Concrete Bag Calculator

Concrete bag calculator: volume ÷ 0.6 cu ft per bag

A concrete bag calculator tells you how many 80-lb bags to buy for a small pour. The formula is bags = (length × width × depth in inches ÷ 12) ÷ 0.6, rounded up. One 80-lb bag makes about 0.6 cubic feet of mixed concrete. Tick the 10% option for spillage and an uneven base; the count is multiplied by 1.10 before rounding. Cost is bags × price per bag.

What the calculator does, step by step

1. Cubic feet: length × width × (depth ÷ 12). Four inches is 0.333 ft.
2. Bags: cubic feet ÷ 0.6.
3. Allowance: optional × 1.10.
4. Round up to whole bags.
5. Cost: bags × price.

Internally the tool goes through cubic yards (cubic feet ÷ 27, then back × 27), so the result is the same as dividing cubic feet by 0.6 directly.

Worked examples

1. Default slab: 10 × 10 ft, 4 in thick, $6 per bag

• Volume: 10 × 10 × 0.333 = 33.3 cu ft (1.23 yd³)
• Bags: 33.3 ÷ 0.6 = 55.6, rounded up to 56 bags, costing $336.00
• With 10% on: 55.6 × 1.10 = 61.1, rounded up to 62 bags, costing $372.00

At 56 bags you are past one cubic yard. That is about 4,480 lb of dry mix, so it is worth getting a ready-mix quote too.

2. Small equipment pad: 4 × 4 ft, 4 in thick, $6 per bag

• Volume: 4 × 4 × 0.333 = 5.33 cu ft
• Bags: 5.33 ÷ 0.6 = 8.9, rounded up to 9 bags ($54)
• With 10% on: 10 bags ($60)

Bag yield and slab reference

Bag size	Yield per bag	Bags per cubic yard
40 lb	about 0.30 cu ft	90
60 lb	about 0.45 cu ft	60
80 lb	about 0.60 cu ft	45

Slab (4 in thick)	Cubic feet	80-lb bags	With 10%
4 × 4 ft	5.3	9	10
10 × 10 ft	33.3	56	62
12 × 12 ft	48.0	80	88
20 × 20 ft	133.3	223	245

Yields vary a little by brand and mix type, so check the bag label. High-strength and fast-setting mixes can yield slightly differently.

How to measure for bagged concrete

• Footprint: measure the inside of the forms, not the outside of the boards.
• Depth: measure the concrete thickness only. A 4 in gravel base under the slab is not part of the bag count.
• Check depth in several spots. If the base dips, use the average depth, or the deepest reading for a safe order.
• Round holes are cylinders. Post holes and sonotube piers use πr²h, not length × width. The post hole concrete calculator handles that shape.

Bags or a truck: comparing the work

Each 80 lb bag is one lift, one cut, one mix, and one dump. The default 56 bag slab means moving 4,480 lb of dry mix by hand, plus water, before any finishing starts. A small electric mixer handles one or two bags per batch, so 56 bags is roughly 28 to 56 batches.

Concrete starts to stiffen as soon as it is mixed, so a long slab poured in many small batches can show joints where batches meet. For anything over about a yard, compare the bag total with a ready-mix quote that includes any short-load fee. For a few post holes or a small pad, bags are usually the simpler choice.

Common bagged concrete mistakes

• Entering depth in feet. The depth box is in inches. Typing 0.33 gives a result 12 times too small.
• Mixing 60-lb and 80-lb counts. A store swap from 80-lb to 60-lb bags means a third more bags.
• Adding water until it pours easily. Soupy concrete is weaker and shrinks more. Use the amount on the bag.
• Pouring too much by hand. Over about a yard, mixing time becomes the limit and the first batches start setting before the last ones arrive.
• Ignoring control joints. Slabs crack as they shrink. Joints placed at regular intervals tell them where to crack.
• Forgetting the tools. A wheelbarrow or small mixer, a hoe, a screed board, and a float are needed for every bag-mixed pour.

Related calculators

For bigger pours in cubic yards, use the concrete slab calculator. If you are buying a specific brand, the Quikrete calculator runs the same bag math.