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Concrete Calculator

Calculate cubic yards for slabs, footings, columns and stairs, with waste allowance.

What are you pouring?
Slab dimensions
ft
ft
in

Four inches is standard for a patio or garage floor; five to six for a driveway.

Order
%

Ten percent covers normal subgrade variation. Go higher over rough or uncompacted fill.

$/yd³

Delivered price before short-load, washout or Saturday fees.

Order this much concrete

3.50cubic yards

2.96 yd³ of finished volume plus 10% waste, rounded up to the quarter yard the plant will actually sell you.
Finished volume2.963 cubic yards before waste
80.0 ft³
Estimated material cost3.50 yd³ at $165 per yard
$578
Ready-mix trucksA full load is 10 yd³
1 truck
60 lb bags0.45 ft³ each
196 bags
80 lb bags0.60 ft³ each
147 bags

Round up, and order the extra

Running short mid-pour is the one mistake you cannot fix. The concrete already placed begins to set while you wait for a second truck, and what you get is a cold joint — a permanent plane of weakness right through the middle of the slab. A quarter yard of extra concrete costs less than a tenth of what removing and repouring costs. Pour the surplus into a splash pad, a step or a post hole.

This is past the point where bags make sense

3.50 cubic yards is 147 eighty-pound bags — roughly 5.9 tons to move, open and mix by hand. Beyond about half a yard, ready-mix is cheaper once you count labor, and far more consistent: hand-mixed batches vary in water content from batch to batch, and water content is what sets the final strength. Pay the short-load fee.
How the volume was derived
StepFigureDetail
Slab area240.0 sq ft20.00 ft × 12.00 ft
Thickness in feet0.333 ft4.00 in ÷ 12
Finished volume80.00 ft³Sum of the geometry above
Converted to yards2.963 yd³27 cubic feet per cubic yard
Waste allowance at 10%0.296 yd³Subgrade variation, spillage, form deflection
Rounded to the quarter yard3.50 yd³Ready-mix is sold in quarter-yard increments
Order3.50 yd³$578
Supply options for this pour
OptionQuantityTotal weightNotes
Ready-mix delivery3.50 yd³7.1 tonsShort load — expect a fee of roughly $60–150 per yard below a full truck
80 lb bags147 bags5.9 tons3.5 pallets at 42 bags each
60 lb bags196 bags5.9 tons3.5 pallets at 56 bags each

Volume only. Reinforcement, vapor barrier, base gravel, forms and finishing are separate line items, and on a slab the compacted base course usually costs more to get right than the concrete does. Concrete weight is figured at 4,050 lb per cubic yard, which is typical for a normal-weight mix.

What this assumes

  • One cubic yard equals 27 cubic feet.
  • Default waste allowance is 10%, appropriate for typical subgrade variation.
  • Bag yields assume 0.45 cubic feet per 60 lb bag and 0.60 cubic feet per 80 lb bag.

Important

Ready-mix is sold in fractional yards with a short-load fee below a full truck. Ordering short and needing a second truck creates a cold joint — always round up.

Understanding the result

Always round up

Running out of concrete mid-pour creates a cold joint, a permanent plane of weakness in the slab. Concrete is cheap relative to a failed pour, so order the next quarter yard up. Excess can go into a sidewalk pad or a footing.

Bags versus ready-mix

Bag mix is practical up to roughly half a cubic yard — about 40 sixty-pound bags. Beyond that the labor and the risk of inconsistent mixing make ready-mix both cheaper and better. A short-load fee is worth paying.

Subgrade preparation affects volume

An uneven subgrade can add 10 to 15% more concrete than a flat, well-compacted base. The 10% waste factor in most estimates covers minor variation, not a badly prepared site. Screed to consistent depth and compact the base before you order.

Reinforcement does not change the volume

Rebar and wire mesh displace negligible concrete volume — do not deduct for them. However, thickened edges and footings around the perimeter add significant volume beyond a simple length × width × thickness slab calculation. Include every pour segment in the total.

Common questions

How many bags of concrete for a 10x10 slab?
A 10×10 slab at 4 inches thick is 3.7 cubic yards before waste. With 10% waste, order 4.1 cubic yards — about 55 eighty-pound bags or 74 sixty-pound bags. Ready-mix is cheaper and more consistent at this volume; bag mix is practical only for small patches.
How much does a yard of concrete cover at 4 inches?
One cubic yard covers 81 square feet at 4 inches thick (27 cubic feet ÷ 0.33 feet depth). A 12×12 foot patio (144 square feet) needs 1.78 cubic yards — round up to 2 yards with waste. At 6 inches thick, one yard covers only 54 square feet.
How many 80 lb bags of concrete per cubic yard?
It takes 45 eighty-pound bags to make one cubic yard. Each bag yields 0.60 cubic feet; 27 cubic feet per yard ÷ 0.60 = 45 bags. Sixty-pound bags require 60 per yard. A half-yard project needs 23 eighty-pound bags.
How much concrete for a 4x8 footing?
A 4×8 footing 12 inches wide and 8 inches deep: 0.67 × 1.0 × 8 = 5.33 cubic feet, or 0.20 cubic yards. Ten linear feet of the same footing needs 0.25 cubic yards. Footings are often poured separately from the slab — calculate each pour independently.
How many cubic yards for a 20x20 driveway?
A 20×20 driveway at 4 inches thick is 4.94 cubic yards. Add 10% waste and order 5.5 cubic yards — one full ready-mix truck. At 6 inches thick for vehicle traffic, the same area needs 7.4 cubic yards before waste.
How much concrete for fence post holes?
A 10-inch diameter hole 3 feet deep holds 1.6 cubic feet, or 0.06 cubic yards. Twenty post holes need 1.2 cubic yards total. Fast-setting post mix in 50-pound bags (0.375 cubic feet each) requires about 4 bags per hole — 80 bags for 20 holes.
How thick should a concrete patio be?
A residential patio for foot traffic only needs 4 inches. A driveway or area with vehicle traffic needs 5 to 6 inches. Add 12-inch-deep footings around the perimeter if the slab is not on a thickened edge. Rebar or wire mesh at mid-depth prevents cracking.
How much does a cubic yard of concrete weigh?
Wet concrete weighs approximately 4,000 pounds per cubic yard. A full 10-cubic-yard truck carries 40,000 pounds. Plan access for the truck — a standard rear-discharge mixer needs 9 feet of width and a stable approach. Wheelbarrow from the street adds significant labor.
How many bags of concrete for 1 cubic foot?
One cubic foot requires 1.67 eighty-pound bags or 2.2 sixty-pound bags. A single fence post hole 10 inches diameter by 2 feet deep (1.1 cubic feet) needs 2 eighty-pound bags. Round up — running short mid-pour creates a cold joint.
How much concrete for a 12 inch sonotube column?
A 12-inch diameter column 8 feet tall holds 6.3 cubic feet, or 0.23 cubic yards. Six deck support columns at 8 feet need 1.4 cubic yards before waste. Sonotubes are measured by their inside diameter; a 12-inch tube has a 12-inch ID.

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