Concrete Slab Calculator
A concrete slab calculator finds cubic yards as length × width × depth (in) ÷ 12 ÷ 27. A 10 × 10 ft slab at 4 in needs 1.23 yd³, or 1.36 yd³ with 10% extra.
How much concrete do I need for a slab?
This concrete slab calculator multiplies length × width × thickness in feet, then divides by 27 to give cubic yards, the unit ready-mix is ordered in. The formula is cubic yards = length × width × (thickness in inches ÷ 12) ÷ 27. A 10 × 10 ft patio poured 4 inches thick needs 1.23 cubic yards.
Enter the slab length and width in feet, the thickness in inches, and the price per yard from your supplier. Tick the 10% box to cover uneven subgrade and spillage. The result is shown to two decimal places. Suppliers usually sell in quarter or half yards, so round up when you call.
The formula step by step
- Thickness to feet: inches ÷ 12. A 4-inch slab is 0.333 ft and a 6-inch slab is 0.5 ft.
- Cubic feet: length × width × thickness in feet.
- Cubic yards: cubic feet ÷ 27, because a cubic yard is 3 × 3 × 3 ft.
- With waste: cubic yards × 1.10.
- Cost: cubic yards × your price per yard.
Worked examples
- Patio: 10 ft × 10 ft, 4 in thick, $150 per yard
- Cubic feet: 10 × 10 × 0.333 = 33.3 ft³
- Cubic yards: 33.3 ÷ 27 = 1.23 yd³, costing $185.19
- With 10% extra: 1.36 yd³, costing $203.70
- Two-car garage floor: 24 ft × 24 ft, 5 in thick, $150 per yard
- Cubic feet: 24 × 24 × 0.4167 = 240 ft³
- Cubic yards: 240 ÷ 27 = 8.89 yd³, costing $1,333.33
- With 10% extra: 9.78 yd³, costing $1,466.67
Example 2 is close to the capacity of many ready-mix trucks, which commonly carry somewhere around 8 to 10 yards. Ask your supplier whether it fits in one load.
Slab coverage by thickness
| Thickness | Cubic yards per 100 sq ft | Sq ft covered by 1 yd³ | Typical use |
|---|---|---|---|
| 3.5 in | 1.08 | 93 | Interior floor over gravel |
| 4 in | 1.23 | 81 | Patio, walkway, shed pad |
| 5 in | 1.54 | 65 | Garage floor, car driveway |
| 6 in | 1.85 | 54 | Heavier driveway, hot tub pad |
| 8 in | 2.47 | 41 | RV or truck pad |
For thickness by use, see the concrete thickness calculator.
How to measure a slab
- Measure inside the forms. The forms set the finished edge. Use the inside dimension, not the size of the dug-out area.
- Check thickness in several spots. Pull a string across the forms and measure down to the base at a few points. A slab that is meant to be 4 inches but averages 4.5 inches uses 12.5% more concrete.
- Add thickened edges separately. Many garage and shed slabs have a deeper perimeter footing. Work out that strip as its own length × width × depth and add it in.
- Break up L-shapes. Split them into rectangles, run each one, and add the cubic yards.
Common slab mistakes
- Forgetting the base. A slab poured on soft or unprepared soil cracks and settles. Plan a compacted gravel layer with the gravel base calculator.
- Ordering exactly the calculated amount. Low spots in the base swallow concrete fast. Over 400 sq ft, a 1/2-inch dip across the slab is about 0.6 yd³.
- Leaving out reinforcement and joints. Rebar or mesh holds cracks tight. Control joints, cut to about a quarter of the slab depth, decide where cracks form. Size steel with the rebar calculator.
- Pouring in extreme weather. Hot sun, wind, or freezing nights spoil the surface. Plan the pour date and the curing.
For small pours, the concrete bag calculator converts these volumes into 60-lb or 80-lb bags. For a full installed price on a driveway, see the concrete driveway cost calculator. For a patio or pad, the concrete slab cost calculator adds labor, forms, and finishing to the material price.
Ready-mix or bags?
Bags make sense for a post footing, a small landing, or a pour you can mix and place before the first batch stiffens. Once a job passes about one cubic yard, you are mixing 45 of the 80-lb bags by hand or in a small mixer. A single truck delivers the same volume in one steady pour with consistent strength, so ready-mix usually wins on time and quality.
Frequently asked questions
How thick should a concrete slab be?
4 inches is the common thickness for patios, walkways, and shed pads that carry foot traffic and light loads. Driveways and garage floors usually go 5 to 6 inches, and pads for RVs or heavy trucks often go thicker. Local code and soil conditions can require more.
How many cubic yards is a 10 × 10 slab?
At 4 inches thick, 10 × 10 × 0.333 = 33.3 ft³, and 33.3 ÷ 27 = 1.23 cubic yards. At 6 inches it is 1.85 yd³. Add 5 to 10 percent before you order, so 1.36 yd³ at 4 inches with the 10% box ticked.
Why does this calculator return cubic yards instead of bags?
Ready-mix trucks are ordered and priced by the cubic yard. One cubic yard is 27 ft³, which equals 45 of the 80-lb bags at 0.6 ft³ each. For small pours, the concrete bag calculator converts the same volume into bag counts.
Is it normal to order more concrete than the calculated volume?
Yes. Subgrade is never perfectly flat, forms bow slightly, and some concrete stays in the chute and pump. Running short mid-pour forces a cold joint or a costly short-load second truck. Most people order 5 to 10 percent extra.
What's the minimum order for a ready-mix truck?
It varies by supplier. Many accept small orders but add a short-load fee below a set number of yards. Call ahead and ask for their minimum and fee. For pours under about one yard, bagged concrete is often simpler.
Does this include rebar, gravel base, or forms?
No. It covers concrete volume and its price only. A compacted gravel base, vapor barrier, rebar or wire mesh, and form lumber are separate items. Use the gravel base and rebar calculators for those.
Estimates only. Verify quantities with your supplier before purchasing.