Concrete Bag Calculator
Find how many bags of concrete you need for a slab, pad or post holes, in every bag size, with waste margin, cost and a ready-mix check.
Built by Toolsque Team · last updated October 15, 2026 · checked against our testing process
| Bag | Makes | Bags needed |
|---|
Yields are the figures printed on standard concrete mix bags (an 80 lb bag makes 0.60 cubic feet). The 90 lb and metric bag yields are worked out from weight, so check the bag.
How to use the concrete bag calculator
- Pick what you are pouring: a slab or pad, round post holes, or a volume you already know.
- Enter the size in feet and inches, or tap a common slab size.
- Choose the bag size you plan to buy and how much extra to allow for waste.
- Read the bag count. The table shows the count for every bag size, and the cost if you enter a price per bag.
How many bags of concrete are in a yard?
A cubic yard is 27 cubic feet, and each bag says how much concrete it makes. Divide one by the other:
| Bag size | Makes (cu ft) | Bags per cubic yard | Covers at 4 in thick |
|---|---|---|---|
| 40 lb | 0.30 | 90 | 0.9 sq ft |
| 50 lb | 0.375 | 72 | 1.1 sq ft |
| 60 lb | 0.45 | 60 | 1.35 sq ft |
| 80 lb | 0.60 | 45 | 1.8 sq ft |
| 90 lb | 0.675 | 40 | 2 sq ft |
So it takes 45 bags of 80 lb concrete to make one cubic yard, or 60 bags of 60 lb, or 90 bags of 40 lb. The last column is how much ground one bag covers at a normal 4 inch slab thickness: one 80 lb bag covers about 1.8 square feet.
Bags for common slabs and pads
| Pour (4 in thick) | Volume | 80 lb bags | With 10% extra | 60 lb bags with 10% extra |
|---|---|---|---|---|
| 8 x 8 ft pad | 21.3 cu ft | 36 | 40 | 53 |
| 10 x 10 ft slab | 33.3 cu ft | 56 | 62 | 82 |
| 4 x 30 ft walkway | 40 cu ft | 67 | 74 | 98 |
| 12 x 12 ft pad | 48 cu ft | 80 | 88 | 118 |
| 10 x 20 ft slab | 66.7 cu ft | 112 | 123 | 163 |
For any other size, enter the length, width and thickness above. The calculator rounds up to whole bags and shows the count for every bag size at once, so you can compare what is on the shelf.
How to work out the number of bags yourself
- Find the volume in cubic feet. For a slab: length in feet × width in feet × thickness in inches ÷ 12. A 10 by 10 slab 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet.
- Divide by the bag yield. 33.3 ÷ 0.60 = 55.6 bags of 80 lb mix.
- Round up and add a margin. Ground is never perfectly level and some mix is always lost, so add 5 to 10 percent: 55.6 × 1.1 = 61.1, so buy 62 bags.
For a round hole, such as a fence or deck post footing, the volume is 3.1416 × radius × radius × depth, all in inches, divided by 1,728 to get cubic feet. A 10 inch hole 36 inches deep holds 1.64 cubic feet, a little under 3 bags of 80 lb mix. The post itself takes up some of that room: a 4x4 post fills about a quarter of a cubic foot over that depth. For whole fence lines, the fence post concrete calculator adds up every hole and the spare bags.
Why some sources say 45 bags a yard and others 51
The 45 comes from the yield printed on the bag: an 80 lb bag makes 0.60 cubic feet of placed concrete, because the water you add takes up room too. The 51 comes from a different method, dividing the dry weight by the weight of a cubic foot of hardened concrete, about 150 lb, which gives 0.53 cubic feet a bag. Use the yield on the bag you are buying, since that is what the maker measured for that mix, and keep the extra margin for waste. If you are short of a few bags halfway through a pour, the join between batches can show and weaken the slab, so running over is cheaper than running out.
Bags or ready-mix?
Bags make sense for post footings, small pads, steps and repairs. Past about one cubic yard they get hard work fast: a yard of 80 lb bags is 45 bags, 3,600 pounds of mix to lift, mix and place before the first batch starts to set. For a full driveway or patio, ready-mix delivered by truck is usually cheaper per yard and far quicker; many suppliers take small loads for an extra fee. The calculator flags this when your pour passes one yard.
A slab also needs a compacted base under it, usually 4 inches of gravel. The gravel calculator works out that layer, and the tons to cubic yards calculator converts it if the quarry sells by the ton.
How far does one bag go at other thicknesses?
An 80 lb bag covers about 2.1 square feet at 3.5 inches, 1.8 at 4 inches, 1.4 at 5 inches and 1.2 at 6 inches. Thicker slabs are needed under vehicles, which is why a driveway takes far more bags per square foot than a garden path.
Frequently Asked Questions
How many 80 lb bags of concrete are in a yard?
45 bags. Each 80 lb bag makes 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45.
How many 60 lb bags of concrete make a yard?
60 bags, since each 60 lb bag makes 0.45 cubic feet.
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick, 56 bags of 80 lb mix, or 62 with a 10 percent margin. In 60 lb bags it is 82 with the same margin.
How much does an 80 lb bag of concrete cover?
About 1.8 square feet at 4 inches thick, or 2.1 square feet at 3.5 inches.
How many cubic feet are in an 80 lb bag of concrete?
0.60 cubic feet once mixed. A 60 lb bag makes 0.45 and a 50 lb bag 0.375.
How many bags of concrete do I need per fence post?
For a 10 inch hole 36 inches deep, about 3 bags of 80 lb mix per post, a little less once the post is in.