Concrete CalculatorFree online other calculator — instant results, no sign-up
Concrete volume and bags for a slab.
About the Concrete Calculator
This calculator determines the volume of concrete required for a rectangular slab and estimates the equivalent volume in cubic yards and standard 40 kg pre-mixed bags. It is useful for builders, homeowners, and contractors estimating materials for patios, footings, and foundations.
- Length
- The length of the slab in meters (m), typically greater than 0.
- Width
- The width of the slab in meters (m), typically greater than 0.
- Thickness
- The depth or thickness of the slab in centimeters (cm), typically between 5 and 30 cm.
Formula and method
Volume (m³) = Length (m) × Width (m) × (Thickness (cm) / 100) Cubic yards = Volume (m³) × 1.30795 40 kg bags = ceil((Volume (m³) × 2400 / 40) × 1.1)
- LengthSlab length in meters
- WidthSlab width in meters
- ThicknessSlab thickness in centimeters
- VolumeCalculated concrete volume in cubic meters
- 2400Assumed concrete density in kilograms per cubic meter (kg/m³)
- 1.110% allowance factor added for waste and spillage
- ceilRounding up to the nearest whole integer
The calculator first converts thickness from centimeters to meters and multiplies by length and width to find the volume in cubic meters. It then converts this volume to cubic yards using a factor of 1.30795 and calculates the required number of 40 kg dry mix bags assuming a cured concrete density of 2,400 kg/m³ with a 10% waste buffer, rounding up to the next whole bag.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Length3 m
- Width3 m
- Thickness10 cm
Results
- Volume0.9 m³
- Cubic yards1.18
- 40 kg bags60
How to use the Concrete Calculator
- 1Divide the thickness in centimeters by 100 to convert to meters.
- 2Multiply length by width by converted thickness to compute total volume in cubic meters.
- 3Multiply the cubic meters by 1.30795 to obtain cubic yards.
- 4Calculate the total mass using 2,400 kg/m³, divide by 40 kg per bag, apply a 10% waste factor, and round up to the nearest whole bag.
Reading your result
The volume in cubic meters and cubic yards provides the amount to order for ready-mix delivery trucks. The bag count indicates how many 40 kg pre-mixed bags to purchase at a supplier, which includes a built-in 10% safety margin for uneven ground and spillage.
Limitations
The calculation assumes a uniform rectangular slab on a completely flat, level base without considering grade variations, thickened edges, or footing trenches. It also uses a standard estimated density of 2,400 kg/m³ and a fixed 10% waste factor, which may differ based on specific mix designs or sub-base conditions.
Common questions
Why is a 10% buffer included in the bag calculation?+
What density is used for the concrete bag estimate?+
Can this calculator be used for circular slabs or footings?+
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
You might also like
Results are estimates for informational purposes and are not financial, medical or legal advice.