Free Concrete Calculator: Yards and Bags
Pick what you’re pouring, enter the size, and get the concrete in cubic yards for a ready-mix order or in 80 lb and 60 lb bags. The waste allowance covers spillage and uneven grade. These are the same rules LaraKey’s takeoff calculator uses.
Concrete to order
Estimates only. Measure on site and check with your supplier before you order.
How to calculate concrete
Slab cu ft = Length × Width × (Thickness in ÷ 12)
Footing cu ft = Length × (Width in ÷ 12) × (Depth in ÷ 12)
Wall cu ft = Length × Height × (Thickness in ÷ 12)
Post holes cu ft = Holes × π × (Diameter in ÷ 24)² × (Depth in ÷ 12)
Cubic yards = cu ft ÷ 27, plus waste, ordered by the quarter yard.
Bags = cu ft ÷ bag yield (0.6 cu ft for 80 lb, 0.45 cu ft for 60 lb), plus waste, rounded up.
Examples
A 12 × 20 ft patio, 4 in thick
12 × 20 × 0.333 = 80 cu ft = 2.96 cu yd. With 10% extra, 3.26 cu yd, so order 3.5 yards. In bags: 80 ÷ 0.6 × 1.1 = 147 bags of 80 lb.
10 fence post holes
10 in holes, 36 in deep: 10 × π × 0.4167² × 3 = 16.4 cu ft. With 10%, that’s 30 bags of 80 lb or 40 bags of 60 lb.
Concrete Calculator: common questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb mix (0.6 cu ft each) or 60 bags of 60 lb mix (0.45 cu ft each), before waste.
How much extra concrete should I order?
Most contractors add 5% to 10% for spillage, uneven grade and form bulge. The calculator uses 10%; change it if you like.
Why does it round to quarter yards?
Ready-mix suppliers sell by the quarter yard (0.25 cu yd), so the calculator rounds up to what you’ll actually order.
How thick should a concrete slab be?
Walkways and patios are usually 4 inches. Driveways are often 4 to 6 inches. Check local code and the load the slab will carry.
When should I use bags instead of ready-mix?
Bags make sense for post holes, small pads and repairs, usually under about a yard. Bigger pours are faster and more consistent with a truck.
Does this include rebar or mesh?
No, just the concrete. LaraKey’s takeoff calculator adds rebar, mesh, vapor barrier and form boards.
How do I measure an odd-shaped slab?
Split it into rectangles, run each one, and add them up.
Are these numbers exact?
They’re estimates based on common rules of thumb. Measure on site and confirm with your supplier before you order.
Talk to a person who knows remodeling.
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