How to use the concrete calculator
- Choose what you are pouring: a slab, path or patio; a continuous wall footing; or a round column or post hole.
- Enter the size. Slab thickness is in inches or centimetres; footings use the footing width and depth; columns use the diameter and height.
- Set how many identical pieces you need, such as ten post holes.
- Keep 10% extra for spillage, an uneven base and form bulges.
- Pick a bag size and add prices to compare bagged premix with ready-mix delivery.
How the volume and bags are worked out
Round column: volume = π × (diameter ÷ 2)² × height
Total = volume × number of pieces × (1 + waste %)
Bags = total ÷ yield per bag, rounded up (about 0.60 cu ft per 80 lb, 0.45 per 60 lb, 0.30 per 40 lb)
One cubic yard is 27 cubic feet, or about 0.765 m³. Bag yields are the figures commonly printed on premix concrete; the calculator scales them by weight for kilogram bags, so a 25 kg bag makes about 11.7 litres. Your bag’s label is the final word.
Worked example
A 10 × 10 ft patio slab, 4 in thick, with 10% extra, using 80 lb bags at $6.75 or ready-mix at $175 per cubic yard:
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft = 1.23 cu yd (33.3 cu ft).
- With 10% extra: 1.36 cu yd (36.7 cu ft).
- An 80 lb bag makes about 0.6 cu ft, so 36.7 ÷ 0.6 = 61.1, which means 62 × 80 lb bags, costing $418.50.
- The same volume from a ready-mix truck: 1.358 × $175 = $237.65, before any delivery or short-load charges.
Tips and common mistakes
- Bags or ready-mix? Mixing a few dozen bags by hand or in a small mixer is realistic for a weekend. Beyond about one cubic yard (0.75 m³), ready-mix is usually easier, and the pour stays consistent.
- Check thickness by use. Paths and patios are commonly 4 in (10 cm) thick, while driveways and slabs that carry vehicles are thicker and need reinforcement. Your local code and the use decide.
- Prepare the base. A compacted gravel base and proper forms matter more to a slab’s life than a little extra thickness.
- Call before digging. In the US, call 811 before digging footings or post holes; in Canada use Click Before You Dig.
- Get permits and inspections where required. Footings for structures are often inspected before the pour.
US, UK and Canada differences
In the US, ready-mix is ordered by the cubic yard and premix comes in 40, 60 and 80 lb bags. Canada and the UK order ready-mix by the cubic metre, and UK merchants sell bagged concrete, usually in 20 or 25 kg bags, along with separate bags of cement, sharp sand and ballast for mixing on site. Suppliers in all three countries often charge extra for small loads, so ask for the minimum order before comparing prices.
Metric units and other countries
In metric mode the volume is in cubic metres and bags are 20, 25, 40 or 50 kg. In Pakistan and India, cement is usually sold in 50 kg bags and concrete is often mixed on site from cement, sand and crushed stone, or ordered from a batching plant by the cubic metre. Use the volume here when you talk to the supplier or mason, and type your local prices.