A concrete volume calculator turns the length, width and thickness of a pour into cubic meters, cubic yards or cubic feet. This concrete calculator also adds a waste allowance and splits the total into cement bags, sand, aggregate and water for the mix ratio you choose. It runs in your browser, shows every formula and step under the result, and lets you copy the answer in one click.
Quick answer: for a rectangular slab, volume = length × width × thickness. In feet and inches, cubic yards = square feet × thickness in inches ÷ 324. Add an allowance for waste before you order. One cubic yard is 27 cubic feet, or 0.7646 cubic meters.
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How to Use This Concrete Calculator
- Choose units and shape. Pick metric or imperial, then a box (slab, footing, wall or beam), a round shape (column or circular pad) or a hollow cylinder. Every size has its own unit (m, cm, mm, ft or in).
- Add shapes. Add each part of the job and the tool sums them into one total.
- Set the mix. Enter the waste percentage, mix ratio, cement bag size and water to cement ratio.
- Read and copy. Check the volume, the materials and the working, then use Copy result.
Concrete Volume Formulas
Every shape uses one of three formulas. N is the number of identical pieces.
| Shape | Formula | Typical use |
|---|---|---|
| Box | V = L × W × H × N | Slabs, footings, walls, beams |
| Round | V = π × (D ÷ 2)² × H × N | Round columns, circular pads |
| Hollow cylinder | V = π ÷ 4 × (D<sub>out</sub>² − D<sub>in</sub>²) × H × N | Rings, pipes, tubes |
All sizes must be in the same unit before you multiply. The calculator converts everything to metres first (1 ft = 0.3048 m and 1 in = 0.0254 m), which avoids mixing inches with feet or millimetres with metres.
Slab Concrete Calculator: Worked Example
Take a slab 10 ft long, 10 ft wide and 4 in thick.
- Thickness in feet: 4 ÷ 12 = 0.3333 ft.
- Volume: 10 × 10 × 0.3333 = 33.33 ft³.
- Cubic yards: 33.33 ÷ 27 = 1.23 yd³.
- With 5 percent waste: 1.23 × 1.05 = 1.30 yd³.
Because 12 × 27 = 324, you can shortcut this: cubic yards = square feet × thickness in inches ÷ 324. In metric, cubic meters = square meters × thickness in millimetres ÷ 1000, so 20 m² at 100 mm is 2 m³. The table applies the shortcut to common sizes at 4 in thickness.
| Slab size (ft) | Area (ft²) | Volume (ft³) | Cubic yards | With 5% extra | With 10% extra |
|---|---|---|---|---|---|
| 10 × 10 | 100 | 33.33 | 1.23 | 1.30 | 1.36 |
| 12 × 12 | 144 | 48.00 | 1.78 | 1.87 | 1.96 |
| 12 × 20 | 240 | 80.00 | 2.96 | 3.11 | 3.26 |
| 16 × 16 | 256 | 85.33 | 3.16 | 3.32 | 3.48 |
| 20 × 20 | 400 | 133.33 | 4.94 | 5.19 | 5.43 |
The 4 in thickness is an example only. Your drawings, engineer or local requirements set the real thickness.
How Much Extra Concrete to Add
Forms bulge, ground is uneven and some concrete is spilled. The National Ready Mixed Concrete Association covers this in its technical note CIP 8, Discrepancies in Yield. It advises ordering at least 4 to 10 percent more than the volume worked out from plan dimensions, and gives an example: a 1/8 in difference in a 4 in slab is about 3 percent.
The calculator starts at 5 percent and accepts 0 to 50. Treat it as a starting value, not a rule. Rough ground or a complex pour may need more.
Cement, Sand and Aggregate: How the Mix Is Calculated
The second half of the tool works as a cement sand aggregate calculator. Each step appears in the Working section under your result.
- Dry volume. Loose materials pack together once water is added, so you need more dry material than finished concrete. The tool multiplies the volume (with waste) by a dry volume factor. The default is 1.54, a widely used site-estimation value (some sources quote 1.54 to 1.57), and you can set 1.2 to 1.8.
- Split by ratio. Material volume = dry volume × its part ÷ the sum of all parts. A 1 : 1.5 : 3 mix has 5.5 parts.
- Convert to weight. Weight = volume × bulk density. Defaults: cement 1,440, sand 1,600 and aggregate 1,500 kg/m³. Real densities change with moisture and material type.
- Count bags and water. Bags = cement weight ÷ bag weight. Water (litres) = cement weight (kg) × water to cement ratio, default 0.5.
