Work out how much concrete you need for a slab, footing, or patio — in cubic yards, cubic meters, and ready-mix bags.
Choose US units (feet and inches) or Metric units (meters and centimeters), then enter the length and width of the slab or footing, along with its thickness. Press "Calculate Concrete Needed" to see the total volume required, plus how many pre-mixed 60 lb or 80 lb concrete bags that translates to if you're not ordering ready-mix by the truckload.
US units: Volume (cubic yards) = (Length ft × Width ft × (Thickness in ÷ 12)) ÷ 27
Metric units: Volume (m³) = Length m × Width m × (Thickness cm ÷ 100)
Bag coverage is estimated using standard yields: one 60 lb bag yields about 0.45 cubic feet of concrete, and one 80 lb bag yields about 0.6 cubic feet.
For a patio slab 10 ft long, 8 ft wide, and 4 inches thick: volume = (10 × 8 × (4 ÷ 12)) ÷ 27 = (10 × 8 × 0.333) ÷ 27 ≈ 0.988 cubic yards. Since 1 cubic yard equals 27 cubic feet, that's about 26.7 cubic feet of concrete — roughly 59 bags at 60 lb each, or 44 bags at 80 lb each. Always round up and add 5–10% extra to cover spillage and an uneven sub-base.
Measure the thickness at a few different points if the ground isn't perfectly level, and use the deepest measurement to avoid running short. For large pours (over 1 cubic yard or so), ready-mix delivered by truck is usually more economical and consistent than mixing dozens of bags by hand.
Should I order extra concrete? Yes — most contractors add a 5–10% buffer to account for spillage, sub-base irregularities, and formwork settling.
Does this work for footings and columns, not just slabs? Yes, as long as the shape is a simple rectangular volume (length × width × thickness), the same formula applies to footings, sidewalks, and pads.
Volume = length x width x depth (or thickness), typically converted to cubic yards for ordering ready-mix concrete in the US.
Divide the total volume needed by the yield per bag (commonly listed on the bag, e.g. 0.6 cubic feet per 80lb bag) to get the bag count.
Yes, most contractors add 5-10% extra to account for spillage, uneven ground, and formwork variations.