structural.
Worked example: a 10 ft by 10 ft patio, 4 in thick
Four inches is one third of a foot. The net volume is 10 x 10 x 0.3333 = 33.33 ft³, which is 1.23 yd³. With a 10 percent waste allowance the order is 36.67 ft³, which is 1.36 yd³, 1.04 m³ or 1,038.3 litres. If you mix from bags, that is 123 bags of 40 lb, 82 of 60 lb, 62 of 80 lb or 84 of 25 kg. At 6.50 per 80 lb bag, the 62 bags cost 403.00.
Worked example: the same slab at 6 in
Make the same 10 ft by 10 ft pad 6 in thick and the net volume rises to 50 ft³, about 1.85 yd³. Adding 10 percent gives 55 ft³, or 2.04 yd³. Going from 4 in to 6 in adds half again as much concrete, so the thickness decision matters more than most other settings on the page.
Worked example: a driveway pad
A pad 20 ft long, 12 ft wide and 6 in thick has a net volume of 20 x 12 x 0.5 = 120 ft³, or 4.44 yd³. With 10 percent waste the order is 132 ft³, or 4.89 yd³. A pour this size is normally ordered as ready-mix by the cubic yard, because hundreds of bags would be hard to mix and place before the first batch sets.
Using known area for irregular shapes
If your patio is an L shape or has curves, split it into rectangles or measure the total area with the Area Calculator, then pick Known area and enter the thickness. For example, 250 ft² at 3 in deep is 62.5 ft³. For a corner cut diagonally, the Triangle and Trapezoid shapes can handle the odd section while the Rectangle handles the rest.
Preparing the base and ordering extra
Concrete ends up filling every low spot, so a base that dips or a trench that is dug deeper than planned uses more than the flat maths says. Spillage and the mix left in the barrow also count. That is why the calculator starts at a 10 percent allowance. You may also need crushed stone under the slab, which you can estimate with the Gravel Calculator or the Road Base Calculator.
Accuracy and limits
These are estimates from the numbers you enter. They do not include reinforcement, curing products or forms, and they assume a uniform thickness. Ready-mix suppliers often have a minimum load and a short-load fee, so ask about both when you order a small volume. For structural slabs, footings under buildings or anything subject to code, have the design checked by a qualified professional.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
If your plan is in feet but your supplier quotes metres, the Length Converter and Volume Converter help. The Concrete Calculator covers columns, stairs and other shapes, and the Concrete Bag Calculator is the place to compare bag sizes.
Frequently asked questions
How much concrete do I need for a 10 ft by 10 ft slab?
At 4 in thick the net volume is 33.33 ft³ (1.23 yd³), and with a 10 percent allowance you should order about 1.36 yd³. At 6 in thick the net is 50 ft³, and the order with 10 percent waste is about 2.04 yd³.
How thick should a concrete slab be?
Typical choices are 4 in for patios, walks and light-use floors, and 6 in for driveways or areas with vehicle loads. Soil, climate, loads and local code all matter, so confirm with your supplier or a qualified professional.
How do I convert slab inches to feet?
You do not have to. Choose inches as the unit for the thickness field and feet for length and width, and the tool converts them for you. By hand, divide inches by 12, so 4 in is 0.3333 ft.
How do I estimate an irregular or curved patio?
Split it into rectangles and triangles and add the results, or measure the total area and use the Known area shape with the thickness. The tool then multiplies area by depth.
Do I need extra concrete for the footing or edges?
If the perimeter is thicker than the middle, calculate the thickened edge as its own rectangle and add it to the slab. A waste allowance on top covers low spots in the base and spillage.
Should I order ready-mix or use bags for a slab?
Small pads can be done with bags, but a 10 ft by 10 ft slab at 4 in already needs about 62 bags of 80 lb. Larger slabs and driveways are usually ordered as ready-mix by the cubic yard, often with a minimum load and short-load fee.