Concrete Slab Calculator

How to calculate concrete for a slab

A slab is a flat box, so its volume is length × width × thickness. The only trap is units: slabs are measured in feet, thickness is given in inches, and ready-mix is sold in cubic yards. This one formula handles all three:

Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324

The 324 is 12 inches per foot × 27 cubic feet per cubic yard. Using it means you never convert the thickness by hand. For cubic feet instead, multiply length × width × thickness in inches and divide by 12.

1. Measure the inside of the forms, not the outside. Round fractions of an inch up to the next ¼ in.
2. Measure the thickness in three or four places. Bases are rarely flat. Use the average, or the deepest reading if the difference is more than ½ in.
3. Run the formula, or enter the numbers in the calculator above. It also gives cubic feet and cubic meters.
4. Add 10% for waste, then round up to the next ¼ yard for ready-mix, or to the next whole bag.

Concrete for common slab sizes

People rarely search for a formula. They search for their slab: “how many yards for a 24×24”, “bags for a 10×10”. This table answers the common sizes at 4 in thick. Every figure comes from the same calculation the tool uses.

At 6 in thick every figure is 1.5 times larger. A 24 × 24 ft garage slab at 6 in is 10.67 yd³, ordered as 11¾ yd³.

The same sizes at 4, 5 and 6 inches

Thickness changes the order more than most people expect. This table gives the exact volume and the order (10% extra, rounded up to ¼ yd³) for sizes commonly poured for patios, shed bases and garages.

How thick should a concrete slab be?

Thickness has the biggest effect on volume, and on cost. Going from 4 in to 6 in uses 50% more concrete.

Bag counts in the 8 in+ row are for exactly 8 in. These are common practice, not code. Garage floors, house slabs and anything structural must follow your plans and local building code. Bagged mixes such as QUIKRETE Concrete Mix are rated for pours 2 in thick or more, so thinner toppings need a different product.

2 in and 3 in slabs

A 2 in or 3 in slab saves concrete but leaves little margin. A 10 × 10 ft area at 3 in needs 0.93 yd³ (42 × 80 lb bags) against 1.23 yd³ at 4 in, and at 2 in it needs 0.62 yd³ (28 bags). Thin pours suit a topping over sound existing concrete or a light garden path, not a pad that will carry a shed, a car or a hot tub. If you are levelling an indoor floor rather than pouring a slab, use the coverage printed on the self-levelling product’s bag instead of this calculator.

Monolithic slabs: slab and footing in one pour

A monolithic slab (also called a turned-down or thickened-edge slab) pours the floor and its perimeter footing at the same time. To estimate it, calculate the flat slab, then add the edge as a beam around the perimeter, counting only the depth below the slab so the overlap is not counted twice.

Edge (ft³) = inside perimeter (ft) × edge width (ft) × depth below slab (ft)

Use the inside perimeter, which is the outside perimeter minus four times the edge width, so the corners are not counted twice.

A smaller 12 × 16 ft shed slab with the same edge works out at 2.37 yd³ for the slab plus 1.28 yd³ for the edge, 3.65 yd³ in all (order 4¼ yd³). Edge width, depth and any frost protection come from your plans and local code; the arithmetic stays the same.

Concrete pad calculator: shed, AC, hot tub and generator pads

A concrete pad is just a small slab, so a concrete pad calculator and a slab calculator use the same formula: length × width × thickness in inches ÷ 324. Enter the pad size and thickness above. The table gives common pad sizes, with 10% extra included in the order and the +10% bag count.

The thicknesses in the table are examples for the arithmetic, not a recommendation for your load. Many AC and generator makers sell or specify a pad, often a precast or composite one, and their installation manual sets the size and height. A hot tub maker’s pre-delivery guide states what base its tub needs, and a shed maker’s foundation sheet gives the pad size; the pad is often a few inches larger than the shed floor. Anything that carries a vehicle, a lift or heavy equipment needs a design from your plans or an engineer.

• Patio. A 12 × 16 ft patio at 4 in is the same volume as the 12 × 16 shed pad: 2.37 yd³, ordered as 2¾ yd³. See concrete patio cost to price it.
• Sidewalk. A 4 ft wide walk at 4 in uses 0.49 yd³ per 10 ft; a 4 × 50 ft walk is 2.47 yd³ (123 × 80 lb bags with extra).
• Small pads in bags. Every pad up to 4 × 4 ft at 4 in fits in ten or fewer 80 lb bags, so bags beat a ready-mix delivery there.

How to measure a concrete slab

Measure the inside of the forms (or the area you will form) in feet, then measure the depth from a string line to the base in several places. Those three numbers, length, width and average thickness, are all the calculator needs.

1. Length and width. Hook a tape on the inside face of one form board and read to the inside face of the other. Do both long sides and both short sides; if they differ, use the larger figure. Convert inches to decimal feet: 6 in is 0.5 ft, 3 in is 0.25 ft.
2. Check it is square. Measure both diagonals. Equal diagonals mean a true rectangle. Unequal diagonals mean the forms are racked, and you should fix them before the pour rather than adjust the estimate.
3. Thickness. Stretch a string tight across the tops of the forms and measure down to the base at three or four points, including the middle. Use the average, or the deepest reading if they differ by more than ½ in.
4. Odd shapes. Split the area into rectangles and circles, measure each, and total them with Add to project. Triangles are half the rectangle around them.
5. Metric. Measure in metres and centimetres and switch the calculator to Metric: a 4 × 4 m slab 10 cm thick is 1.6 m³.

Write the numbers on a sketch with each rectangle labelled. It makes the order easy to check when you call the plant, and it shows where a thickened edge or a step-down needs its own line.

