Concrete order volume and whole bags
Order volume is the rectangular volume plus your extra allowance. The result also shows both cubic metres and cubic yards. Bag count divides the adjusted volume by the stated yield and rounds up to a whole bag.
If you first need the footprint of a slab, the Square Footage Calculator converts rectangular area between square feet and square metres before you choose a thickness.
The formula
Dimensions are converted to metres before multiplying. One foot is 0.3048 metres and one inch is 0.0254 metres. One cubic yard is 0.764554857984 cubic metres. In imperial mode, bag yield is entered in cubic feet. Known area substitutes for length times width. An independently selected bag-yield unit is converted to the same volume basis before dividing; changing units preserves the represented yield.
A 5 m × 4 m slab, 10 cm thick
Convert 10 cm to 0.1 m. The base volume is 5 × 4 × 0.1 = 2 m³. Adding 10% gives an order volume of 2.2 m³. If each bag yields 0.012 m³ of mixed concrete, 2.2 ÷ 0.012 = 183.33, so you need 184 whole bags.
How to use this calculator
- Choose metric or imperial measurements, then dimensions or a known footprint area. Switching units converts current measurements.
- Enter uniform thickness and the number of identical slabs.
- Set the allowance and check bag yield in Advanced options. Yield can use cubic metres, cubic feet or the current measurement system.
- Read live order volumes and whole-bag working. Numeric yield shortcuts remain editable and must match the product label.
Measure the finished slab
Use the inside dimensions of the formwork and the intended finished thickness. Take several depth measurements if the base is uneven; an average thickness can be more useful than a single reading.
For a shape made of rectangles with different dimensions or depths, calculate each section separately and add the volumes. The quantity field repeats identical slabs; this calculation does not model columns, curved forms or nonuniform thickness.
Allowance and bag yield
An extra allowance can account for uneven ground, spills, and variations in measurements. Ten percent is an editable starting value, not a requirement for every project. Select an allowance that fits the actual site and supplier guidance.
Bag weight is not the same as mixed-concrete yield. Two products with the same dry weight can produce different volumes. Check the manufacturer’s stated yield before relying on the bag estimate.
Assumptions & limitations
What this calculation assumes
- The slab is rectangular with a uniform depth.
- The extra allowance is applied to the whole base volume.
- Switching measurement systems converts current dimensions and bag yield to preserve the physical quantities.
What to keep in mind
- This estimate does not determine structural thickness, reinforcement, or concrete strength.
- Supplier order increments, delivery minimums, and product yields can change the quantity you purchase.
- The default bag yield is an example and must be checked for your chosen product.
Common questions
Are cement and concrete the same?
No. Cement is one ingredient in concrete. Use the yield for a concrete mix, not a bag of cement alone.
Why are bags rounded up?
A partial bag count would not provide enough material. Rounding up ensures the estimated volume is covered by whole bags.
What units do I use for thickness?
Use centimetres in metric mode and inches in imperial mode. Length and width use metres or feet respectively.
Sources & further reading
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