Bags of Concrete per Cubic Foot: Exact Count

This calculator tells you how many bags of concrete mix you need for a given volume in cubic feet. Enter the volume and the bag size (in pounds), and it returns the number of bags required, rounded up to whole bags. It also shows the total weight of mix needed.

Formula

One cubic foot of cured concrete weighs about 150 pounds. A bag of concrete mix weighs less than that because it contains dry ingredients (cement, sand, and gravel) that gain weight when water is added. The general rule: a 40-pound bag yields about 0.30 cubic feet, a 60-pound bag yields about 0.45 cubic feet, and an 80-pound bag yields about 0.60 cubic feet.

The formula used here is:

Bags = Volume (cu ft) × 150 lb/cu ft ÷ Bag weight (lb)

Where:

• Volume – the space to fill, in cubic feet (length × width × depth, all in feet).
• 150 lb/cu ft – the average density of cured concrete.
• Bag weight – the dry weight printed on the bag (40, 60, or 80 lb).

The result is rounded up to the next whole bag because you cannot buy a fraction of a bag.

Worked Example

Using the calculator defaults: volume = 1 cubic foot, bag size = 60 lb.

1. Weight of concrete needed = 1 × 150 = 150 lb.
2. Bags needed = 150 ÷ 60 = 2.5 bags.
3. Rounded up = 3 bags.

The calculator shows 3 bags and a total mix weight of 150 lb. That matches the common rule: a 60 lb bag makes 0.45 cu ft, so 1 ÷ 0.45 = 2.22, rounded up to 3.

How Concrete Bag Yield Works

Concrete mix is a blend of Portland cement, fine aggregate (sand), and coarse aggregate (gravel). The dry volume of the bag is larger than the wet volume because water fills the spaces between particles and causes chemical reactions that shrink the mixture slightly as it cures.

Manufacturers publish yield on the bag, usually as cubic feet per bag. For example, a typical 80 lb bag yields about 0.60 cu ft. That means to fill one cubic foot, you need 1 ÷ 0.60 = 1.67 bags of 80 lb mix. The 150 lb/cu ft density is an average; actual density can range from 140 to 160 lb/cu ft depending on the mix design and how much water is added.

For small projects, the difference between the density-based estimate and the bag-yield method is usually less than half a bag. For larger pours, always check the specific mix you buy because some “fast-setting” or “high-strength” mixes have different yields.

How to Measure Volume in Cubic Feet

Before you can calculate bags, you need the volume of the space to fill. Use these formulas:

• Slab or square pad: length (ft) × width (ft) × thickness (ft). For a 4-inch slab, use 0.333 ft.
• Round column: π × radius² (ft) × height (ft). Radius is half the diameter.
• Wall or footing: length (ft) × height (ft) × thickness (ft).
• Irregular shape: break it into rectangles and triangles, calculate each, and add them.

If you measure in inches, convert to feet by dividing by 12. For example, a 6-inch depth is 0.5 ft. A common mistake is mixing units – always work in feet or convert everything first.

Choosing the Right Bag Size

Bag size affects how many trips you make and how easy the mixing is:

• 40 lb bags – yield about 0.30 cu ft each. Good for small repairs, setting posts, or when you need to mix by hand in a wheelbarrow.
• 60 lb bags – yield about 0.45 cu ft. The most common size for DIY projects like walkways and small footings.
• 80 lb bags – yield about 0.60 cu ft. Best for larger pours, but heavy – you may need a mixer or a second person.

For a 1-cubic-foot project, you need 2 bags of 80 lb, 3 bags of 60 lb, or 4 bags of 40 lb (rounding up). Larger bags mean fewer batches, but each batch is harder to mix thoroughly.

Common Mistakes and How to Avoid Them

Unit mix-ups are the most frequent error. Forgetting to convert inches to feet leads to wildly wrong volumes. For example, a 10 ft × 10 ft slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft. If you mistakenly use 4 as the thickness, you get 400 cu ft – a huge overestimate.

Another mistake is not rounding up. If you calculate 2.2 bags, you need 3 bags, not 2. The calculator rounds up automatically.

Also, remember that the bag weight is the dry weight. After adding water, the total weight increases by about 10–15%. That does not change the number of bags, but it matters if you are moving wet concrete.

