Understanding density and bag yield assumptions
| Material property | Typical value |
|---|---|
| Normal-weight concrete density | ~2.4 t/m³ (2,400 kg/m³) |
| 20 kg pre-mixed bag yield | ~0.0095 – 0.011 m³ (varies by product) |
| Recommended ordering margin | 5 – 10% above calculated volume |
- Bag yield varies by manufacturer and mix design — always confirm the exact yield printed on the specific bag before ordering, rather than relying on the default value alone.
- This calculator computes material quantity only. Structural design (slab thickness, reinforcement, footing sizing for load-bearing elements) should be specified or reviewed by a qualified structural engineer or follow local building code requirements.
What does a concrete calculator do?
A concrete calculator converts the three dimensions of a slab, footing or pad — length, width and depth — into a volume in cubic meters, then divides that volume by the yield of a single bag of pre-mixed concrete to determine how many bags to buy. Bag yield is the volume of mixed, cured concrete produced by one bag and is stated on the product packaging; it varies by product and bag weight, so the default value here (0.0095 m³ for a typical 20 kg general-purpose bag) should be checked against the specific product being used.
Concrete's density is commonly approximated at 2.4 tonnes per cubic meter for normal-weight structural concrete, which this calculator uses to estimate the pour's total weight. A 10% margin volume is also shown, reflecting the common trade practice of ordering slightly more material than the theoretical minimum to allow for an uneven subgrade, formwork variance, and minor spillage or waste during placement.
How to use this concrete calculator
- Enter the length and width of the slab or footing in meters.
- Enter the depth (thickness) of the pour in centimeters.
- Enter the yield of the bag you plan to buy, in cubic meters per bag — check the bag's packaging or data sheet, since this varies by product and bag weight.
- Read the total volume, the number of bags needed (rounded up), the volume with a 10% ordering margin, and the approximate weight of the pour.
The formula behind concrete quantity
Volume equals length multiplied by width multiplied by depth (converted to meters). The number of bags equals that volume divided by the bag's stated yield, rounded up to the next whole bag. Weight is estimated using a typical normal-weight concrete density of 2.4 t/m³.
Worked example: a 4 m × 3 m slab at 10 cm depth has a volume of 4 × 3 × 0.10 = 1.2 m³. At a bag yield of 0.0095 m³ per 20 kg bag, that requires 1.2 ÷ 0.0095 ≈ 126.3, rounded up to 127 bags. With a 10% ordering margin the volume is 1.32 m³, and the approximate weight of the pour is 1.2 × 2.4 = 2.88 tonnes.
Common mistakes
- Entering depth in meters instead of centimeters, which changes the volume result by a factor of 100.
- Using a generic bag-yield figure instead of the value printed on the specific product being purchased.
- Ordering the exact calculated volume with no margin, risking a short pour that leaves a visible cold joint.
- Using a slab-depth assumption without accounting for reinforcement, drainage falls, or an uneven subgrade that changes actual concrete volume on site.
Pertanyaan yang sering diajukan
How much concrete do I need for a 4m x 3m slab?
At a 10 cm depth, a 4 m × 3 m slab needs 4 × 3 × 0.10 = 1.2 cubic meters of concrete. Using a typical 20 kg bag yield of 0.0095 m³, that requires about 127 bags, or 1.32 m³ if ordering with a 10% margin.
How many bags of concrete do I need per cubic meter?
It depends on the bag's yield. A standard 20 kg pre-mixed bag typically yields around 0.0095 m³ once mixed, so roughly 105 such bags are needed per cubic meter — always confirm the exact yield on your specific product.
How much does a cubic meter of concrete weigh?
Normal-weight structural concrete is commonly approximated at about 2.4 tonnes (2,400 kg) per cubic meter once cured, though the exact figure varies slightly with the mix design and aggregate used.
Why should I order extra concrete beyond the calculated volume?
A margin of 5–10% above the calculated volume is standard trade practice to account for an uneven subgrade, formwork variance, and minor spillage, avoiding a short pour that would leave a visible cold joint.
Do I need a structural engineer for a concrete slab?
This calculator estimates material quantity only. Slab thickness, reinforcement and footing design for load-bearing or structural elements should be specified by a qualified structural engineer or follow the applicable local building code.
Referensi
- Portland Cement Association (PCA) — typical density and mix-design reference values for normal-weight concrete.
- American Concrete Institute (ACI) — general guidance on concrete volume estimation and standard practice.
- Manufacturer bag data sheets — stated yield (m³ per bag) is printed on pre-mixed concrete bag packaging and varies by product.