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Concrete calculator: bags for slab, footing, and column
Estimate how many concrete bags you need to buy for your pour and the total volume in cubic meters (m³), cubic feet (ft³), or cubic yards (yd³). The tool works with slab, footing, and cylindrical column shapes, in metric (Europe/LatAm) or American (US) units, and already includes a configurable waste margin.
Last reviewed: July 2026 · How we calculate
Input data
Yield varies by product.
Results
Enter the data and press Calculate.
Results are estimates. Always verify manufacturer or supplier specifications before buying.
How the concrete you need is calculated
Calculating concrete is, in essence, calculating a volume and then converting that volume into the number of bags you'll need to buy. The process has three steps and this calculator solves them for you, but it helps to understand them so you can check that the result makes sense:
- Geometric volume. It is obtained by multiplying the dimensions of the piece. A rectangular slab or footing is length × width × thickness. A cylindrical column uses the area of the circle (π × radius²) multiplied by the height.
- Waste adjustment. A percentage (10% by default) is added to the theoretical volume to cover real job-site losses: mixing losses, uneven ground, over-excavation, and small measurement errors.
- Conversion to bags. The adjusted volume is divided by the yield of one bag (how much fresh concrete a bag of premixed cement produces) and rounded up, because in practice you can't buy half a bag.
Formula
For slabs and footings (rectangular):
Adjusted volume = Volume × (1 + Waste ÷ 100)
Bags = round up( Adjusted volume ÷ Yield per bag )
For cylindrical columns, the volume changes to:
In the American system, thickness and diameter are entered in inches and converted to feet (÷ 12) before multiplying, so the volume ends up in cubic feet (ft³). That volume is also shown converted to cubic yards (yd³ = ft³ ÷ 27), the unit that ready-mix trucks are usually sold in.
Practical example
Suppose a slab 3 m long × 2 m wide × 0.10 m thick, with 50 kg bags (yield 0.0225 m³) and 10% waste:
| Geometric volume | 3 × 2 × 0,10 = 0,600 m³ |
|---|---|
| Volume with 10% waste | 0,600 × 1,10 = 0,660 m³ |
| 50 kg bags (0.0225 m³ each) | 0,660 ÷ 0,0225 = 29,3 → 30 bags |
It's exactly the result you'll see if you press "Calculate" with the default values. If you switch to 40 kg bags (yield 0.018 m³), you'll need 37 bags for the same pour; smaller bags yield less volume, so more are needed.
Typical yield by bag size
Yield is the value that most influences the result. These are common reference values for bagged premixed concrete (a dry mix of cement, sand, and gravel to which you only add water):
| 20 kg bag | ≈ 0,009 m³ |
|---|---|
| 30 kg bag | ≈ 0,0135 m³ |
| 40 kg bag | ≈ 0,018 m³ |
| 50 kg bag | ≈ 0,0225 m³ |
| 60 lb bag | ≈ 0,45 ft³ |
| 80 lb bag | ≈ 0,60 ft³ |
If your supplier lists a different yield on the label, enter it in the "Yield per bag" field to fine-tune the estimate.
Concrete types and strength (f'c): what to order for each element
Besides volume, when buying concrete you must choose the strength, expressed as f'c in kg/cm² (or PSI in the US): the compression the concrete withstands after 28 days of curing. Ordering more than needed raises the cost; ordering less compromises the element. Common references for light construction:
| Sidewalks, walkways, and garden bases | f'c 100–150 kg/cm² (≈1,500–2,000 PSI) |
|---|---|
| Interior floor slabs (residential) | f'c 150–200 kg/cm² (≈2,000–3,000 PSI) |
| Light slabs and beams | f'c 200–250 kg/cm² (≈3,000–3,500 PSI) |
| Columns, footings, and structural elements | f'c 250–300 kg/cm² (≈3,500–4,500 PSI) |
Strength doesn't change the volume you calculated above, but it does change the mix proportions and the price per bag or per cubic meter/yard. Premixed bags state their strength on the label; if you mix on site, the cement:sand:gravel ratio and the amount of water determine the final strength. For any structural element (columns, load-bearing slabs), an engineer should define the proportions — this table is a guide for minor works only.
Common mistakes when estimating concrete
- Forgetting the waste. Buying the exact amount almost always leaves the pour short. Keep the margin at 10% or raise it on very uneven ground.
- Mixing units. Don't combine inches of thickness with meters of length. Use a single measurement system; the calculator handles it for you when you change the selector.
- Confusing weight with volume. A 50 kg bag doesn't produce 50 liters of concrete: it produces about 22.5 liters (0.0225 m³). The weight is of the dry material, not the fresh concrete.
- Pouring in parts without planning. Structural elements benefit from a continuous pour; calculate the total and prepare the bags before you start.
Frequently asked questions
How much does a bag of concrete yield?
It depends on the weight. A 50 kg bag yields approximately 0.0225 m³ of mixed concrete; an 80 lb bag yields around 0.60 ft³. Always check the product sheet because the yield changes depending on the brand and the water ratio.
Why should I add waste to the calculation?
A 10% margin covers losses from spills, uneven ground, compaction, and measurement errors. It's cheaper to have a little extra than to run short in the middle of a pour, when you can no longer stop.
Can I use this calculator for any type of concrete?
Yes, but adjust the yield per bag according to the product. High-strength, premixed, or additive-containing mixes may yield a different volume per bag.
How do I calculate the volume of a slab?
Multiply length × width × thickness with all measurements in the same unit. For example, a 4 × 3 m slab with 12 cm (0.12 m) thickness has 4 × 3 × 0.12 = 1.44 m³.
What thickness should I use for a slab?
It depends on the use: light interior floors are usually 8 to 10 cm; sidewalks and patios, 10 to 12 cm; and slabs that support vehicles, 15 cm or more. For structural elements, consult an engineer.
Is it better to buy bags or ready-mix concrete?
For small volumes (a few m³) bags are practical and economical. For large volumes or continuous pours, ready-mix delivered by truck (sold by m³ or yd³) is usually better and saves mixing work.
Curing: the free step that defines final strength
Buying the right bags is half the job; the other half is curing: keeping the concrete moist during the first days so the cement's chemical reaction can complete. Uncured concrete can lose up to 30% of its potential strength, even if you paid for a high-spec mix.
The basics: wet the surface 2–3 times a day for at least 7 days (the first 3 are critical), or cover it with plastic film or damp burlap to retain moisture. In hot or windy weather, water evaporates faster than the cement consumes it — water more often and avoid pouring during peak sun. In cold weather, setting slows down: protect the surface and don't load it early. As a rule of thumb, concrete reaches ~70% of its strength at 7 days and 100% of design strength at 28.