How to figure concrete for a slab
- Measure the length and width in feet. For an L-shaped slab, split it into two rectangles and add both as separate slabs.
- Pick a thickness in inches. Measure the form depth in a few spots, because subgrade is rarely perfectly flat.
- Square feet = length × width.
- Cubic yards = square feet × thickness (in) ÷ 324.
- Add waste if you want a margin, then round up to how your supplier sells.
Why 324? A cubic yard is 27 cubic feet, and a foot is 12 inches, so 27 × 12 = 324. The ÷ 81 shortcut only works at exactly 4 inches. The formula guide walks through the algebra.
Worked example: 10 × 10 ft slab, 4 in thick
- Area: 10 × 10 = 100 sq ft.
- Cubic yards: 100 × 4 ÷ 324 = 1.23 yd³.
- Cubic feet: 1.23 × 27 = 33.33 ft³.
- 80 lb bags: 33.33 ÷ 0.60 = 55.6, which rounds up to 56 bags.
Common slab sizes at a glance
Net volume, no waste, 80 lb bags at 0.60 ft³ each .
| Slab | Thickness | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 4 in | 33.33 | 1.23 | 56 |
| 10 × 10 ft | 6 in | 50.00 | 1.85 | 84 |
| 12 × 12 ft | 4 in | 48.00 | 1.78 | 80 |
| 16 × 16 ft | 4 in | 85.33 | 3.16 | 143 |
| 20 × 20 ft | 4 in | 133.33 | 4.94 | 223 |
| 24 × 24 ft | 4 in | 192.00 | 7.11 | 320 |
At these sizes a ready-mix truck usually beats bags. Compare both with your own prices in the concrete cost calculator.
Typical slab thickness
| Use | Often cited |
|---|---|
| Sidewalk, patio, shed pad | 4 in |
| Driveway, garage floor | 4–6 in |
| Slab-on-ground minimum (IRC) | 3.5 in, as cited by some pages |
These are typical figures, not code. Heavy vehicles, poor soil, or frost may call for more. Check your plans and your local building department.
Gravel base under the slab
A compacted gravel base helps drainage and gives an even surface to pour on. Base volume is the slab area times base depth. A 10 × 10 slab on 4 inches of gravel needs 100 × 4 ÷ 324 = 1.23 yd³ of gravel before compaction. Gravel is often sold by the ton, and tons per yard vary by material, so ask the yard .
Rebar grid for a slab
Open the rebar panel to lay out a simple grid on each rectangular slab. You set bar size, spacing, and edge cover. It returns bars in each direction, linear feet, weight, 20 ft sticks, and tie points. It does not add lap splices unless you enter a lap length. It is a takeoff aid, not a structural design.
Concrete slab estimator tips
- Thickened edges and footings under the slab are extra volume. Add them as their own section in the main concrete calculator.
- Low spots in the subgrade eat concrete. A 4 in slab poured on a base that dips to 4.5 in uses 12.5% more in that area.
- For bags only, the concrete bag calculator shows 40, 50, 60, and 80 lb counts side by side.