# Concrete Slab Calculator

> Enter your slab's length, width and thickness to get cubic yards of concrete, the number of bags and the gravel base under it. A 12 ft × 12 ft slab, 4 in thick, needs about 1.96 cubic yards of concrete with 10% extra.

Canonical: https://receiptscanner.co/tools/concrete-slab-calculator
Updated: 2026-10-26 · Sources re-checked by: 2027-10-26

This is a free interactive calculator that runs in the browser. Nothing you type is stored or sent. Results are estimates.

## How it works

Slab area is length × width. Concrete in cubic feet is area × thickness (in) ÷ 12, and cubic yards are cubic feet ÷ 27. The gravel base is figured the same way with the base depth in place of the slab thickness. The waste percentage is added to both. Bags are the cubic feet divided by what one bag makes, rounded up.

**Cubic yards of concrete per 100 square feet, no waste**

| Thickness | Cubic yards per 100 sq ft |
| --- | --- |
| 3 in | 0.926 |
| 4 in | 1.235 |
| 5 in | 1.543 |
| 6 in | 1.852 |

**Common slab sizes at 4 in thick, with 10% extra**

| Slab size | Area | Cubic yards | 80 lb bags |
| --- | --- | --- | --- |
| 8 ft × 8 ft | 64 sq ft | 0.87 | 40 |
| 10 ft × 10 ft | 100 sq ft | 1.36 | 62 |
| 12 ft × 12 ft | 144 sq ft | 1.96 | 88 |
| 12 ft × 20 ft | 240 sq ft | 3.26 | 147 |
| 20 ft × 20 ft | 400 sq ft | 5.43 | 245 |
| 24 ft × 24 ft | 576 sq ft | 7.82 | 352 |

**What one bag of QUIKRETE Concrete Mix makes**

| Bag size | Yield (cubic feet) |
| --- | --- |
| 40 lb | 0.3 |
| 50 lb | 0.375 |
| 60 lb | 0.45 |
| 80 lb | 0.6 |

## Worked example: 12 ft × 12 ft slab, 4 in thick, 4 in gravel base

1. Area: 12 × 12 = 144 square feet.
2. Concrete: 144 × (4 ÷ 12) = 48 cubic feet = 1.78 cubic yards.
3. Add 10% for waste: 52.8 cubic feet = 1.96 cubic yards. A ready-mix order would be about 2 cubic yards.
4. In bags: 88 bags of 80 lb mix.
5. Gravel base, 4 in deep: 144 × (4 ÷ 12) = 48 cubic feet; with 10% extra that is 1.96 cubic yards.

## FAQ

### How do you calculate concrete for a slab?
Multiply length × width × thickness, with thickness in feet (inches ÷ 12), to get cubic feet. Divide by 27 for cubic yards. A 12 ft × 12 ft slab that is 4 in thick is 48 cubic feet, or 1.78 cubic yards before waste.

### How much concrete do I need for a 12×12 slab?
A 12 ft × 12 ft slab at 4 in thick needs about 1.78 cubic yards, or 1.96 cubic yards with 10% extra. That is about 88 bags of 80 lb mix, so for a slab this size it is worth asking a ready-mix supplier for a quote.

### How many bags of concrete for a 10×10 slab?
A 10 ft × 10 ft slab at 4 in thick, with 10% extra, needs 62 bags of 80 lb mix. The table above shows other common sizes.

### How thick should a concrete slab be?
It depends on what the slab carries and on your local rules. Many home patios and walkways are poured about 4 in thick, and slabs that take vehicles are usually thicker. Sources disagree on exact numbers, so follow your plans, your engineer or your local building department.

### How much gravel do I need under a concrete slab?
Many slabs sit on a compacted gravel base, and the depth depends on your soil, the use and your local rules. Enter the base depth in the calculator and it shows the gravel in cubic yards, using the same length and width as the slab. Set the base depth to 0 if you don't need it.

### Does a slab need rebar or wire mesh?
That depends on the use and the local rules, and this calculator doesn't size reinforcement. Ask your supplier, your engineer or your local building department what your slab needs.

## Sources

- [QUIKRETE Concrete Mix 1101 data sheet (bag yields: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 cubic feet)](https://www.quikrete.com/PDFs/DATA_SHEET-Concrete%20Mix%201101.pdf) (checked 2026-10-26)

This calculator gives an estimate for planning. Slab thickness, base and reinforcement depend on your soil, use and local rules. Check your plans and a supplier before you order.