Concrete Calculator
Enter your slab or column dimensions and we work out the concrete volume (m³ and yd³), the number of bags you need, and the total cost.
Total cost
£158.40
- Volume with wastage (m³)
- 1.32
- Volume with wastage (yd³)
- 1.73
- Bags needed (80 lb premix)
- 78
- Bags needed (25 kg UK)
- 110
Slab volume = length × width × thickness. Column volume = π × (diameter/2)² × height. Neat volume × (1 + wastage%) gives the order quantity; cost = volume × price/m³. Bag counts use NRMCA premix yields (80 lb ≈ 0.0170 m³) and a representative 25 kg UK bag yield (~0.012 m³).
How to use this calculator
Pick the shape: a slab/footing is a rectangular prism (length × width × thickness), while a column is a cylinder (you enter the diameter as "length" and the column height as "thickness"). Width is ignored for columns. Set wastage — 10% is the industry default, drop it to 5% for a tightly formed pour, raise it to 15% for irregular formwork or pumped delivery. Price per m³ is the supplier’s ready-mix rate; for bagged concrete, divide bag price by bag yield to get an effective price per m³.
How the calculation works
For a slab: V = length × width × thickness. For a column: V = π × (diameter/2)² × height. The neat volume is then multiplied by (1 + wastage%) to give the order quantity. Cost = volume × price per m³. Bag counts use NRMCA published yields — an 80 lb premix bag yields about 0.6 ft³ (0.0170 m³); a UK 25 kg general-purpose bag yields roughly 0.012 m³. Both are rounded up because you can’t buy a fraction of a bag.
Worked example
A 4 m × 3 m garage slab, 100 mm thick, with 10% wastage and £120/m³ ready-mix: neat volume 4 × 3 × 0.10 = 1.20 m³; with wastage 1.20 × 1.10 = 1.32 m³ (≈ 1.73 yd³). At £120/m³ that’s £158.40. In bags: 1.32 ÷ 0.0170 = 77.7 → 78 bags of 80 lb premix, or 1.32 ÷ 0.012 = 110 bags of 25 kg. For most slabs above 0.5 m³, ready-mix is cheaper than bags.
Frequently asked questions
How much wastage should I add when ordering concrete?
10% is a safe default for a typical residential slab — it covers spillage, formwork irregularities and minor shrinkage. Drop to 5% only if your formwork is very tight and you’ve measured carefully. Raise to 15% for pumped deliveries (some is left in the hose), awkwardly shaped pours, or first-time DIY pours where you’re likely to over-fill in places. Running short mid-pour is far more expensive than over-ordering by a few %.
Should I order ready-mix or buy bags?
Below about 0.5 m³ (≈ 30 × 80 lb bags or ≈ 40 × 25 kg bags), bags are usually more practical — the minimum delivery charge for ready-mix often exceeds the saving. Above 1 m³, ready-mix is almost always cheaper, fresher and faster. Between 0.5 and 1 m³ it depends on local supplier minimums and whether you have help on site to mix bags fast enough before the first batch sets.
How is the column volume calculated?
A column or post is treated as a cylinder: V = π × r² × h, where r is half the diameter you entered. So a 0.4 m diameter column 3 m tall: V = π × 0.20² × 3 ≈ 0.377 m³ neat, or 0.415 m³ with 10% wastage. The "width" input is ignored when shape is column.
How do I convert from cubic yards to cubic metres?
1 yard = 0.9144 m exactly, so 1 yd³ = 0.9144³ ≈ 0.7646 m³, and 1 m³ ≈ 1.308 yd³. The calculator shows both. Quick check: an 8 yd³ truckload of concrete is about 6.1 m³.
Why does the bag count round up?
You can’t buy a fraction of a bag. If 77.7 bags would be exactly enough, you need to buy 78. The same applies to 25 kg bags: 109.6 → 110. The rounding is on top of the wastage allowance, so the buffer is real, not artificial.
What strength concrete should I use?
This calculator is volume-only — strength (e.g. C25/30 in the UK, 3000 psi in the US) is a separate spec. For a domestic garage slab or path, C20/25 or 3000 psi is typical. For structural footings, follow the engineer’s spec exactly. Strength affects price per m³ (stronger mixes cost more) but not the volume needed.