Concrete Calculator by Square Footage: Yards and Bags from Area
Enter your total square footage and slab thickness to get the concrete you need in cubic yards, cubic feet, and bags. It's built for slabs, patios, driveways, and floors where you already know the area but not the volume.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How to Calculate Concrete from Square Footage
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
cubic feet = area_ft² × (thickness_in ÷ 12); cubic yards = cubic feet ÷ 27
- ·area_ft² = total surface area of the pour (square feet)
- ·thickness_in = slab depth (inches)
- ·12 = inches per foot, converting depth to feet
- ·27 = cubic feet per cubic yard
- ·default 10% waste is added to the yardage
If you already know the square footage, you only need the thickness to get volume. Depth is the variable that quietly drives cost, so confirm it before ordering.
Worked examples
Example 1: 400 ft² area at 4 inches
- Thickness 4 in ÷ 12 = 0.333 ft
- Volume = 400 × 0.333 = 133.3 ft³
- Cubic yards = 133.3 ÷ 27 = 4.94
- Add 10% waste ≈ 5.4 yd³
Result: ≈ 5.4 cubic yards
Example 2: 12 × 12 ft patio (144 ft²) at 4 inches
- Area = 12 × 12 = 144 ft²
- Thickness 4 in ÷ 12 = 0.333 ft
- Volume = 144 × 0.333 = 48 ft³
- Cubic yards = 48 ÷ 27 = 1.78, plus 10% ≈ 2.0 yd³
Result: ≈ 2.0 cubic yards
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Square Footage Concrete FAQs
How much concrete do I need for a 400 square foot slab?
At 4 inches thick, 400 square feet needs about 133 cubic feet, or roughly 4.9 cubic yards of concrete.
How do I convert square feet to cubic yards of concrete?
Multiply the area in square feet by the thickness in feet to get cubic feet, then divide by 27. For example, 100 sq ft at 4 inches (0.333 ft) is 33.3 cubic feet, or about 1.23 cubic yards.
How many cubic yards of concrete for 1000 square feet?
At a standard 4-inch thickness, 1000 square feet requires about 333 cubic feet, or roughly 12.3 cubic yards.
What thickness should my concrete slab be?
Patios and walkways are typically 4 inches thick, while driveways and areas carrying heavier loads are usually 5 to 6 inches.
Does this calculator account for waste?
It gives the exact volume, so add about 5 to 10 percent extra to cover spillage, subgrade variation, and forming.
How much concrete for a 12x12 patio?
A 12 by 12 foot area is 144 square feet; at 4 inches thick that's about 48 cubic feet, or roughly 1.8 cubic yards.
I only know my square footage, not length and width. Is that enough?
Yes. Volume depends only on area and thickness, so square footage plus the depth in inches gives you everything needed. Enter the area directly and the calculator multiplies by the thickness in feet, then divides by 27 for yards.
How does thickness change the yardage for the same area?
Volume scales directly with depth: a 400 ft² slab needs about 4.9 yd³ at 4 inches but roughly 7.4 yd³ at 6 inches. Every extra inch adds meaningful concrete and cost. Confirm the required thickness for your use before ordering.
What thickness should I use for a patio versus a driveway?
A walking patio is typically fine at 4 inches, while a driveway that carries vehicles should be 5 to 6 inches, often with rebar. The heavier the load, the thicker the slab. Enter the specific thickness for each area rather than a single default.
How do I total up several areas with different thicknesses?
Calculate each area at its own thickness, then add the cubic yards together. A 4-inch patio and a 6-inch shed pad are two separate volumes summed. This is more accurate than blending them into one average depth.
Does square footage alone tell me the cost?
Not by itself. Cost follows volume, which needs thickness, and then the price per yard plus any delivery fee. Two slabs with identical square footage but different depths can differ substantially in price.
How accurate does my area measurement need to be?
Measure to the nearest half foot and square up corners; small errors in area translate directly into concrete quantity. For irregular shapes, split them into rectangles, find each area, and add. The built-in 10% waste absorbs minor measuring slack.
How many 80 lb bags would a given area take?
Divide the area's cubic feet by 0.60. A 144 ft² patio at 4 inches is 48 ft³, or about 80 bags, which is near the point where ready-mix becomes more practical than bags. Above roughly one cubic yard, a truck is usually cheaper and faster.