Concrete Slab Calculator
0 yd³
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Enter the length, width and thickness of a concrete slab, pad or floor to get cubic yards for a ready-mix order or 80, 60, 50 or 40 lb bags. The math is length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 = cubic yards, so a 10' × 10' × 4" slab takes 1.23 yd³.
The concrete slab calculator has one shape and no tabs: three measurements and Calculate. Type them the way they read on the tape, 12' 6 1/2", down to 1/16. Thickness is read in inches unless you type the foot mark.
How to calculate a concrete slab
How to calculate concrete for a slab: multiply length by width by thickness, then convert to cubic yards. That's all a concrete slab material calculator does.
length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 = cubic yards
Dividing by 12 turns inches of thickness into feet. Dividing by 27 turns cubic feet into yards, since a cubic yard is 3 ft × 3 ft × 3 ft. Ready-mix plants sell by the yard, so that's the number to give them when you order yards of concrete for a slab.
For the 10' × 10' × 4" slab: 10 × 10 × 4 ÷ 12 = 33.33 ft³, and 33.33 ÷ 27 = 1.23 yd³. That's 0.94 m³ if you switch the site to metric. The line under the result also gives 5,000 lb, at 150 lb/ft³, the figure WSDOT uses in design. Tons and kilograms are on the page for the weight of a concrete slab.
Small pour? Buy as switches the big number to bags without pressing Calculate again, which is the other half of any concrete calculator for slab work. Bags are counted by yield and rounded up. Quikrete lists 0.60 ft³ per 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, so the same slab is 56 bags of 80 lb, 75 of 60 lb, 89 of 50 lb or 112 of 40 lb.
We add 10% by default, because running short in the middle of a pour costs more than a few spare bags. With 10% the slab comes to 1.36 yd³, or 62 bags of 80 lb, and the result is labeled incl. 10% extra. Set Extra % anywhere from 0 to 50; 0 means no extra.
Enter your price per yard or per bag to see the cost of the material.
For a thickened edge, add the edge as a footing on the Concrete Calculator and keep both lines in your project. My project shows the total per material at the bottom.
Slab size in square feet
Already know the area? Then the concrete slab calculator square feet math is one step shorter: square feet × thickness (in) ÷ 12 ÷ 27 = cubic yards. The 10' × 10' slab is 100 ft², and at 4" it's the same 1.23 yd³.
Turned around, one yard of concrete covers 81 ft² at 4". At 3½" it covers 92.6 ft². At 6", only 54 ft².
The tool asks for length and width. For an L-shaped slab, split it into rectangles, calculate each one and add both to your project. Odd shapes are easier to measure first with the square footage calculator.
Concrete pad calculator
A pad is a small slab, so the concrete pad calculator is the same three fields. Hot tub makers list a minimum thickness — check the manual for your model. For a shed or an AC unit, use the same slab math with the thickness from your plans.
Two pads the same size? Enter one and set Quantity to 2. Two 10' × 10' × 4" pads take 66.67 ft³, or 2.47 yd³: 112 bags of 80 lb with no extra. They go into My project as one line, 2 slabs 10' × 10'.
Concrete floor slab
A concrete floor in a garage or basement is a slab on ground, and this page works as a concrete floor calculator with the same fields. The International Residential Code sets 3½ in as the minimum for a concrete floor on ground. Your plans may call for more, so check your local code before you set the thickness.
How thick should a concrete slab be?
Look under the Thickness field. The hint there reads 4" patio, walk · 4" driveway, 5–6" for trucks and RVs, so the concrete slab thickness calculator puts the answer right next to the number you enter.
Where those numbers come from: Quikrete's patio and sidewalk projects use 4". For a driveway, Concrete Network gives 4" minimum for passenger cars and 5–6" for heavy trucks or RVs.
Thickness moves the order fast. A yard covers 81 ft² at 4" and 54 ft² at 6", so going from 4" to 6" takes half again as much concrete.
Concrete for common slab sizes
All slabs 4" thick. Bags are 80 lb at Quikrete's 0.60 ft³ per bag.
| Slab size | Area | Yards, no extra | Yards, 10% extra | 80 lb bags, no extra | 80 lb bags, 10% extra |
|---|---|---|---|---|---|
| 10' × 10' | 100 ft² | 1.23 yd³ | 1.36 yd³ | 56 | 62 |
| 10' × 12' | 120 ft² | 1.48 yd³ | 1.63 yd³ | 67 | 74 |
| 12' × 12' | 144 ft² | 1.78 yd³ | 1.96 yd³ | 80 | 88 |
| 20' × 20' | 400 ft² | 4.94 yd³ | 5.43 yd³ | 223 | 245 |
| 20' × 30' | 600 ft² | 7.41 yd³ | 8.15 yd³ | 334 | 367 |
| 30' × 40' | 1,200 ft² | 14.81 yd³ | 16.30 yd³ | 667 | 734 |
| 40' × 60' | 2,400 ft² | 29.63 yd³ | 32.59 yd³ | 1,334 | 1,467 |
A different thickness or size? Enter it in the tool above.
Gravel base under a slab
The International Residential Code calls for a 4" base of clean graded sand, gravel or crushed stone where the slab is below grade. Check your local code.
This page doesn't count the stone. The gravel calculator gives the base in tons or yards. Add both results to your project and the slab and its gravel sit in one list, ready to print. The numbers you enter stay in your browser; no account needed.
FAQ
How much concrete is needed for a 20x20 pad?
At 4" thick, a 20x20 slab or pad takes 4.94 yd³, or 5.43 yd³ with 10% extra.
How many yards of concrete do I need for a 12x12 slab?
1.78 yd³ at 4" thick, or 1.96 yd³ with 10% extra.
How many yards is a 40x60 concrete slab?
At 4" thick, 29.63 yd³, or 32.59 yd³ with 10% extra.
How many bags of concrete for a 12x12 slab?
80 bags of 80 lb at 4" with no extra: 48 ft³ ÷ 0.60, using Quikrete's 0.60 ft³ per 80 lb bag. With 10% extra it's 88. Other bag sizes are on the concrete bag calculator.
Results are estimates for ordering material. Check the quantity with your supplier before you order.