Concrete Bags
Free Concrete Bags calculator with clear step-by-step results.
About Concrete Bags
Slab volume is length by width by thickness, grossed up for the sub-base irregularity and spillage that every pour loses, then divided by the mixed yield printed on the bag. Bag count is rounded up because you cannot buy a partial bag, and the mass figure is a reality check on handling.
Formulas
Slab volume
Volume m3 = length x width x thickness mm / 1000
Bags required
Bags = ceil(volume x 1000 x (1 + waste) / yield litres per bag)
Worked example
With the default values below, Concrete Bags returns Bags of premixed concrete of 58. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Slab lengthminimum 0.1 m
- 3 m
- Slab widthminimum 0.1 m
- 2.4 m
- Slab thicknessaccepts 20–600 mm
- 100 mm
- Yield per bagminimum 1 L
- 13.5 L
- Waste and over-dig allowanceaccepts 0–40 %
- 8 %
Results
- Bags of premixed concrete
- 58 bags
- Net concrete volume
- 0.720 m3
- Volume including waste allowance
- 0.778 m3
- Approximate mass to carry (20 kg bags)
- 1,160 kg
Step by step
- 1Values used
- 2Slab volume
- 3Bags required
- 4Bags of premixed concrete= 58 bags
- 5Net concrete volume= 0.720 m3
- 6Volume including waste allowance= 0.778 m3
- 7Approximate mass to carry (20 kg bags)= 1,160 kg
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How much does a 20 kg bag yield?
Around 9-10 litres of mixed concrete; a 25 kg bag gives about 12 litres and a 40 lb bag roughly 0.3 cubic feet. Check the bag, as yields differ by mix.
When should I order ready-mix instead?
Above roughly 0.5 cubic metres - about forty 20 kg bags - hand mixing becomes slow enough that cold joints form between batches.