How to use the concrete slab calculator calculator
- 1
Measure the slab area
Measure the length and width at the inside face of the forms, because that is the space the concrete will actually fill. For a circular pad, measure the diameter through the center. If you already know the square footage, choose the known-area option instead.
- 2
Split an irregular slab into sections
An L-shaped or stepped slab is easier and more accurate as two or three rectangles than as one averaged shape. Add a section for each part and the calculator totals them before converting.
- 3
Enter the slab thickness
Type the thickness from your project specification and pick its unit. Inches and feet both work. This calculator does not suggest a thickness, because the right value depends on loads, subgrade, reinforcement, and local code.
- 4
Add an allowance if you want one
An optional percentage covers an uneven subgrade, slight form movement, and spillage. Choose a figure that reflects how well the base was prepared and how confident you are in the measurements.
- 5
Add bags or pricing if you need them
Enter the yield printed on your concrete bag for a bag count, or your quoted price per cubic yard or per bag for a cost estimate. Enter both to see the arithmetic difference between the two routes.
Formula
Slab cubic yards = (length(ft) x width(ft) x thickness(ft)) / 27
Convert the thickness from inches to feet by dividing by 12. The one-step version is square feet times thickness in inches divided by 324.
Worked example
A garage slab 24 feet by 24 feet, poured 5 inches thick
- 1Find the area: 24 ft x 24 ft = 576 square feet.
- 2Convert the thickness: 5 in / 12 = 0.4167 ft.
- 3Find the volume: 576 sq ft x 0.4167 ft = 240 cubic feet.
- 4Convert to cubic yards: 240 / 27 = 8.89 cubic yards.
- 5Confirm with the shortcut: 576 x 5 / 324 = 8.89 cubic yards.
The slab needs about 8.89 cubic yards, or 240 cubic feet, before any allowance.
Worked examples
Circular slab
A round pad 10 feet in diameter at 5 inches thick
- 1Radius: 10 / 2 = 5 ft.
- 2Area: pi x 5^2 = 78.54 square feet.
- 3Thickness in feet: 5 / 12 = 0.4167 ft.
- 4Volume: 78.54 x 0.4167 = 32.72 cubic feet.
- 5Cubic yards: 32.72 / 27 = 1.21 cubic yards.
The circular slab needs about 1.21 cubic yards.
Slab plus a landing
The same 24 ft x 24 ft slab with a 4 ft x 12 ft landing, both 5 inches thick
- 1Main slab area: 24 x 24 = 576 square feet.
- 2Landing area: 4 x 12 = 48 square feet.
- 3Total area: 576 + 48 = 624 square feet.
- 4Apply the shortcut once: 624 x 5 / 324 = 9.63 cubic yards.
Together the two sections need about 9.63 cubic yards. Adding the areas before converting avoids the rounding error you get from converting each piece separately.
How to measure a slab
Measure to the inside face of the forms. The forms define the volume that concrete will occupy, and measuring to the outside adds the thickness of the form boards to every dimension. On a large slab that difference is small in percentage terms but can still be a meaningful fraction of a cubic yard.
Take the measurement in at least two places along each dimension. Forms rarely end up perfectly parallel, and a slab that is 24 feet at one end and 24 feet 4 inches at the other is common. Using the larger figure, or the average of the two, is safer than assuming the drawing was followed exactly.
Check the diagonals if the slab is meant to be square or rectangular. Equal diagonals confirm the corners are square, which means your length times width figure is genuinely the area rather than the area of a parallelogram that happens to share those side lengths.
Thickness and volume move together
Volume scales directly with thickness, which makes thickness the single most sensitive input in a slab estimate. Going from 4 inches to 5 inches on the same footprint is a 25 percent increase in concrete. On a 576 square foot slab that is the difference between 7.11 and 8.89 cubic yards, close to two full cubic yards.
That sensitivity cuts both ways. An error in the area of a foot or two changes the answer slightly, but a thickness entered in inches where feet were expected changes it by a factor of twelve. If a result looks implausible, check the thickness field first.
It also means the thickness on your drawings is the thickness to enter, not a value borrowed from a similar-looking project. YardCalc does not recommend slab thicknesses. What a slab needs depends on the loads it will carry, the subgrade beneath it, the reinforcement in it, the local climate, and the applicable code, all of which belong to your project specification or a qualified designer.
Slabs poured in several sections
Many slabs are not a single rectangle. A patio may wrap around a corner of the house, a shed slab may have a thickened apron at the door, a driveway may open out where it meets the street. Each of these is straightforward once you stop trying to describe it with one length and one width.
Divide the outline into rectangles, triangles, and circle segments that you can measure directly, then let the calculator add them. If the same shape repeats, such as four identical equipment pads, use the quantity field on that section rather than adding it four times.
