Concrete & Masonry
Concrete Calculator
Estimate concrete for a rectangular or circular slab, a tube, or multiple post holes and columns. Get volume, localized bag counts, and a rounded ready-mix order.
- 3 project shapes
- Imperial & metric
- Bag & ready-mix estimates
Choose a shape and enter measurements
Select the shape that matches your pour. Only its measurements appear; waste allowance and purchasing units apply to every shape.
Estimate result
How this estimate is calculated
Rectangular slabs use length × width × thickness. Circular slabs, tubes, post holes, and columns use the cylinder formula π × radius² × depth; matching holes or columns multiply that result by quantity. The calculator then applies the selected waste allowance.
Rectangle: L × W × T · Circle: π × r² × depth · Multiple holes: π × r² × depth × quantity
Post-hole estimates treat each hole or column as a full cylinder and do not subtract the volume of a post. Imperial results use approximately 0.60 cubic feet per 80 lb bag and 0.45 cubic feet per 60 lb bag. Metric results use approximately 11.6 litres per 25 kg bag. Every bag count and ready-mix suggestion rounds up.
Concrete Calculator FAQs
How much concrete do I need for a 10 × 12 ft slab that is 4 inches thick?
The raw volume is 1.48 cubic yards. With the default 10% waste allowance, this calculator estimates 1.63 cubic yards, or 74 80 lb bags.
How does the calculator estimate a circular slab or concrete tube?
It divides the diameter by two to get the radius, then uses π × radius² × thickness or height. The selected waste allowance is added after the raw cylindrical volume is calculated.
How do I calculate concrete for several post holes?
Choose Post Holes or Columns, enter one hole’s diameter and concrete-filled depth, then enter the number of identical holes. The calculator multiplies the cylindrical volume by that quantity before adding waste.
Should I add extra concrete for waste?
Usually, yes. A 5% to 10% allowance is common for simple pours. This calculator starts at 10% for a typical DIY project, but you can reduce or increase it for the conditions.
How many 80 lb bags make one cubic yard of concrete?
At approximately 0.60 cubic feet per bag, one cubic yard (27 cubic feet) needs 45 bags before allowing for waste.
When should I use ready-mix instead of bags?
For larger projects, ready-mix can reduce mixing time and physical handling. Ask local suppliers about their minimum order, delivery, and short-load fees before deciding.
Suggested supplies
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