Bags of Concrete per m3: How Many You Need

This calculator works out how many bags of concrete mix you need for a given volume in cubic metres. Enter the volume you need to fill and the bag size you plan to buy, and it returns the number of bags required, rounded up to whole bags.

Formula

The standard calculation uses the approximate density of cured concrete: about 2,400 kg per cubic metre. The formula is:

Bags needed = ceil( (Volume in m3 × 2,400 kg/m3) ÷ Bag weight in kg )

• Volume (m3) – the space you need to fill, in cubic metres.
• 2,400 kg/m3 – the typical density of normal-weight concrete.
• Bag weight (kg) – the dry weight of one bag of concrete mix (e.g., 20 kg, 25 kg, 30 kg).

The result is rounded up because you cannot buy a fraction of a bag.

Worked Example

Using the default values: volume = 1 m3, bag size = 20 kg.

1. Total mix weight = 1 × 2,400 = 2,400 kg.
2. Bags = 2,400 ÷ 20 = 120 bags.
3. Rounded up: 120 bags.

So for 1 cubic metre of concrete with 20 kg bags, you need exactly 120 bags. For a 25 kg bag, it would be 96 bags; for a 30 kg bag, 80 bags.

Why 2,400 kg per Cubic Metre?

The density of concrete varies with the mix design. Normal-weight concrete made with typical aggregates (gravel and sand) falls in the range of 2,200 to 2,500 kg/m3, with 2,400 kg/m3 being the commonly used average for estimation. Lightweight concrete (using expanded clay or shale) can be as low as 1,600 kg/m3, while heavy concrete (with barite or magnetite) can exceed 3,000 kg/m3. For most DIY projects—slabs, footings, paths—2,400 kg/m3 is accurate enough.

Bagged concrete mix is a dry blend of cement, sand, and aggregate. The bag weight refers to the dry mix, not the cured concrete. When water is added, the mix undergoes a chemical reaction that binds everything together, and the final volume is slightly less than the dry volume. The 2,400 kg/m3 figure applies to the cured concrete, which is what you need to fill the formwork.

How to Measure Your Volume

Before you can calculate bags, you need the volume of the space to fill. For simple shapes:

• Rectangular slab: length × width × thickness (all in metres).
• Circular pad: π × radius² × thickness.
• Footing trench: length × width × depth.

For example, a path 5 m long, 1 m wide, and 0.1 m thick is 5 × 1 × 0.1 = 0.5 m3. With 25 kg bags, that would be 0.5 × 2,400 ÷ 25 = 48 bags.

For irregular shapes, break them into rectangles or triangles, calculate each, and add them together. Always measure in metres to get cubic metres directly. If you measure in centimetres, convert by dividing by 100 for each dimension.

Common Mistakes and How to Avoid Them

One frequent error is confusing dry bag volume with wet concrete volume. A 20 kg bag of mix does not produce 20 kg of concrete—it produces roughly 0.0083 m3 of cured concrete (20 ÷ 2400). Some people use the bag’s stated “yield” (often around 0.01 m3 for a 20 kg bag) which is close but not exact.

Another mistake is forgetting to account for waste. On-site you will spill some, lose some to the ground, or make a slight error in the form. It is wise to add 5–10% to your calculated volume. For a 1 m3 project, that means buying 5–10% more bags.

Also, do not mix units. If you measure the slab in feet, convert to metres first (1 foot = 0.3048 m) or use a cubic feet calculator. Mixing feet and metres will give wildly wrong results.

Choosing the Right Bag Size

Bag sizes commonly available at home improvement stores are 20 kg, 25 kg, and 30 kg. Larger bags (40 kg or more) are sometimes sold, but they are heavy to carry and mix. The right size depends on the project scale:

• Small repairs (under 0.05 m3): a 20 kg bag is manageable.
• Medium projects (0.05–0.2 m3): 25 kg bags balance weight and number of trips.
• Large pours (over 0.2 m3): consider ordering ready-mix concrete instead of bags—it is often cheaper and less labour-intensive.

For a 1 m3 slab, you would need about 80–120 bags depending on size. That is a lot of mixing by hand; a concrete mixer or ready-mix delivery is more practical for volumes above 0.3 m3.

Cost Considerations

The cost per cubic metre from bags is typically higher than ready-mix concrete because you are paying for packaging, transport, and convenience. However, bags are ideal for small jobs where ready-mix has a minimum delivery charge. To estimate cost, multiply the number of bags by the price per bag. For a 1 m3 project with 20 kg bags, that is 120 bags—at a typical retail price of $5–$8 per bag, the total could be $600–$960. Ready-mix for the same volume might cost $150–$300 plus delivery, but you need a crew to pour it quickly.

Note that prices vary by region and store, so check local rates. The calculator gives you the quantity; you can apply your local price.

Safety and Practical Tips

Concrete mix contains cement, which is caustic. When mixing, wear gloves, long sleeves, and eye protection. Avoid breathing the dust—use a dust mask if you are in a confined space. Wet concrete can cause skin burns; wash any contact with water immediately.

When mixing, follow the bag instructions for water. Too much water weakens the concrete; too little makes it hard to work. The mix should be workable but not soupy. Use a wheelbarrow or a mixer for large batches. Pour the concrete within 30–60 minutes of mixing, as it begins to set.

For structural work (foundations, load-bearing walls), consult a professional engineer. The 2,400 kg/m3 density is for normal-weight concrete; if you use a special mix, adjust accordingly.

Frequently Asked Questions

How many 20kg bags of concrete do I need for 1m3?
Using the standard density of 2,400 kg/m3, you need 120 bags of 20 kg mix. That is 2,400 kg of dry mix in total.

Does the bag size affect the number of bags?
Yes, inversely. A 25 kg bag yields more concrete per bag, so you need fewer bags. For 1 m3, you need 96 bags of 25 kg, or 80 bags of 30 kg.

Is the density of concrete always 2,400 kg/m3?
No, it ranges from about 2,200 to 2,500 kg/m3 for normal concrete. Lightweight concrete is lower, heavy concrete is higher. For DIY estimation, 2,400 is a safe average.

Should I add extra bags for waste?
Yes, adding 5–10% is recommended. For a 1 m3 pour, that means buying 6–12 extra bags depending on your confidence and the site conditions.

Can I use this calculator for mortar or screed?
No. Mortar and screed have different densities and mix proportions. Mortar is about 2,100 kg/m3, screed about 2,200 kg/m3. Use a dedicated calculator or adjust the density in the formula.

What if my project is in cubic feet?
Convert cubic feet to cubic metres by multiplying by 0.0283168. For example, 10 cubic feet is 0.283 m3. Then use the calculator.

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in v | Volume needed | m3 | 1 | 0
in b | Bag size | kg | 20 | 0
out bags | Bags needed | bags | ceil(v * 2400 / b) | 0
out weight | Total mix weight | kg | v * 2400 | 0