How Many Bags of Concrete Do I Need?

Bagged concrete is the go-to for small pours: a patio slab, a shed pad, a set of fence posts. The hard part is knowing how many bags to haul home. Buy too few and you are mixing a cold joint into a half-finished slab; buy too many and you are storing heavy bags that go hard in a damp garage.

Step 1: Work out the volume

Concrete is measured by volume. For a rectangular slab or pad, multiply the length and width in feet by the thickness in inches, then divide by 12 to get cubic feet:

Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12

A 10 × 10 ft patio slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet. To convert to cubic yards, the unit ready-mix is sold in, divide by 27: 33.3 ÷ 27 = 1.23 cubic yards.

For odd shapes, split the area into rectangles and add them up. For a circular pad, use π × radius² for the area in square feet, then multiply by the thickness in feet.

How thick should the slab be?

Thickness drives the total as much as area does. Common starting points:

These are typical values, not a structural design. Footings, frost depth, and driveway requirements are set by local building codes, so check with your local building department before you pour anything that carries a structure.

Step 2: Check the yield on the bag

Every bag of concrete mix lists its yield: the volume of concrete it makes once mixed. That number, not the weight, is what you divide by. Typical yields for standard concrete mix are:

Yields vary by brand and product. Fast-setting mixes, high-strength mixes, and mortar or sand mixes can differ noticeably, so use the figure printed on the bag you are actually buying.

Step 3: Divide, add an allowance, and round up

Bags = cubic feet ÷ yield per bag

Then add an allowance for real-world losses. Subgrades are never perfectly flat, forms bow slightly under load, and some concrete always ends up on the ground or stuck in the mixer. An allowance of 5 to 10% is a common starting point: nearer 5% for a neat pour into rigid forms, and nearer 10% or more over uneven ground. Round the result up to whole bags.

Worked example: a 10 × 10 ft patio slab

The slab is 4 inches thick, so the volume is 33.3 cubic feet, as calculated above. Here is how that translates into bags:

Notice the weight: close to 2.5 tons either way. That is more than most pickup trucks can safely carry in one trip, so check your vehicle’s payload rating or arrange store delivery.

Example: setting fence posts

Post holes are cylinders, so the volume is π × radius² × depth, minus the space the post takes up. For a 10-inch diameter hole, 3 feet deep, holding a 4 × 4 post (actual size 3.5 × 3.5 in):

• Hole: 3.1416 × (5 ÷ 12)² × 3 = 1.64 cu ft
• Post: (3.5 × 3.5 ÷ 144) × 3 = 0.26 cu ft
• Concrete needed: about 1.38 cu ft per hole

That is about 2.3 bags of 80 lb standard mix per hole, or about 3.7 bags of a 50 lb fast-setting mix with a 0.375 cu ft yield. Round up per hole or across the whole job, and buy a spare bag. Running short halfway through a fence line is common.

The Fence Calculator estimates post concrete for a whole fence line, and the Concrete Calculator handles groups of holes of any size.

Bags or ready-mix?

Bags are practical for small jobs and for spots a truck cannot reach. As the volume grows, mixing by hand or with a small mixer becomes slow, and a slab should be placed in one continuous pour. Many people switch to ready-mix somewhere around 1 cubic yard, which is 45 bags of 80 lb mix.

• Bags: no minimum order, flexible timing, and you can mix in batches. They are heavy to transport and labor-intensive to mix.
• Ready-mix: consistent concrete delivered all at once, sold by the cubic yard. Small orders often carry a short-load fee, and you need crew and tools ready when the truck arrives.

For comparison, a 12 × 20 ft driveway pad 5 inches thick is 100 cubic feet, or 3.7 cubic yards. That would take about 167 bags of 80 lb mix, which is why driveways are almost always poured from a truck.

Tips before you buy

• Read the yield, not the weight. Two 60 lb bags from different products may not make the same volume.
• Measure after the forms are set. The inside of the forms is the real pour size, and depth is often deeper than planned where the ground dips.
• Keep bags dry. Store them off the ground and covered. Moisture starts the cement curing in the bag.
• Plan the mix rate. With a small mixer, a typical crew goes through bags steadily but not quickly. Have enough hands so the first concrete placed has not set before the last arrives.
• Follow the bag’s water ratio. Extra water makes mixing easier but weakens the finished concrete.

Frequently asked questions