Concrete Slab Calculator
Enter the dimensions of your slab or footing and get the concrete volume in cubic yards or cubic meters right away. The visual editor resizes a sketch as you type, and an optional section estimates a compacted gravel base under the slab — a common detail in many regions. Everything here is an estimate for planning; the thickness, reinforcement and whether you need a base at all come from your plans, your soil and your local building authority.
Step by step
How to use this calculator
- Pick the shape. A slab is a flat pad measured by length and width in feet and thickness in inches. A footing is a long strip measured by length in feet and by width and depth in inches.
- Fill in the fields on the drawing. The sketch resizes as you type, so you can quickly see whether the proportions look like the real thing.
- Open the gravel base section if you want one. Enter a base depth (4 inches is a common example) and a density in tons per cubic yard from your supplier's price list.
- Read the results. You get the concrete volume, the gravel volume and weight, and how much area one cubic yard covers at your slab thickness. For other shapes, open the main concrete calculator.
Formulas
- Slab: volume in ft³ = length (ft) × width (ft) × thickness (in) ÷ 12. Cubic yards = ft³ ÷ 27.
- Footing: volume in ft³ = length (ft) × (width in ÷ 12) × (depth in ÷ 12).
- Gravel base: ft³ = area (ft²) × base depth (in) ÷ 12; weight = yd³ × density (tons per yd³).
- Coverage: one cubic yard spread at a given depth covers about 324 ÷ depth in inches, in square feet. At 4 inches that is about 81 ft².
- Metric: length (m) × width (m) × thickness (m) gives cubic meters directly; 1 m³ ≈ 1.31 yd³ and 1 yd³ ≈ 0.76 m³.
Worked example
Worked Example
Backyard patio, 10 × 10 ft, 4 inches thick
10 × 10 × 4 ÷ 12 = 33.3 ft³, which is 33.3 ÷ 27 = 1.24 cubic yards. With a 10% allowance for uneven ground, plan on roughly 1.36 yd³ — a volume where many people mix bags or order a short truck load.
Strip footing, 20 ft long, 16 in wide, 8 in deep
20 × (16 ÷ 12) × (8 ÷ 12) = 17.8 ft³, or 0.66 yd³. Adding 10% brings it to about 0.72 yd³, still within bag territory for a patient weekend crew.
Metric shed base, 3 × 3 m, 10 cm thick
3 × 3 × 0.10 = 0.9 m³, which is about 1.18 cubic yards. If your supplier quotes in yards, ask for a little extra rather than rounding down.
How much gravel under a slab?
A compacted gravel base under a slab does two jobs: it helps water drain away from the concrete, and it gives the pour a uniform surface to sit on instead of soft or rocky soil. Common practice for patios and shed pads is around 4 inches of compacted gravel; driveways and poor soils often call for more. In cold climates the right depth also depends on frost conditions, which vary a lot by region.
The calculator turns your base depth into a volume and an approximate weight, using a gravel density you can edit. Densities between 1.3 and 1.5 short tons per cubic yard are common examples for crushed gravel — ask your supplier what they sell, because it varies by material and region. Having the tonnage handy makes it easier to compare deliveries priced by the ton against those priced by the yard.
How to square a slab form
Slab forms are usually rectangles, and a rectangle that is not square will show up as odd cuts in whatever you build on top. The classic check needs only a tape measure:
- Set the form boards and stake the four corners.
- Measure 3 ft along one side from a corner and mark it. Measure 4 ft along the other side from the same corner and mark it.
- The distance between the two marks should be 5 ft. If it is not, nudge the boards and measure again — this is the 3-4-5 rule.
- On larger forms, measure both diagonals corner to corner. Equal diagonals mean the form is square.
Recheck the diagonals after staking and before the pour, since kicking a brace is easier than fixing a crooked slab.
Knowledge base
Frequently Asked Questions
How thick should a concrete slab be?
Common examples are around 4 inches for patio and shed pads, and thicker for driveways and anything carrying vehicles. The right thickness depends on the load, the soil and local requirements, so treat any number here as a planning example and confirm with your plans or local building authority.
How much gravel do I need under a 10 × 10 slab?
At a 4-inch base depth, a 10 × 10 ft slab sits on 100 ft², which is 33.3 ft³ or about 1.23 cubic yards of gravel. Using an example density of 1.4 tons per cubic yard, that is roughly 1.7 tons. Enter your own depth and your supplier's density in the gravel section for a closer estimate.
Does this calculator add a wastage allowance?
This page shows the plain volume so you can see the base number. The main concrete calculator on the home page includes a wastage slider — 5 to 10% extra is a common example for spillage, uneven subgrade and over-excavation.
Should I use bags or a ready-mix truck for a slab?
Small pads are often poured from bags, while anything over a yard or two usually goes to a truck. Where the line sits depends on your prices, your time and your back. The concrete cost calculator compares both options with your own numbers.
My slab is not a simple rectangle. What do I do?
Split it into rectangles, run each one through the calculator as its own slab, and add the volumes. For projects with several different shapes at once, the home calculator accepts multiple pours in one project.
How do I convert between cubic yards and cubic meters?
Multiply cubic yards by 0.76 to get cubic meters, or multiply cubic meters by 1.31 to get cubic yards. The calculator shows both units so you can check whichever way your supplier quotes.
This calculator is an estimating tool for planning only. Quantities depend on site conditions, wastage and suppliers, and are not a quotation. Thickness, footing depth, reinforcement and mix strength must come from your plans, an engineer or your local building authority.
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