Finally, do not assume all mixes are the same. Read the bag label for the exact yield – some “high-strength” mixes have slightly different densities. The 150 lb/cu ft figure is a safe average for most standard mixes.

Cost and Quantity Planning

While prices vary by region and brand, you can estimate cost once you know the number of bags. Multiply bags by the local price per bag. For a typical 10 ft × 10 ft × 4-inch slab (33.3 cu ft), you would need about 56 bags of 60 lb mix (33.3 ÷ 0.45 = 74, actually 74 bags – let’s correct that). Wait, recalculate: 33.3 cu ft ÷ 0.45 cu ft per 60 lb bag = 74 bags. That is a lot; an 80 lb bag at 0.60 cu ft gives 55.5 bags, so 56 bags. The calculator’s density method gives 33.3 × 150 = 4995 lb ÷ 60 = 83.25 bags, which is higher because the bag yield is based on the actual mix, not the cured density. For planning, use the bag-yield numbers printed on the bag, not the density method, for large pours. The density method is a quick estimate but can overestimate by 10–15% because it assumes the dry mix weighs the same as cured concrete per cubic foot.

To be accurate, use the yield on the bag: 40 lb = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft. For the slab above, 33.3 ÷ 0.60 = 55.5, so 56 bags of 80 lb. That is the number to buy.

Safety and Mixing Tips

Concrete mix contains Portland cement, which is caustic. Wear gloves, long sleeves, and eye protection when handling dry mix or wet concrete. Avoid inhaling dust – use a dust mask if mixing in a confined space.

Mix only as much as you can place in about 20–30 minutes. Once water is added, the mix begins to set. For a 60 lb bag, add about 3.5–4 quarts of water, but always follow the bag instructions. Too much water weakens the concrete; too little makes it hard to work.

For small batches, a wheelbarrow and a hoe work. For more than a few bags, rent or borrow a portable mixer. Clean tools immediately after use – cured concrete is very hard to remove.

Alternatives to Bagged Mix

For very large pours (more than 1 cubic yard), it is often cheaper and easier to order ready-mix concrete delivered by truck. One cubic yard equals 27 cubic feet. A typical 10 ft × 10 ft × 4-inch slab is about 1.23 cubic yards, which is on the edge – many DIYers still use bags for that, but it means about 56 bags of 80 lb. For anything larger, ready-mix is usually more cost-effective and saves labor.

Another alternative is using a concrete mix with larger aggregate, which yields more volume per bag but is harder to hand-trowel. For small repairs, consider concrete patch or resurfacer, which have different yields and are not structural.

Frequently Asked Questions

How many 80 lb bags of concrete make a cubic foot?

One 80 lb bag yields about 0.60 cubic feet. So you need 1 ÷ 0.60 = 1.67 bags, meaning 2 bags to make a full cubic foot.

How many 60 lb bags of concrete make a cubic foot?

A 60 lb bag yields about 0.45 cubic feet. You need 1 ÷ 0.45 = 2.22 bags, so 3 bags to make a cubic foot.

How many 40 lb bags of concrete make a cubic foot?

A 40 lb bag yields about 0.30 cubic feet. You need 1 ÷ 0.30 = 3.33 bags, so 4 bags to make a cubic foot.

Why does the calculator use 150 lb per cubic foot?

That is the average density of cured concrete. The dry mix in a bag is lighter, but the yield is based on the volume of concrete you get after adding water. The calculator uses the density to estimate, but for exact numbers, check the bag label.

Can I mix bags in a wheelbarrow?

Yes, for one or two bags at a time. A 60 lb bag is manageable, but an 80 lb bag is heavy – you may need a second person to lift it. Use a sturdy wheelbarrow and a hoe or shovel.

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

Typically about 3.5 to 4 quarts of water per 60 lb bag, but always follow the instructions on the bag. Adding too much water weakens the concrete.

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in v | Volume | cu ft | 1 | 0
sel b | Bag size | 40 lb=40, 60 lb=60, 80 lb=80 | 60
out bags | Bags needed | bags | ceil(v * 150 / b) | 0
out weight | Total mix weight | lb | v * 150 | 0