One caution applies to slabs that are thicker in places. A slab with a thickened edge or an integral footing is really two volumes: the main slab area at slab thickness, plus the extra depth along the perimeter. Calculate the thickened part separately, either as its own section here or with the concrete footing calculator, and add it to the slab figure.
It is worth writing the sections down as you measure rather than holding them in your head. A slab that is measured in one visit and ordered in another is exactly where a landing or a step gets dropped from the total, and a forgotten section is a far more common cause of running short than an arithmetic error. Keeping the list also means you can answer a supplier who asks how the figure was arrived at, and you can revise one section later without remeasuring the whole slab.
Ready-mix and bags for a slab
Larger slabs are commonly placed from a ready-mix truck, because a continuous placement avoids cold joints between batches and because mixing that volume by hand is impractical. Smaller pads are often done with bags. Where the crossover sits depends on your site access, how many people are working, and what you can get delivered.
The calculator prices both from figures you enter. For ready-mix it needs your quoted price per cubic yard; for bags it needs the yield printed on the bag and the price per bag. Nothing is assumed, because bag yields differ by product and concrete pricing varies by mix, quantity, and location.
One practical point that the arithmetic does not show: a slab that must be placed in one continuous pour sets a floor on how fast the concrete has to arrive. That is a scheduling question for your supplier rather than a calculation, and it is worth settling before the order is confirmed. See the ready-mix concrete calculator for the ordering side.
Common mistakes
The recurring problems on slab estimates:
- Measuring to the outside of the forms instead of the inside face.
- Entering the thickness in inches without converting, or entering feet where inches were meant.
- Treating an L-shaped slab as one averaged rectangle.
- Ignoring a thickened edge or integral footing, which can add a surprising amount on a small slab.
- Assuming a bag yield rather than reading it off the bag.
- Ordering the exact calculated figure with no allowance on a subgrade that is visibly uneven.
- Rounding each section separately and then adding the rounded results.
Slab area to concrete cubic yards by thickness
Cubic yards for a given slab area at a given thickness. Thickness values are columns for reference only and are not recommendations.
| Area | 2 in | 3 in | 4 in | 5 in | 6 in | 8 in | 12 in |
|---|---|---|---|---|---|---|---|
| 25 sq ft | 0.154 | 0.231 | 0.309 | 0.386 | 0.463 | 0.617 | 0.926 |
| 50 sq ft | 0.309 | 0.463 | 0.617 | 0.772 | 0.926 | 1.23 | 1.85 |
| 100 sq ft | 0.617 | 0.926 | 1.23 | 1.54 | 1.85 | 2.47 | 3.7 |
| 150 sq ft | 0.926 | 1.39 | 1.85 | 2.31 | 2.78 | 3.7 | 5.56 |
| 200 sq ft | 1.23 | 1.85 | 2.47 | 3.09 | 3.7 | 4.94 | 7.41 |
| 300 sq ft | 1.85 | 2.78 | 3.7 | 4.63 | 5.56 | 7.41 | 11.11 |
| 500 sq ft | 3.09 | 4.63 | 6.17 | 7.72 | 9.26 | 12.35 | 18.52 |
| 750 sq ft | 4.63 | 6.94 | 9.26 | 11.57 | 13.89 | 18.52 | 27.78 |
| 1,000 sq ft | 6.17 | 9.26 | 12.35 | 15.43 | 18.52 | 24.69 | 37.04 |
Frequently asked questions
- How much concrete do I need for a 12 x 12 slab?
- A 12 by 12 foot slab is 144 square feet. At 4 inches that is 144 x 4 / 324, which is 1.78 cubic yards. At 5 inches it is 2.22 cubic yards, and at 6 inches it is 2.67 cubic yards. Use the thickness your project specifies.
- How many bags of concrete for a slab?
- Work out the slab volume in cubic feet, then divide by the yield printed on the bag and round up. A 144 square foot slab at 4 inches is 48 cubic feet, which is 80 bags at a 0.6 cubic foot yield or 107 bags at a 0.45 cubic foot yield. Read the yield off the bag you plan to buy.
- How thick should a concrete slab be?
- This calculator does not prescribe a thickness. Slab thickness depends on the loads, the subgrade, the reinforcement, the climate, and local code, so it should come from your project drawings, your designer, or your building department.
- Do I measure inside or outside the forms?
- Inside. The inside faces define the space the concrete will fill. Measuring outside adds the form thickness to each dimension and overstates the volume.
- How do I handle a slab with a thickened edge?
- Calculate the main slab at its own thickness, then calculate the thickened perimeter as a separate volume and add it. The footing calculator handles the perimeter part if it is a defined strip.
- Can I calculate a circular slab here?
- Yes. Choose the circle shape and enter the diameter measured through the center. The calculator uses pi times the radius squared for the area, then multiplies by the thickness.
Slab volume is calculated from your measurements using exact geometry, with one cubic yard treated as exactly 27 cubic feet. No slab thickness, bag yield, or price is assumed. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.