Concrete Calculator
Choose what you are pouring, enter its dimensions in any mix of units, and see how much concrete it takes as a volume, a weight and a bag count. Add a price and the same figures become a cost estimate.
Created by the Reeliy Editorial Team. Last updated: Oct 19, 2026
How to use the concrete calculator
- Pick the concrete form: rectangular slab, wall, footing, rectangular column, round column (which also covers a round slab or footing), steps, or curb and gutter.
- Enter each dimension with its own unit. Mixing inches for thickness with feet for length is fine, the calculator converts everything before it multiplies.
- For identical pieces such as four columns or three footings, set the quantity. Steps and curbs are always one piece.
- Optionally describe the material you will buy. Choose bags or bulk, type the bag size and, if the bag states it, the yield, or type a density directly. A price turns the answer into a cost.
- Press Calculate. The results list the volume of one piece, the total volume, the weight, the number of bags if you chose bags, and the cost if you entered a price.
Use the Unit system switch first. It decides whether volumes are reported in cubic yards and cubic feet or in cubic meters, and whether weights come out in pounds and tons or kilograms and tonnes.
Volume formulas for each shape
Every concrete quantity starts as a volume. The shapes differ only in how the cross-section is described.
| Form | Volume of one piece |
|---|---|
| Slab, wall, footing, rectangular column | length × width × height (thickness) |
| Round column, round slab or footing | π × (diameter ÷ 2)2 × height |
| Steps | width × [platform depth × steps × rise + run × rise × steps × (steps − 1) ÷ 2] |
| Curb and gutter | length × [curb depth × (curb height + flag thickness) + gutter width × flag thickness] |
For the steps formula, the number of steps includes the platform. The first term is the solid block under the platform, as tall as all the risers together. The second adds the blocks in front of it: the lowest one is one rise high, the next two rises, and so on, which sums to steps × (steps − 1) ÷ 2 rises.
volume = length × width × thicknessKeep all three in the same unit before you multiply.
Worked example: a patio slab
A patio measures 12 ft by 10 ft and is poured 4 inches thick. Four inches is 1/3 ft, so the volume is 12 × 10 × 1/3 = 40 ft3. Dividing by 27 gives 1.481 yd3.
At the calculator's default density of 2,130 kg/m3 (about 133 lb/ft3) that slab weighs 5,319 lb, or 2.66 short tons. With 80 lb bags that each make 0.6 ft3, you would buy 40 ÷ 0.6 = 66.7, so 67 bags.
Worked example in metric units
A garage apron of 3 m by 4 m at 10 cm thick holds 3 × 4 × 0.10 = 1.2 m3. If a 25 kg bag is labelled as making 0.012 m3, the bags carry a density of 25 ÷ 0.012 ≈ 2,083 kg/m3, so the pour weighs 2,500 kg and takes 100 bags.
Cubic yards, cubic feet and cubic meters
Ready-mix is sold by the cubic yard in the United States and by the cubic meter elsewhere. One cubic yard is 27 cubic feet, or about 0.7646 m3. One cubic meter is about 35.31 ft3 or 1.308 yd3. For other volume units see the volume converter.
| Slab | Volume (ft3) | Volume (yd3) | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft, 4 in thick | 33.33 | 1.23 | 56 |
| 12 × 12 ft, 4 in thick | 48 | 1.78 | 80 |
| 20 × 20 ft, 4 in thick | 133.33 | 4.94 | 223 |
| 24 × 24 ft, 4 in thick | 192 | 7.11 | 320 |
| 10 × 10 ft, 6 in thick | 50 | 1.85 | 84 |
| 20 × 20 ft, 6 in thick | 200 | 7.41 | 334 |
Weight and density
Weight is volume times density. Normal-weight structural concrete falls between roughly 1,900 and 2,400 kg/m3 (120 to 150 lb/ft3); lightweight and heavyweight mixes sit outside that range. When you give no density the calculator assumes 2,130 kg/m3.
There are two ways to override it. Type a density if you know it, or enter the bag size and bag yield from the label and the calculator divides one by the other. If both are present, the bag figures win.
A cubic yard at the default density weighs about 3,590 lb, a little under two short tons. That matters for a trailer or a wheelbarrow route: a single cubic foot is already about 133 lb.
Bags or a ready-mix truck?
Bagged mix makes sense for small jobs, remote sites and pours you can stage over several days. With yields of 0.6, 0.45 and 0.3 ft3, a cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.
Beyond a cubic yard or so, hand mixing gets slow enough that the first batch can start to set before the last is placed. A truck delivers the whole quantity at once, although suppliers usually set a minimum load and charge extra for small ones. Compare the two with the price fields: enter a price per bag, or the bulk price per cubic yard (cubic meter), and the results show the total cost, plus the cost of a single piece when you pour more than one.
Avoiding a shortfall
- Order extra. Allow 5 to 10 percent for uneven ground, forms that bow outward and spillage. Running a few bags short in the middle of a pour leaves a cold joint.
- Measure the real thickness. A slab poured on a rough subgrade is thicker than its forms suggest. Average several readings.
- Do not forget the base. The gravel or crushed stone under a slab is a separate material, see the gravel calculator and the sand calculator.
- Check the unit on every field. A 4 in thickness entered as 4 ft is twelve times too much concrete.
These are planning estimates. Mix design, reinforcement and structural thickness must come from your local code, the supplier or an engineer. See the disclaimer.
Frequently asked questions
How many bags of concrete are in a cubic yard?
It depends on the bag. A cubic yard is 27 ft3, so you need 45 bags that yield 0.6 ft3 each (80 lb), 60 bags at 0.45 ft3 (60 lb) or 90 bags at 0.3 ft3 (40 lb). Check the yield printed on your brand.
How much does a cubic yard of concrete weigh?
Normal concrete weighs between about 3,200 and 4,000 lb per cubic yard. With the calculator's default density of 2,130 kg/m3 the figure is roughly 3,590 lb, or 1.8 short tons.
How much extra concrete should I order?
Add 5 to 10 percent to the calculated volume. The extra covers an uneven base, slightly oversized forms and material left in the mixer or truck chute.
Can I use the round-column option for a round slab?
Yes. The round form computes π × (diameter ÷ 2)2 × height, which is the volume of any cylinder, so it fits a circular slab, a pier hole or a round footing as well as a column.
Cite this calculator and page
If you use this page in your work, you can cite it as:
Reeliy Editorial Team. (2026). Concrete Calculator. Reeliy. https://reeliy.com/calculators/concrete-calculator“Concrete Calculator.” Reeliy, 2026, https://reeliy.com/calculators/concrete-calculator.