VolunaroConcrete calculator
How the calculation works
- Area: we add up every shape you enter and subtract the ones marked as cut-outs. A round column or post hole is a circle.
- Volume: area × thickness gives the net volume of concrete. We then add the waste margin for uneven ground and spills, and show the result in cubic yards, the unit ready-mix is sold in.
- Bags: volume divided by the yield of the bag size you pick (or the yield printed on your bag), rounded up, because you cannot buy part of a bag.
- Bag yields are approximate and depend on the mix: they are listed under «Default values and sources».
Worked example
A 10 × 10 ft slab, 4 in thick, with the default 10 % margin and 80 lb bags of mix.
- Area
- 100.0 sq ft
- Concrete to order
- 1.36 cu yd
- Bags of mix
- 62
Default values and sources
| Value | Default | Type | Sources |
|---|---|---|---|
| Waste margin for concrete | 10 % | Our default (editable) | — |
| Yield of a 40 lb bag of concrete mix | 0.3 cu ft | Typical published value | Quikrete |
| Yield of a 60 lb bag of concrete mix | 0.45 cu ft | Typical published value | Quikrete |
| Yield of a 80 lb bag of concrete mix | 0.6 cu ft | Typical published value | Quikrete |
Limitations
- Bag yields are approximate and belong to one manufacturer's mix: use the yield printed on your bag if it differs. For more than a few dozen bags, ready-mix delivered by truck is usually the practical choice: ask the supplier about minimum loads.
- This estimates volume only. It does not size footings, rebar, gravel sub-base or forms, and it does not account for ground that is uneven or soft: follow local code and ask a professional for structural work.
Results are estimates for planning and ordering, not professional, engineering or legal advice. Confirm quantities, sizes and local building requirements with your supplier or contractor before you buy or build.