Concrete Bag Cubic Feet: How Much You Get

One 60-pound bag of concrete mix yields about 0.45 cubic feet of wet concrete, and a standard 80-pound bag yields about 0.60 cubic feet. These are the two most common bag sizes, and the yield is printed on every bag, but it varies slightly by brand and mix. The calculator below works out the volume you need for a slab, footing, or post hole, then tells you how many bags to buy.

Formula

The volume of a rectangular slab is:

Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)

Since depth is usually measured in inches, convert it first: Depth (ft) = Depth (in) ÷ 12. The bag yield is the weight of the dry mix divided by the pounds per cubic foot of the wet concrete. Most standard concrete mixes produce about 133.3 lb of wet concrete per cubic foot, so:

Bag yield (cu ft) = Bag weight (lb) ÷ 133.3

For a 60 lb bag: 60 ÷ 133.3 ≈ 0.45 cu ft. For an 80 lb bag: 80 ÷ 133.3 ≈ 0.60 cu ft. A 40 lb bag gives about 0.30 cu ft, and a 90 lb bag (often sold for high-strength mixes) gives about 0.68 cu ft. These are industry-standard approximations; the exact yield on the bag label is the most reliable number.

Worked Example

Using the calculator defaults: a slab that is 10 ft long, 4 ft wide, and 4 inches deep, with 60 lb bags.

1. Convert depth: 4 in ÷ 12 = 0.333 ft
2. Volume = 10 × 4 × 0.333 = 13.32 cu ft
3. Bag yield = 60 ÷ 133.3 = 0.45 cu ft
4. Bags needed = 13.32 ÷ 0.45 = 29.6 → round up to 30 bags

The calculator rounds up to 30 bags. Always round up, never down, because a short pour is much harder to fix than having a bag or two left over.

Why Bag Yield Varies by Brand

Different manufacturers use slightly different formulas. A 60 lb bag of a standard all-purpose mix might yield 0.45 cu ft, but a high-strength mix or a fast-setting mix could yield 0.44 or 0.46 cu ft. The difference comes from the density of the aggregates (sand and gravel) and the amount of cement. The bag label always prints the yield in cubic feet – check it before you calculate. For critical projects, use the number on the bag rather than a fixed rule of thumb.

Also note that the yield is for wet concrete. The dry powder takes up more space than the finished wet concrete because water fills the voids between particles. So you cannot measure the dry volume of the bag and expect that to be the pour volume.

How to Measure a Project That Isn’t a Simple Slab

For a rectangular slab, the formula above is all you need. But many projects are round or irregular:

• Round column footing: Volume = π × (radius)² × height. Use radius in feet, height in feet.
• Post hole: Subtract the volume of the post from the volume of the hole. For a round hole and round post, both are cylinders.
• Triangular area: Volume = (base × height ÷ 2) × depth.
• Irregular shape: Break it into rectangles and triangles, calculate each, then add.

For a post hole, the hole is usually wider at the top than the bottom. If you want to be precise, measure the diameter at the top and bottom, average them, and use that average in the cylinder formula. Most DIY calculators ignore this taper, which is fine for small projects.

How Many Bags Do You Need for Common Projects?

Here are typical bag counts for a few common DIY projects using 60 lb bags (0.45 cu ft each):

• 4×4 slab, 4 inches deep: 1.33 cu ft → 3 bags
• 2×2 pad, 4 inches deep: 0.33 cu ft → 1 bag
• 10-inch diameter post hole, 24 inches deep: 1.31 cu ft → 3 bags
• 18-inch diameter footing, 12 inches deep: 1.77 cu ft → 4 bags

These are for standard mixes. If you use 80 lb bags, divide the bag count by about 1.33 (since 0.60 ÷ 0.45 = 1.33). For example, the 4×4 slab needs 30 bags of 60 lb, or about 23 bags of 80 lb.

Common Mistakes and How to Avoid Them

The most frequent error is mixing up units. Depth is often measured in inches, but the formula needs feet. A 4-inch deep slab is 0.333 ft, not 4 ft – that mistake would make you buy 12 times more concrete than you need.

