How to Calculate Concrete for a Slab: Step-by-Step Guide
Pouring a concrete slab is one of the most common DIY and professional construction tasks. Whether you’re building a patio, shed foundation, or garage floor, getting the right amount of concrete is crucial. Too little means a failed pour with weak cold joints. Too much means wasted money. This guide shows you exactly how to calculate concrete for any slab.
The Formula You Need
The formula for calculating concrete volume for a rectangular slab is straightforward:
Volume (cubic feet) = Length (ft) x Width (ft) x Thickness (ft)
Then convert to cubic yards by dividing by 27:
Cubic Yards = Cubic Feet / 27
The key detail most people miss is the unit conversion for thickness. Slab thickness is usually measured in inches, but the formula requires feet. Divide your inches by 12 to convert.
For example, a 4-inch thick slab: 4 / 12 = 0.333 feet.
Worked Example: 10x10 Patio
Let’s calculate concrete for a standard 10-foot by 10-foot patio at 4 inches thick.
- Convert thickness: 4 in / 12 = 0.333 ft
- Calculate volume: 10 x 10 x 0.333 = 33.3 cubic feet
- Convert to cubic yards: 33.3 / 27 = 1.23 cubic yards
- Add 10% safety margin: 1.23 x 1.10 = 1.36 cubic yards
If you’re using bags instead of ready-mix:
- 80-lb bags: 1.23 x 45 = about 56 bags
- 60-lb bags: 1.23 x 60 = about 74 bags
- 40-lb bags: 1.23 x 90 = about 111 bags
At an average cost of $150 per cubic yard for ready-mix, this project would cost approximately $204 for concrete alone.
Quick Reference Table
Here are pre-calculated volumes for common slab sizes at 4-inch thickness:
| Slab Size | Cubic Feet | Cubic Yards | 80-lb Bags |
|---|---|---|---|
| 8 x 8 ft | 21.3 | 0.79 | 36 |
| 8 x 10 ft | 26.7 | 0.99 | 45 |
| 10 x 10 ft | 33.3 | 1.23 | 56 |
| 10 x 12 ft | 40.0 | 1.48 | 67 |
| 12 x 12 ft | 48.0 | 1.78 | 80 |
| 10 x 20 ft | 66.7 | 2.47 | 111 |
| 12 x 20 ft | 80.0 | 2.96 | 133 |
| 20 x 20 ft | 133.3 | 4.94 | 222 |
Choosing the Right Thickness
The thickness of your slab depends on its intended use:
- Walkways and small patios: 4 inches is the standard minimum. This handles foot traffic, chairs, and tables without issue.
- Driveways: 5 to 6 inches. Passenger cars weigh 3,000-5,000 pounds, and the slab needs extra thickness to distribute that weight.
- Garage floors: 5 to 6 inches. Similar loads to driveways, with the added consideration of stored heavy items.
- Shed foundations: 4 inches for small sheds, 5 inches for larger ones.
- Hot tub pads: 6 inches minimum. A full hot tub can weigh 5,000+ pounds.
Going thinner than recommended saves very little money (the difference between 4” and 5” on a 10x10 slab is only 0.31 cubic yards) but dramatically increases the risk of cracking and failure.
Bags vs. Ready-Mix: Which Should You Choose?
Use bags when:
- Your project requires less than 1 cubic yard
- You have time and physical ability to mix by hand or with a mixer
- Access is limited (narrow gate, no truck access)
- The project involves small, separate pours (fence posts, small pads)
Use ready-mix delivery when:
- Your project requires more than 1 cubic yard
- You need a consistent mix quality
- Time is limited (concrete must be poured and finished before it sets)
- The project area is accessible by truck
Ready-mix is almost always cheaper per cubic yard than bags. A cubic yard of bagged concrete (using 80-lb bags at $6 each) costs about $270, while ready-mix is typically $120-$200 per yard.
Common Mistakes to Avoid
Forgetting the safety margin. Always add 5-10% to your calculated volume. Subgrade is never perfectly flat, forms can bow, and some concrete always gets wasted.
Not accounting for slope. If your slab site has a slope, one end will be thicker than the other. Measure the thickness at the thickest point and use that for your calculation, or average the thick and thin ends.
Ordering too late on a hot day. Concrete begins setting immediately after mixing. On hot days (above 90 degrees F), it sets even faster. Have everything ready before the truck arrives.
Skipping the base. A compacted gravel base (4 inches minimum) is essential for drainage and preventing settling. Factor the excavation depth into your project planning: total depth = slab thickness + base thickness.
Using Our Calculator
Rather than doing this math by hand every time, use our free concrete calculator to get instant results. Enter your dimensions, choose imperial or metric units, and see your volume, bag count, and cost estimate immediately. It supports rectangular slabs, round slabs, tubes, and stairs.