Understanding your result
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.
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 units.
- Enter length, width, and uniform thickness.
- Set an extra allowance. Open Advanced options and check the yield per bag against the product label.
- Calculate the order volume and rounded-up bag count.
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, calculate each section separately and add their volumes. This calculator assumes a uniform rectangular slab and does not model footings, columns, or curved forms.
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.