The table shows what these defaults give for each concrete mix ratio in the tool, per 1 m³ of finished concrete, before waste.
| Mix | Ratio | Cement bags (50 kg) | Cement (kg) | Sand (m³) | Aggregate (m³) |
|---|---|---|---|---|---|
| M5 | 1 : 5 : 10 | 2.77 | 138.6 | 0.481 | 0.963 |
| M7.5 | 1 : 4 : 8 | 3.41 | 170.6 | 0.474 | 0.948 |
| M10 | 1 : 3 : 6 | 4.44 | 221.8 | 0.462 | 0.924 |
| M15 | 1 : 2 : 4 | 6.34 | 316.8 | 0.440 | 0.880 |
| M20 | 1 : 1.5 : 3 | 8.06 | 403.2 | 0.420 | 0.840 |
| M25 | 1 : 1 : 2 | 11.09 | 554.4 | 0.385 | 0.770 |
Estimates only, based on the default assumptions above. The ratios are common nominal ratios, so your project documents decide which mix applies.
Worked Example: 5 m × 4 m × 150 mm Slab in M20
This uses 5 percent waste, a 1 : 1.5 : 3 mix, 50 kg bags and the default assumptions.
| Step | Calculation | Result |
|---|---|---|
| Slab volume | 5 × 4 × 0.15 | 3.000 m³ |
| With 5% waste | 3.000 × 1.05 | 3.150 m³ |
| Dry volume | 3.150 × 1.54 | 4.851 m³ |
| Cement | 4.851 × 1 ÷ 5.5 = 0.882 m³, then × 1,440 | 1,270.1 kg, or 25.40 bags |
| Sand | 4.851 × 1.5 ÷ 5.5 = 1.323 m³, then × 1,600 | 2,116.8 kg |
| Aggregate | 4.851 × 3 ÷ 5.5 = 2.646 m³, then × 1,500 | 3,969.0 kg |
| Water | 1,270.08 × 0.5 | 635.0 L |
Cement comes to 25.40 bags, so buy 26 whole bags. The tool shows both figures.
Frequently Asked Questions
How do I calculate concrete volume?
Multiply length × width × thickness for a box shape, with all sizes in the same unit. For a round shape use π × (diameter ÷ 2)² × height. Then add an allowance for waste.
How many cubic yards of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 ft × 10 ft slab needs 1.23 yd³. With 5 percent extra it is 1.30 yd³, and with 10 percent extra it is 1.36 yd³.
How many bags of cement do I need per cubic meter?
It depends on the mix. With the calculator's default assumptions, a 1 : 1.5 : 3 mix needs about 8.06 bags of 50 kg cement per cubic meter of finished concrete, before waste. Treat that as an estimate.
What is the dry volume factor?
It converts the volume of finished concrete into the larger volume of loose dry materials needed to make it. The calculator starts at 1.54 and lets you change it from 1.2 to 1.8.
What is the water to cement ratio?
It is the weight of water divided by the weight of cement. The calculator starts at 0.5 and accepts 0.25 to 1. Use the value your mix design specifies.
Can I use this concrete calculator for columns and footings?
Yes. Use the round shape for circular columns, the box shape for square columns and footings, and add several shapes to get one total.
Is the result exact?
No. It is an estimate that depends on your measurements and the assumptions you choose. Confirm it with your supplier or engineer.
Quick Summary
Measure each shape in one unit system, multiply to get the volume, and add a waste allowance. The calculator does this for slabs, columns and hollow shapes, then converts the total into cement bags, sand, aggregate and water, and shows every step. Use the defaults as starting points and confirm final quantities with your supplier or engineer.
Important Limits and Disclaimer
This calculator and the figures in this article are estimates for planning and ordering only. They are not structural, engineering, architectural or mix design advice, and they do not check strength, durability, building codes or local regulations. Results depend entirely on the sizes and assumptions you enter. The waste allowance, dry volume factor, bulk densities, mix ratios and water to cement ratio are editable starting values that may not match your materials, site or project. The tool does not deduct for reinforcement, openings or voids, and it does not account for pumping losses, ground conditions or form movement.
Always confirm final quantities and the mix with your ready-mix supplier, a qualified engineer or the project specification before you order, buy materials or pour. Nothing here replaces drawings, specifications or professional judgment.
The calculator and this article are provided "as is" without warranty of any kind. To the fullest extent permitted by law, the publisher is not responsible for any loss, cost, delay, shortage, excess or damage that results from using or relying on these results.