How Much Concrete Do You Need for a Slab?
To find how much concrete a slab needs, multiply the length by the width by the thickness, all in feet, then divide by 27 to get cubic yards. The divide-by-27 step converts cubic feet into cubic yards, which is how ready-mix concrete is ordered. A 10 by 10 foot slab at 4 inches thick works out to about 1.25 cubic yards, or roughly 56 bags of 80-pound mix. Add a waste factor and you have your order.
The formula
Concrete volume is just area times depth, kept in the same units. The only trick is turning inches of thickness into feet before you multiply.
- Convert thickness to feet. Divide the inches by 12. A 4-inch slab is 4 divided by 12, which is 0.33 feet.
- Multiply length by width by thickness. That gives cubic feet.
- Divide by 27. That converts cubic feet to cubic yards.
- Add 5 to 10 percent for spillage, uneven ground, and over-excavation.
For a 10 by 10 foot slab at 4 inches: 10 x 10 x 0.33 = 33 cubic feet. Divide by 27 and you get 1.22 cubic yards. Add 10 percent and order about 1.35 yards.
Concrete by common slab size
Here is the volume for popular slab sizes at a standard 4-inch thickness, before adding waste.
| Slab size | Area | Cubic yards (4") | 80-lb bags |
|---|---|---|---|
| 8 x 8 ft | 64 sq ft | 0.8 | 36 |
| 10 x 10 ft | 100 sq ft | 1.25 | 56 |
| 12 x 12 ft | 144 sq ft | 1.8 | 80 |
| 20 x 20 ft | 400 sq ft | 4.9 | 220 |
| 24 x 24 ft | 576 sq ft | 7.1 | 320 |
| 30 x 30 ft | 900 sq ft | 11.1 | 500 |
One 80-pound bag of concrete makes about 0.6 cubic feet, so it takes roughly 45 bags to make a single cubic yard. That is why bags only make sense for small slabs. Anything past a yard or two is cheaper and far less work as a ready-mix delivery.
Bags or ready-mix
The break-even between bags and a truck sits around one to two cubic yards. Under that, bagged mix from a hardware store is convenient and you avoid delivery and short-load fees. Over it, mixing dozens of bags by hand is slow, and a ready-mix truck is both cheaper per yard and far easier on your back. For a 10 by 10 shed pad, bags are reasonable. For a 20 by 20 slab needing nearly 5 yards, order a truck.
Adjusting for thickness and edges
Thicker slabs need proportionally more concrete, so a 6-inch slab uses 50 percent more than a 4-inch slab over the same area. Thickened edges and footings add volume too. If your slab has a 6-inch perimeter footing, calculate that strip separately and add it to the field total so you do not come up short.
The fastest way to get an exact number, including bags, yards, and a waste allowance, is to run your dimensions through the concrete slab calculator. For bag counts by size, the concrete bag calculator breaks it down by 40, 60, and 80-pound bags. For a broader walkthrough, see our guide on how much concrete you need.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length by width by thickness in feet, then divide by 27 for cubic yards. For a 4-inch slab, use 0.33 feet as the thickness. Add 5 to 10 percent for waste.
How many bags of concrete do I need for a 10x10 slab?
About 56 bags of 80-pound mix for a 4-inch slab, or roughly 1.25 cubic yards. That is near the point where a ready-mix delivery becomes the easier option.
How many cubic yards is a 20x20 concrete slab?
About 4.9 cubic yards at 4 inches thick, before waste. Add 10 percent and order around 5.4 yards.
Should I order concrete by the bag or by the truck?
Bags make sense under one to two cubic yards. Above that, ready-mix delivery is cheaper per yard and much less labor than mixing bags by hand.