How to measure

Pick the shape. A patio, shed pad or sidewalk is a slab. A strip under a wall is a footing. A fence or deck post hole is a round column.
Measure length and width in feet. You can type feet and inches the way you’d say them, like 10' 6" or 10 1/2.
Measure thickness in inches. Use the depth of your forms, not the depth you hope to pour. Quikrete’s slab instructions use 4 inches of concrete over 3–4 inches of gravel.
Count identical pieces. Ten identical post holes? Enter one and set How many to 10.

Slab: length and width in feet, thickness in inches. Footings work the same way, with width and depth in inches.

How this calculator works

The calculator finds the volume of your shape, adds extra for waste, then converts it to cubic yards and to bags.

Rectangle (slab, footing): volume = length × width × thickness, with inches divided by 12 to make feet.
Round (column, round slab): volume = π × (diameter ÷ 2)² × height.
Order volume = volume × (1 + waste % ÷ 100).
Cubic yards = cubic feet ÷ 27, because a cubic yard is 3 × 3 × 3 feet.
Bags = order volume ÷ yield per bag, rounded up to a whole bag.

Example. Shape: Slab or rectangle · Length 10 ft · Width 10 ft · Thickness 4 in · How many 1 · Extra for waste and spillage 10% · Bag size: 80 lb

You need about 1.36 cubic yards of concrete. That's 62 80-lb bags.

Check the yield on the bag. The defaults come from Quikrete’s data sheet for its standard Concrete Mix: 0.30 cubic feet for a 40-lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. Other products can differ, so enter your bag’s number under Other.
Bags or ready-mix? At 0.60 cubic feet per 80-lb bag, one cubic yard is 45 bags (27 ÷ 0.60). That’s a lot of mixing by hand. For bigger pours, compare the bag count with a ready-mix quote for the cubic yards shown.
Waste. The default adds 10% on top of the exact volume. Change it to suit your job. Running short in the middle of a pour is worse than a leftover bag.
Plan control joints. Quikrete says a 4-inch slab needs control joints at least every 10 feet in each direction, cut at least ¼ of the slab’s depth.

How many 80-lb bags make a cubic yard?

About 45, at 0.60 cubic feet per bag (27 ÷ 0.60). Check your bag's label, because yields vary by mix.

How many 60-lb bags make a cubic yard?

About 60, at 0.45 cubic feet per bag (27 ÷ 0.45).

How much concrete do I need for a 10 × 10 slab, 4 inches thick?

About 1.23 cubic yards before waste. With 10% extra that's 1.36 cubic yards, or 62 bags of 80-lb mix.

Can I type feet and inches?

Yes. Type 10' 6", 10 ft 6 in, 10 1/2 or 10.5: they all mean the same length. Metric works too: 3.2 m or 320 cm.