Concrete Slab Calculator
Estimate ready-mix concrete for a rectangular slab or circular footing. Calculate volume in cubic yards and cubic metres, add a waste allowance, estimate premix bag quantities and work out an optional material cost.
Your concrete volume will appear here.
How much concrete do I need for a slab?
Measure the slab length, width and thickness, then choose feet or metres. For a circular footing or pier, enter the diameter and depth. The calculator converts all dimensions to a consistent unit, calculates the geometric volume, and adds your chosen overage percentage.
Concrete volume formulas
Rectangular slab: length × width × thickness.
Circular footing: π × (diameter ÷ 2)² × depth.
Order volume: base volume × (1 + waste allowance ÷ 100).
The result is shown in cubic feet, cubic yards and cubic metres. One cubic yard equals 27 cubic feet. Premix bag estimates use the yield you enter; check the exact yield printed on the manufacturer’s bag because products differ.
Worked example: 20 ft × 10 ft slab
A 20 ft long, 10 ft wide slab with a 4-inch thickness has a base volume of approximately 2.47 cubic yards. Adding a 5% planning allowance gives about 2.59 cubic yards. Actual order quantities should be checked against site levels, forms, sub-base and the supplier’s minimum load.
Ready-mix or premix bags?
Ready-mix is typically ordered by volume, often in cubic yards in the United States. Premix bags are purchased by bag count and each product has a stated yield. Enter the yield from the product label to estimate how many bags would be needed. This is a quantity comparison, not a recommendation about which supply method is suitable.
Frequently asked questions
How many bags of concrete do I need?
Divide the required volume in cubic feet by the yield per bag, then round up to a whole bag. Use the yield on the exact product label.
How much extra concrete should I order?
The allowance depends on excavation accuracy, ground conditions, formwork and project shape. The default 5% is only a starting estimate; confirm the quantity with your contractor or supplier.
Can I use this calculator for footings?
Choose circular footing for a round pier or column base. For continuous strip footings, calculate each rectangular section separately or consult the construction drawings.
Does this calculate reinforcement or structural strength?
No. It estimates concrete volume only. Reinforcement, mix design, load capacity, curing and structural dimensions must come from project specifications and qualified professionals.
Related tools: Gravel Calculator · Deck Material Calculator.
Construction disclaimer: This is an approximate quantity estimator, not a structural design, concrete mix specification or permit document. Confirm dimensions and order quantities before purchase.