Concrete Slab Calculator

How to Use This Calculator

1. Select your project type (patio, shed base, driveway, garage floor, or custom) so the calculator can suggest a standard thickness, or choose custom and enter your own.
2. Enter the length and width of your slab. Switch to UK units if you're working in metres and centimetres.
3. Confirm or adjust the thickness. Standard residential slabs are 4 inches; driveways and garage floors typically need 5 to 6 inches.
4. Add a waste or overage percentage. We default to 10%, which covers spillage and an uneven subgrade.
5. Read your results instantly: cubic yards or cubic metres needed, the number of pre-mixed bags at common bag sizes, the estimated weight, and a cost estimate both in total and per square foot.
6. If your slab isn't a perfect rectangle, break it into smaller rectangular sections, run each one separately, and add the totals together.

How We Calculate Concrete for a Slab

A slab's volume comes down to three numbers: length, width, and thickness. The only step that trips people up is that thickness is almost always given in inches while length and width are given in feet, so everything has to be converted to the same unit before multiplying.

Formula (US): Volume (cubic feet) = Length (ft) x Width (ft) x (Thickness (in) / 12); Cubic Yards = Cubic Feet / 27

Worked example: A 12 ft by 20 ft driveway slab poured 5 inches thick works out as follows. 12 x 20 x (5 / 12) = 100 cubic feet. 100 / 27 = 3.7 cubic yards. Add 10% waste and round up to about 4.1 cubic yards for ordering.

Formula (UK): Volume (cubic metres) = Length (m) x Width (m) x Thickness (m)

Worked example: A 4 m by 5 m patio slab poured 100 mm (0.1 m) thick works out as follows. 4 x 5 x 0.1 = 2 cubic metres.

How Thick Should Your Slab Be

Slab thickness is the single biggest factor in how much concrete you'll need, and it should match how the slab will actually be used, not just be guessed at.

For anything supporting a structure or heavy, repeated vehicle weight, check your local building code before finalizing thickness. Frost depth and soil type both affect minimum requirements in many regions.

Subgrade and Gravel Base

A slab is only as good as what's underneath it. Most residential slabs sit on a compacted gravel base, typically 4 inches deep, which improves drainage and reduces the chance of cracking from ground movement or frost heave.

If you're calculating gravel for your base layer separately from the concrete itself, use the same length and width as your slab with the gravel depth in place of concrete thickness, then use our Gravel Calculator to convert that volume into tons for ordering. Skipping or skimping on the gravel base is one of the most common causes of slab cracking and settling over time.

Reinforcement: Wire Mesh vs Rebar for Slabs

Reinforcement doesn't change how much concrete you need, but it does affect how long your slab lasts and what it can support.

• Wire mesh works well for slabs 4 inches thick carrying light to moderate loads, such as patios, walkways, and sheds. It helps control minor surface cracking as the slab cures and settles.
• Rebar is recommended for driveways, garage floors, and any slab over 5 inches thick, or anything supporting vehicle weight. It provides much stronger structural support against cracking under load.
• Fiber-reinforced concrete is a newer option that blends synthetic fibers into the mix itself, often used alongside or instead of wire mesh on residential flatwork.

For sizing and spacing rebar correctly, use our Rebar Weight Calculator.

What Is a Monolithic Slab

A monolithic slab is a foundation style where the slab floor and its surrounding footing are poured together in a single continuous pour, rather than pouring the footing first and the slab separately. It's common in warmer climates without deep frost lines, since it's faster to build and uses less material than a separate footing and slab. If you're calculating concrete for a monolithic slab, add your footing's extra depth around the perimeter as its own section using the calculator above, then add that total to your main slab volume.

How Many Bags Do You Need for a Slab

A 10 ft by 10 ft slab at 4 inches thick needs about 1.23 cubic yards, which works out to roughly 56 bags at 80 lb. For anything over about 2 cubic yards, ready-mix delivery is usually more practical than mixing dozens of bags by hand.

How Much Does a Concrete Slab Weigh

Standard concrete weighs about 150 lbs per cubic foot, or roughly 4,050 lbs per cubic yard. A typical 10 by 10 ft slab at 4 inches thick weighs around 5,000 lbs once fully cured. This matters if you're planning a slab on an elevated structure, a deck, or anywhere weight load needs to be confirmed against what the structure below can support.

How Much Does a Concrete Slab Cost

Costs vary by region, site prep needs, reinforcement type, and finish (broom finish vs. stamped or decorative concrete costs more). Labor typically makes up more of the total cost than the material itself, especially on smaller pours. For UK pricing, expect roughly £60 to £100 per square metre for a standard residential slab, before labor. For a full cost breakdown across other project types, including driveways and foundations specifically, see our Foundation Cost Calculator.

For Strength Grade and Curing Time

Slabs follow the same PSI strength and curing guidelines as any other concrete pour. See our Concrete Calculator for the full PSI strength table by project type and the complete curing timeline.

Tips for Pouring a Slab

1. Never skip the gravel base. A compacted 4-inch gravel layer prevents most of the cracking and settling issues homeowners run into later.
2. Match your thickness to your project type, not just the cheapest option. Under-pouring a driveway is one of the most common and expensive mistakes to fix later.
3. Add control joints every 8 to 10 feet on larger slabs to guide where cracks form, rather than leaving it to chance.
4. Check the weather forecast. Avoid pouring in freezing temperatures or during heavy rain, both of which can weaken the cure.
5. Order 10% more than your calculated total for any slab, since running short mid-pour is far more costly than having a little extra.

Frequently Asked Questions