Ordering concrete is a one-shot job: run short and you have a cold joint, order too much and you pay for concrete you have to dump. Measure carefully, add 5–10% for uneven subgrade, spillage and form bulge, and remember that ready-mix plants deliver in quarter- or half-yard increments. Small jobs under about 1 yd³ are usually cheaper with premixed bags.
How to use the Concrete Calculator
- Shape of pour
- Length
- Width
- Thickness / depth
- Diameter (outer)
- Inner diameter
- Height / depth
- Curb depth
- Curb height
- Gutter width
- Flag (gutter) thickness
- Quantity
- Waste
- Concrete type
- Premix bag size
- Price per bag
- Ready-mix price
- Every result field is editable: type into Concrete needed and Thickness / depth plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the shape of the pour and enter its dimensions in any unit, the number of identical pours and a waste allowance. The calculator converts the volume to cubic yards, cubic feet and cubic metres, weighs it with the density you choose, counts the premix bags and prices both bags and ready-mix.
Premixed bag yields
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 25 kg | ≈ 0.0117 m³ | ≈ 85 per m³ |
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Shape of pour | Choose one: Slab, wall or square footing, Round column or hole, Round tube (hollow column), Curb and gutter |
| Length | Units: ft, m, in, yd, cm |
| Width | Units: ft, m, in, yd, cm |
| Thickness / depth | Units: in, cm, mm, ft, m |
| Diameter (outer) | Units: in, cm, mm, ft, m |
| Inner diameter | Units: in, cm, mm, ft, m |
| Height / depth | Units: ft, m, in, yd, cm |
| Curb depth | Units: in, cm, mm, ft, m |
| Curb height | Units: in, cm, mm, ft, m |
| Gutter width | Units: in, cm, mm, ft, m |
| Flag (gutter) thickness | Units: in, cm, mm, ft, m |
| Quantity | A number |
| Waste | In % |
| Concrete type | Choose one: Normal-weight (2,400 kg/m³), Reinforced (2,500 kg/m³), Lightweight (1,850 kg/m³) |
| Premix bag size | Choose one: 40 lb bag (0.30 ft³), 50 lb bag (0.375 ft³), 60 lb bag (0.45 ft³), 80 lb bag (0.60 ft³) … |
| Price per bag | Units: /piece |
| Ready-mix price | Units: /yd³, /m³, /ft³ |
What each result means
- Concrete needed = [Volume of one pour]×[Quantity]×(1 + [Waste]÷100)
- Weight = [Concrete needed]×[Concrete type]
- Premix bags = ceil([Concrete needed]÷[Premix bag size])
- Cost with bags = [Premix bags]×[Price per bag]
- Cost with ready-mix = [Concrete needed]×[Ready-mix price]
Formula
Slab / wall / footing: V = L × W × TColumn or hole: V = π/4 × D² × HTube: V = π/4 × (D² − d²) × HCurb and gutter: V = L × (curb depth × curb height + gutter width × flag thickness)Total = V × quantity × (1 + waste)Worked example
Inputs — Shape of pour: Slab, wall or square footing; Length: 10 ft; Width: 10 ft; Thickness / depth: 4 in; Diameter (outer): 12 in; Inner diameter: 8 in; Height / depth: 4 ft; Curb depth: 6 in; Curb height: 18 in; Gutter width: 12 in; Flag (gutter) thickness: 6 in; Quantity: 1; Waste: 7 %; Concrete type: Normal-weight (2,400 kg/m³); Premix bag size: 80 lb bag (0.60 ft³); Price per bag: $6.5 /piece; Ready-mix price: $150 /yd³.
Results — Volume of one pour = 1.235 yd³; Concrete needed = 1.321 yd³; Weight = 5,344 lb; Premix bags = 60; Cost with bags = $390.00; Cost with ready-mix = $198.15.
Key terms
- Ready-mix
- Concrete batched at a plant and delivered in a mixer truck, sold by the cubic yard or cubic metre.
Frequently asked questions
How much concrete do I need for a 10 × 10 slab?
At 4 in thick, 33.3 ft³ = 1.23 yd³; with 7% waste order about 1.32 yd³ (1¼–1½ yd³).
How many 80 lb bags make a cubic yard?
45 bags (each yields about 0.6 ft³). 60 lb bags: 60 per yard.
How thick should a concrete slab be?
4 in for patios and walks, 5–6 in for driveways and garage floors carrying cars, more for heavy trucks.
When should I use ready-mix instead of bags?
Above roughly 1 yd³, ready-mix is cheaper and much faster; many suppliers have a short-load fee under 3–4 yd³.
What formula does the Concrete Calculator use?
Slab / wall / footing: V = L × W × T.