The short answer
Multiply length × width × thickness in feet to get cubic feet, divide by 27 for cubic yards and add about 10% for spillage and an uneven base. A 10 × 10 ft slab that is 4 inches thick is 33.3 cu ft, or 1.36 cubic yards with the extra. In bags, that is 62 bags of 80 lb mix.
Step 1: Work out the volume
Measure the length and width of the forms in feet. Convert the thickness from inches to feet by dividing by 12, so 4 inches is 0.333 ft. Multiply the three numbers to get cubic feet, then divide by 27 to get cubic yards, which is the unit ready-mix suppliers use.
Cubic feet = length × width × (thickness in inches ÷ 12)
Cubic yards = cubic feet ÷ 27
For a slab that is not a rectangle, split it into rectangles and add their volumes. For a footing or thickened edge, work out the extra trench volume separately and add it.
Step 2: Add extra
Family Handyman suggests adding 10% to allow for spillage and variations in the depth of the base. Their 10 × 10 ft, 4 inch example works out to 1.3 cubic yards before the extra. Ordering a little too much costs far less than a second delivery or a cold joint in the middle of the slab.
Worked example: a 10 × 10 ft slab, 4 inches thick
- Thickness: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 cu ft.
- Add 10%: 33.33 × 1.10 = 36.67 cu ft.
- Cubic yards: 36.67 ÷ 27 = 1.36 cubic yards of ready-mix.
- Bags: an 80 lb bag yields about 0.60 cu ft, so 36.67 ÷ 0.60 = 61.1, rounded up to 62 bags.
| Slab | Volume | With 10% extra | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 33.33 cu ft | 36.67 cu ft | 1.36 | 62 |
| 12 × 12 ft | 48 cu ft | 52.8 cu ft | 1.96 | 88 |
| 4 × 25 ft walkway | 33.33 cu ft | 36.67 cu ft | 1.36 | 62 |
How many bags make a cubic yard
Quikrete lists the yield of its concrete mix as about 0.60 cu ft for an 80 lb bag, 0.45 cu ft for a 60 lb bag and 0.30 cu ft for a 40 lb bag. A cubic yard is 27 cu ft, so it takes 45 bags of 80 lb mix, 60 bags of 60 lb mix or 90 bags of 40 lb mix.
The bag size changes the count a lot, so the concrete calculator shows all three. Yields are approximate and assume no waste, which is why the extra allowance matters.
Bags or a truck?
Family Handyman says ordering ready-mix by truck is the most expensive option per project, but it makes sense for patios that need more than one cubic yard. A 12 × 12 ft slab needs 88 bags, which is a lot of lifting, mixing and time before the concrete starts to set.
- Under about half a yard: bags are practical.
- Between half a yard and one yard: bags work if you have a mixer and helpers, and a small delivery may cost less than you expect.
- Over one yard: ask for a quote on a delivery, including any short-load fee.
Choosing the thickness
Home Depot lists 3 to 4 inches for walkways, 4 inches for patios and sheds, 4 to 5 inches for garage floors and 5 to 6 inches for driveways. It also notes that the International Residential Code sets a minimum of 3.5 inches for residential slabs. Angi adds that expansive clay soil is a reason to reinforce or step up to 5 inches. Family Handyman suggests 6 to 8 inches under a hot tub or gazebo.
Thickness drives the volume directly: going from 4 to 6 inches on a 10 × 10 ft slab raises the order from 1.36 to 2.04 cubic yards, so decide before you order.
What to settle before the pour
- The base: a compacted layer of crushed stone under the slab changes how deep you dig, and it also changes how much gravel to order. The gravel calculator covers that.
- Reinforcement: Home Depot says wire mesh or rebar belongs in the upper third of the slab, and that patios can go without but a hot tub needs it.
- Control joints: Home Depot recommends spacing them at 2 to 3 times the slab thickness in feet, so every 8 to 12 ft for a 4 inch slab.
- Ask your supplier about strength and slump for the intended use.
Common mistakes
- Entering the thickness in inches into a formula that expects feet.
- Forgetting the thickened edges or footings.
- Ordering exactly the calculated volume with no extra.
- Mixing up bag sizes: 62 bags of 80 lb mix is 82 bags of 60 lb mix.
Calculators for this job
These calculators do the arithmetic from this guide. Each card shows the result for a typical project; open one to enter your own measurements.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
A 10 × 10 ft slab that is 4 inches thick is 33.3 cu ft. With 10% extra that is 36.7 cu ft, which takes 62 bags of 80 lb mix, 82 bags of 60 lb mix or 123 bags of 40 lb mix.
How many 80 lb bags are in a cubic yard?
An 80 lb bag yields about 0.60 cu ft and a cubic yard is 27 cu ft, so it takes 45 bags.
How much extra concrete should I order?
Family Handyman suggests adding 10% for spillage and variations in depth. Ordering slightly more than you need is much cheaper than running short.
How thick should a concrete patio be?
Home Depot lists 4 inches for patios and sheds, and Angi says most professionals suggest 4 inches. Go thicker under heavy loads such as a hot tub, and check your local code.
Sources
- Family Handyman, How to Pour a Concrete Slab Successfully
- Family Handyman, Pouring Concrete Patio: A First Timer's Guide
- The Home Depot, Concrete Slab Thickness and Control Joint Spacing
- The Home Depot, QUIKRETE 80 lb. Concrete Mix
- Angi, How Thick Should a Concrete Patio Be?