Another common mistake is forgetting to round up. If your calculation says 29.6 bags, you need 30 bags. You cannot buy 0.6 of a bag. Always round up to the next whole number.

Also, do not forget that the bag yield is for wet concrete, not dry powder. If you measure the dry powder volume, you will overestimate how much concrete you get. The bag label gives the wet yield – trust that.

Finally, for a slab that will bear weight (like a shed base or a patio), consider adding a margin of 5–10% for uneven ground, spillage, or slight over-excavation. For a post hole, the actual hole is often bigger than you planned because of rocks or loose soil. Adding one extra bag is cheap insurance.

Cost and Practicality: Bags vs. Ready-Mix

Bagged concrete is convenient for small projects, but it is much more expensive per cubic foot than ordering a ready-mix truck. A typical 60 lb bag costs a few dollars and yields 0.45 cu ft, so you are paying roughly $6–$8 per cubic foot. A ready-mix truck delivers about 10 cubic yards (270 cu ft) for a few hundred dollars, which works out to about $1–$2 per cubic foot – but you have to order a minimum volume, often 1 cubic yard (27 cu ft).

For projects under about 1 cubic yard, bags are usually the practical choice. For anything larger, a ready-mix truck is cheaper and saves hours of mixing. The break-even point depends on local prices and your labor, but a good rule of thumb is: if you need more than 20–30 bags, at least call a ready-mix company for a quote.

Safety and Mixing Tips

Concrete mix contains Portland cement, which is caustic. Wear gloves, long sleeves, and eye protection when mixing and pouring. Dry cement dust can irritate your lungs, so a dust mask is a good idea, especially in a confined space. If wet concrete gets on your skin, rinse it off promptly – it can cause chemical burns if left in contact.

When mixing, use the amount of water recommended on the bag. Too much water weakens the concrete; too little makes it hard to work. A good consistency is like thick oatmeal – it should hold its shape but be workable. Mix in a wheelbarrow or a mixing tub, not on the ground, to avoid picking up dirt that weakens the mix.

For cold weather (below 40°F), concrete cures much slower. For hot weather, it can dry too fast and crack. In both cases, follow the bag instructions for curing – usually keeping the surface damp for a few days. Do not walk on the concrete until it has set enough to support weight, which is typically 24–48 hours, but check the bag for the specific mix.

Frequently Asked Questions

How many cubic feet are in a 60 lb bag of concrete?

A 60 lb bag of standard concrete mix yields about 0.45 cubic feet of wet concrete. This is the most common bag size for DIY projects. The exact yield is printed on the bag and can vary slightly by brand.

How many 80 lb bags of concrete do I need for 1 cubic yard?

One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, so you need 27 ÷ 0.60 = 45 bags. For 60 lb bags (0.45 cu ft), you need 27 ÷ 0.45 = 60 bags.

How many cubic feet does a 40 lb bag of concrete make?

A 40 lb bag of standard mix yields about 0.30 cubic feet of wet concrete. This size is convenient for small repairs like filling a post hole or a small stepping stone.

How much water do I add to a 60 lb bag of concrete?

Most 60 lb bags require about 3 quarts (0.75 gallons) of water, but always check the bag – some mixes need more or less. Add water gradually and mix until the concrete is uniform and workable.

Can I use bagged concrete for a shed foundation?

Yes, for a small shed (up to about 8×10 feet), a 4-inch thick slab made from bagged concrete is fine. For larger sheds, consider a ready-mix truck or a footing design. The key is proper ground preparation – compacted gravel base and a form to hold the concrete.

Is it cheaper to mix your own concrete from scratch?

Buying cement, sand, and gravel separately is usually cheaper per cubic foot than buying pre-mixed bags, but it requires measuring and mixing the ingredients yourself. For most DIYers, the convenience of bagged mix is worth the extra cost, especially for small projects.

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in l | Length | ft | 10 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 4 | 0
in bag | Bag size | lb | 60 | 0
out vol | Wet concrete volume | cu ft | (l * w * (d / 12)) | 2
out bags | Bags needed | bags | ceil((l * w * (d / 12)) / (bag / 133.333)) | 0