Round, L-shaped and odd slabs

Choose Round slab in the calculator for circular pads such as fire-pit pads and round patios. The volume is π × radius² × thickness. At 4 in thick:

For L-shaped or irregular slabs, split the shape into rectangles, calculate each, and add them with Add to project. An L made of a 20 × 12 ft and a 10 × 8 ft rectangle at 4 in is 2.96 + 0.99 = 3.95 yd³ (order 4½ yd³). For a triangle, calculate the rectangle around it and halve the result.

Metric slab volumes

Outside the US, slabs are measured in metres and centimetres and ready-mix is sold by the cubic metre. The calculation is simpler because there is no 324:

Cubic metres = length (m) × width (m) × thickness (cm) ÷ 100

Switch the calculator to Metric to enter metres and centimetres directly. Weights use 150 lb/ft³ (about 2,400 kg/m³); your supplier can confirm the density of its mix.

How much does a concrete slab weigh?

About 50 lb per square foot at 4 in thick (75 lb at 6 in), using 150 lb/ft³ for normal concrete, so a 12 × 12 ft slab at 4 in weighs about 7,200 lb (3,266 kg). For other sizes, densities and debris tonnage, use the concrete weight calculator; weight does not tell you how much load a slab can carry.

Slope and drainage

Outdoor slabs are usually formed with a slight fall so water runs off. Many builders use about ⅛–¼ in per foot away from buildings; your plans or local code set the figure. A 12 ft patio at ⅛ in per foot drops 1.5 in from one side to the other, and at ¼ in per foot it drops 3 in. A 20 ft slab drops 2.5 in or 5 in.

Slope does not change the volume when the thickness stays the same, because the whole slab tilts with its base. It only adds concrete if the base is flat and you build the fall into the concrete. In that case measure the thickness at the high and low edges and enter the average.

Prepare the base before you measure

The base affects the order almost as much as the size. A gravel base ½ in low across a 20 × 20 ft slab adds 0.62 yd³ of concrete; 1 in low across a 24 × 24 ft slab adds 1.78 yd³.

• Remove topsoil and organic material down to firm subgrade.
• Spread compactable gravel and compact it in layers. The gravel calculator gives tons for the base.
• Set forms with the fall described above, so water drains away from buildings.
• Add a vapor barrier under interior slabs and reinforcement where your plans call for it. The rebar calculator sizes a grid.
• Re-measure depth from a string line stretched across the forms the day before the pour, and update the calculator if the base has moved.

Ready-mix or bags for a slab?

Ready-mix trucks deliver concrete already mixed and priced by the cubic yard. Bags are mixed on site. The volume usually decides it:

• Under about ½ yd³ (around 23 × 80 lb bags): bags are practical, and you avoid delivery charges.
• About ½–1 yd³: compare a small ready-mix delivery with buying 23–45 bags and renting a mixer.
• Over 1 yd³: ready-mix is usually far less work. One yard is 45 bags, about 3,600 lb of dry mix to lift and mix, and big pours mixed by hand risk cold joints.

Three yards in bags is 135 × 80 lb or 180 × 60 lb. Ask your plant about its minimum order and any short-load charge before you decide; both vary by supplier. To price either option, use the cost to pour a concrete slab calculator with your local quotes.

Ordering concrete for a slab

When you call the plant, give them the order quantity, not the exact volume. For the 12 × 12 ft pad above that means “2 yards”, not “1.78”. Have these ready:

• Quantity in cubic yards (or m³), already rounded up to the plant’s increment.
• Use and thickness, for example “4 in exterior patio slab”. The plant can suggest a mix for that use; your plans may specify one.
• Access. How close the truck can get to the forms. If it cannot reach them, ask about a pump or plan wheelbarrow runs.
• Timing. Book the pour for when the crew, tools and finishing help are on site, and check the forms and base the day before.

If the slab needs several trucks, ask the plant to space the loads so each arrives before the previous concrete starts to set. A 30 × 40 ft slab at 4 in is 16½ yd³ to order, so plan to place it in sections from one side to the other.

Common mistakes when estimating a concrete slab

1. Treating inches as feet. 12 × 12 × 4 gives 576, which is not cubic feet. Divide by 324 for yards or by 12 for cubic feet; the right answer is 48 ft³ (1.78 yd³).
2. Measuring the outside of the forms. Form boards are about 1½ in thick, so outside dimensions add about 3 in each way and overstate the pour.
3. Trusting a nominal 4 in. A base ½ in low across a 12 × 16 ft slab adds 0.30 yd³. Check depth in several places from a string line.
4. Forgetting the thickened edge. On a 24 × 24 ft monolithic slab the edge adds 2.27 yd³, about a quarter of the pour.
5. Ordering the exact volume. With no allowance, a small spill or form bulge leaves you short and creates a cold joint. Add 10%, then round up to the next ¼ yd³.
6. Mixing up bag sizes. 45 × 80 lb bags make a yard, but it takes 60 × 60 lb bags. Use the yield printed on the bag you buy.

Slab calculator vs other concrete calculators: which one to use

• This slab calculator: any flat rectangle or circle with one thickness, including patios, pads, sidewalks, shed bases and garage floors.
• Concrete driveway calculator: a long slab with an apron, flares and parking pads, usually 4–6 in thick.
• Concrete footing calculator: trench footings and perimeter footings poured separately from the slab.
• Concrete bag calculator: you already know the volume and only need counts for 40, 50, 60, 80 or 90 lb bags.
• Concrete steps calculator: poured stairs and landings, where each step is a different height.
• Concrete patio calculator and concrete sidewalk calculator: patio sizes, round and L-shaped patios, and walks priced by the foot.
• Concrete slab thickness calculator: you know how much concrete you have and want the thickness it gives.
• Concrete curb calculator: curb and gutter by the linear foot.
• Metric concrete calculator: metres, centimetres and cubic metres for every shape.