Quick answer
Concrete volume = length × width × thickness ÷ 12 × quantity × (1 + waste), converted to cubic yards by dividing cubic feet by 27. A 20 × 10 ft slab 4 inches thick with 10% waste needs 2.716 yd³ (73.33 ft³), or 123 bags of 80 lb mix.
Enter length, width, thickness and waste. The calculator returns cubic yards, cubic feet, cubic metres and the number of bags.
Related tools: Concrete Steps Calculator: Concrete volume for solid stacked steps. Concrete Weight Calculator: Weight of a volume of concrete from its density. Post Hole Concrete Calculator: Concrete per post hole after the post's volume.
How to use the Concrete Volume Calculator
- Enter length (ft) (above 0).
- Enter width (ft) (above 0).
- Enter thickness (in) (above 0).
- Enter waste allowance (%) (between 0 and 100). It starts at 10.
- Optionally, enter number of identical slabs (a whole number, 1 or more).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Length | A number, in ft | > 0 |
| Width | A number, in ft | > 0 |
| Thickness | A number, in in | > 0 |
| Waste allowance | A number, in % | ≥ 0, ≤ 100, starts at 10 |
| Number of identical slabs (optional) | A whole number | ≥ 1 |
Formula and method
Formula
volume (ft³) = length × width × thickness ÷ 12 × quantity × (1 + waste); yd³ = ft³ ÷ 27
IEEE-754 double precision; shown to the decimals indicated.
Bag counts use typical label yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³; confirm on your product. Plan geometry only; confirm quantities with your supplier and local code before ordering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Length = 20; Width = 10; Thickness = 4; Waste allowance = 10.
| Result | Value |
|---|---|
| Concrete needed | 2.716 yd³ |
| Cubic feet | 73.33 |
| Cubic meters | 2.077 |
| 80 lb bags | 123 |
| 60 lb bags | 163 |
| 40 lb bags | 245 |
Example 2
Inputs: Length = 12; Width = 12; Thickness = 6; Waste allowance = 5.
| Result | Value |
|---|---|
| Concrete needed | 2.8 yd³ |
| Cubic feet | 75.6 |
| Cubic meters | 2.141 |
| 80 lb bags | 126 |
| 60 lb bags | 168 |
| 40 lb bags | 252 |
Example 3
Inputs: Length = 10; Width = 10; Thickness = 4; Waste allowance = 0; Number of identical slabs = 3.
| Result | Value |
|---|---|
| Concrete needed | 3.704 yd³ |
| Cubic feet | 100 |
| Cubic meters | 2.832 |
| 80 lb bags | 167 |
| 60 lb bags | 223 |
| 40 lb bags | 334 |
The first line is the headline result (concrete needed). Below it the concrete Volume Calculator lists cubic feet, cubic meters, 80 lb bags, 60 lb bags and 40 lb bags, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The calculation
20 × 10 × (4 ÷ 12) = 66.67 ft³; adding 10% gives 73.33 ft³, and 73.33 ÷ 27 = 2.716 yd³. Ready-mix is ordered by the yard, so round up and ask the supplier about minimums.
Waste and subgrade
The waste allowance covers spillage and an uneven subgrade. A sloped or soft base increases the amount of concrete needed. For small jobs, bags are practical: 123 bags of 80 lb (0.6 ft³ each) is a lot of mixing.
How the result changes with length
Holding every other input at the first worked example, this table changes only length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Length | Concrete needed | Cubic feet | Cubic meters |
|---|---|---|---|
| 10 | 1.358 yd³ | 36.67 | 1.038 |
| 20 | 2.716 yd³ | 73.33 | 2.077 |
| 40 | 5.432 yd³ | 146.67 | 4.153 |
| 80 | 10.864 yd³ | 293.33 | 8.306 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Length: entering "0" shows "Length must be greater than 0." beside that field and no result.
- Thickness: entering "0" shows "Thickness must be greater than 0." beside that field and no result.
Choosing between the Concrete Volume Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Concrete Steps Calculator](/construction/concrete-steps-calculator/) | number of steps, riser height and tread depth | Concrete volume |
| [Concrete Weight Calculator](/construction/concrete-weight-calculator/) | volume, volume unit and density | Weight |
| [Post Hole Concrete Calculator](/construction/post-hole-concrete-calculator/) | hole diameter, hole depth and post shape | Concrete per hole |
| [Concrete Mix Calculator](/construction/concrete-mix-calculator/) | volume of concrete, volume unit and nominal mix | Cement |
| [Concrete Reinforcing Mesh Calculator](/construction/concrete-reinforcing-mesh-calculator/) | slab area, sheet width and sheet length | Sheets needed |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Waste covers spillage and uneven subgrade
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for length and thickness to confirm this.
Frequently asked questions
How thick should a slab be?
Patios and walkways are often 4 inches; driveways 5 to 6.
How many yards is 100 ft²?
At 4 inches thick, about 1.23 yd³ before waste.
Do I need reinforcement?
Often mesh or rebar, depending on use.
What does the Concrete Volume Calculator calculate?
Cubic yards of concrete and bags needed for a slab, footing or pad. It returns Concrete needed, Cubic feet, Cubic meters, 80 lb bags and 60 lb bags.
What formula does the Concrete Volume Calculator use?
volume (ft³) = length × width × thickness ÷ 12 × quantity × (1 + waste); yd³ = ft³ ÷ 27. Bag counts use typical label yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³; confirm on your product.
What do I need to enter in the concrete Volume Calculator?
You enter Length (ft), Width (ft), Thickness (in) and Waste allowance (%). Optional: Number of identical slabs.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Waste covers spillage and uneven subgrade.
What is an example of a concrete Volume result?
With length 20, width 10, thickness 4 and waste allowance 10, the concrete needed is 2.716 yd³.
Where does the method come from?
The method follows: OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem; NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L); Typical pre-mixed bag yields as printed on manufacturers' bags (varies by brand and product; check your bag).
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Waste covers spillage and uneven subgrade.
Related calculators
- Concrete Steps Calculator — Concrete volume for solid stacked steps.
- Concrete Weight Calculator — Weight of a volume of concrete from its density.
- Post Hole Concrete Calculator — Concrete per post hole after the post's volume.
- Concrete Mix Calculator — Cement, sand and aggregate for a nominal volume mix such as 1:2:4.
- Concrete Reinforcing Mesh Calculator — Mesh sheets for a slab, allowing for overlaps.
- Concrete Block Fill Calculator — Grout or concrete needed to fill block cores.
- Beer Volume Converter — Convert kegs, cans, bottles, pints and litres of beer.
- Excavation Volume Calculator — Soil volume to dig for a pit, trench or foundation, with an optional swell factor.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
- Typical pre-mixed bag yields as printed on manufacturers' bags (varies by brand and product; check your bag)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.