Concrete Slab Calculator

1. Enter the slab length and width in feet and the thickness in inches.
2. Set the number of slabs if you are pouring more than one of the same size, and a 5 to 10 percent waste allowance.
3. Pick a bag size if you want a bag count for hand mixing.
4. Read the result: cubic yards is the ordering number; bags, weight and area help you plan the pour.

Here is what each result line means:

A concrete slab calculator turns the length, width and thickness of a flat slab into the amount of concrete to order. It is the sizing step for any flatwork: a patio, walkway, driveway, shed base or garage floor.

Concrete is sold by the cubic yard for ready-mix or by the bag for small pours, so the tool converts feet and inches into both and adds a waste allowance for an uneven sub-base. It handles rectangular slabs; the general concrete calculator in the related tools below covers footings, round piers and mixed shapes. To size what goes into the mix, use the concrete mix calculator, the cement calculator for the binder, or the water cement ratio calculator for mix water.

A slab is a rectangular box, so its volume is length times width times thickness. Because concrete is priced in cubic yards and one cubic yard is 27 cubic feet, the tool finds cubic feet and divides by 27.

1. Convert thickness from inches to feet by dividing by 12 (a 4 inch slab is 0.333 feet).
2. Multiply length by width by that thickness, then by the number of slabs, to get cubic feet.
3. Add the waste allowance, then divide by 27 for cubic yards and multiply by 0.0283168 for cubic meters.
4. Divide cubic feet by the bag yield for a bag count, and multiply by 150 pounds per cubic foot for the weight.

Suppose you are pouring a patio slab 12 feet by 12 feet at 4 inches thick, with 10 percent waste.

Calculation: thickness is 4 / 12 = 0.333 ft. Volume = 12 x 12 x 0.333 = 48 cu ft. In cubic yards that is 48 / 27 = 1.78 cu yd. Adding 10 percent waste gives 52.8 cu ft, or 1.96 cu yd, about 88 eighty-pound bags and roughly 3.96 short tons over 144 square feet.

The lesson: at nearly two cubic yards, this patio is on the edge where a short-load ready-mix delivery beats mixing 88 bags by hand.

Patios and Walkways

Commonly 4 inches, which suits foot traffic and garden furniture. This is the default most flatwork starts from.

Driveways and Parking Pads

Usually 5 to 6 inches for cars and light trucks, more for heavier vehicles, often with reinforcement. Thickness drives volume, so moving from 4 to 6 inches adds half as much concrete again.

Garage and Shed Floors

Typically 4 inches for a shed, 5 to 6 inches for a garage or workshop that parks or stores heavy loads.

These are common figures, not a specification. The required thickness, reinforcement and sub-base for a load-bearing slab come from your plans, local code or an engineer.

Thickness

Thickness multiplies the whole area, so it moves the volume more than any other input. Confirm it before you order, because an extra inch across a large slab is a lot of concrete.

Sub-base and Over-excavation

A soft or uneven sub-base soaks up concrete where it dips below grade. This is the main reason real slabs use more than the raw geometry, and why a waste allowance exists.

Thickened Edges and Footings

A slab with a thickened edge or an integral footing holds more than a flat rectangle. Work those parts out separately and add them.

Waste and Spillage

Concrete is lost to the forms, the barrow and finishing. A 5 to 10 percent allowance covers it; add more for a rough base or a long barrow run.

Most slabs past a cubic yard are easier and cheaper as ready-mix than as bags. The crossover is usually one to two cubic yards, which for a 4 inch slab is roughly 80 to 160 square feet.

Check the cubic yards above against this guide; for the full delivered price, the concrete cost calculator in the related tools prices a pour by the yard.

Ordering Ready-mix for Flatwork

Size the order in cubic yards and add waste before you call the supplier, so you neither run short nor pay for concrete you cannot place.

Pricing a Bag Job

For a small slab, the bag count tells you how many bags to buy and whether hand mixing is realistic.

Planning the Pour

The weight and area help you plan barrow loads, access, finishing and how many hands you need on the day.

1. Mixing Up Feet and Inches

Entering thickness in feet instead of inches shrinks the result twelvefold. Keep length and width in feet and thickness in inches, as the fields are labelled.

2. Forgetting a Waste Allowance

Ordering the exact geometric volume leaves nothing for an uneven sub-base or spillage. Running short mid-pour means a cold joint or a second delivery.

3. Ignoring a Thickened Edge

A slab with a thickened perimeter or a footing holds more than a flat rectangle. Add those parts so you do not under-order.

4. Using the Wrong Thickness for the Use

A driveway at patio thickness cracks under vehicles. Use the thickness your plans or code call for, not the thinnest that fits the budget.

5. Treating the Estimate as a Structural Spec

Volume is not strength. Reinforcement, control joints and mix for a load-bearing slab come from a code or an engineer, not from this tool.

The arithmetic is exact; the usefulness depends on the dimensions, the thickness and the waste allowance you enter.

A 12 by 12 foot slab at 4 inches thick needs about 1.78 cubic yards, or 1.96 cubic yards with a 10 percent waste allowance. That is roughly 88 eighty-pound bags, which is why most people order ready-mix at this size. Enter your thickness above for the exact figure.

Commonly 4 inches for patios and walkways, and 5 to 6 inches for driveways and garage floors that carry vehicles. The required thickness for a load-bearing slab comes from your plans or local code; this tool estimates volume, not the thickness.

It depends on the size and bag. A 10 by 10 foot slab at 4 inches is about 62 eighty-pound bags with waste; a 12 by 12 at 4 inches is about 88. The tool gives the bag count for your slab and bag size, rounded up.

Multiply length by width by thickness, with thickness in feet, to get cubic feet, then divide by 27 for cubic yards and add 5 to 10 percent waste. For a 12 by 12 foot slab at 4 inches that is 12 x 12 x 0.333 = 48 cubic feet, or about 1.78 cubic yards before waste.

About 150 pounds per cubic foot, or roughly 4,050 pounds a cubic yard, for normal-weight concrete. A 12 by 12 foot patio at 4 inches weighs close to 4 short tons. The tool shows the weight so you can plan the sub-base and access.

Yes, a 5 to 10 percent waste allowance. Flatwork loses concrete to an uneven sub-base, the forms and finishing, and running short mid-pour risks a cold joint. The allowance gives you that margin without guessing.

For a small slab under about a cubic yard, bags can be cheaper and simpler. Past one to two cubic yards, ready-mix usually wins on cost and effort, since a slab that size is dozens of bags to mix by hand.

No. It estimates the concrete for the slab itself. The gravel or crushed-stone sub-base under the slab is a separate material; size that from its own depth and area.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering for flatwork).
• American Cement Association (American Cement Association; what concrete and cement are).