Concrete Volume Math: How Many Bags You Actually Need for a Slab, Footing or Column

Concrete is priced two different ways depending on scale. Ready-mix arrives by the cubic yard on a truck; bagged mix is sold by the 60- or 80-pound sack at a hardware store. Almost every concrete mistake on a small job comes from converting between those units incorrectly, or from skipping the waste allowance. Get the volume right first, then apply overage — not the other way around.

The arithmetic below is simple multiplication, but the units are not consistent, so a general-purpose online calculator helps only if you convert everything to feet before multiplying.

The Three Shapes

Pour type	Formula (all dimensions in feet)	Notes
Slab	length x width x thickness	Thickness is usually 4 in = 0.333 ft
Footing / strip	length x width x depth	Measure the trench, not the form
Column / pier	pi x radius squared x height	Diameter is measured in inches — halve and convert

Cubic yards = cubic feet ÷ 27. That divisor is the only conversion you need for ordering ready-mix.

Worked Example 1: A 12 x 20 Foot Patio Slab

1. Thickness: 4 inches = 4 ÷ 12 = 0.333 ft
2. Volume: 12 x 20 x 0.333 = 80 cubic feet
3. Cubic yards: 80 ÷ 27 = 2.96 yd3
4. With 10% overage: 2.96 x 1.10 = 3.26 yd3 → order 3.5 yards

If that slab were 6 inches thick instead — typical for a driveway that will take vehicle weight — the volume doubles to 5.93 cubic yards, and with overage you would order 6.5 yards. Thickness is the dimension that most often gets underestimated.

Worked Example 2: A Deck Footing Trench

1. Trench: 40 ft long, 16 in wide, 12 in deep
2. Convert width and depth to feet: 16 in = 1.333 ft; 12 in = 1.0 ft
3. Volume: 40 x 1.333 x 1.0 = 53.3 cubic feet
4. Cubic yards: 53.3 ÷ 27 = 1.98 yd3 → plus 10% = 2.18 yd3, order 2.25 yards

Bags vs Ready-Mix: The Conversion Table

Bagged concrete is typically quoted by yield. An 80-pound bag yields about 0.60 cubic feet; a 60-pound bag yields about 0.45 cubic feet. These are the numbers to divide your volume by.

Volume needed	60 lb bags	80 lb bags
1 cubic foot	3	2
1 cubic yard (27 ft3)	60	45
0.5 cubic yard	30	23
A 12 x 20 x 4 in slab (80 ft3)	178	134

Those counts are the practical decision point. A 12 x 20 slab needs 134 eighty-pound bags — a pallet and a half, over 10,000 pounds of material, mixed by hand. Beyond about 1 cubic yard, ready-mix delivery is cheaper and dramatically faster even after the short-load fee, which is often $50-$150 for loads under 3 yards.

Worked Example 3: A Round Column

1. Diameter 10 in → radius 5 in → 0.4167 ft
2. Area: 3.1416 x 0.4167 x 0.4167 = 0.545 sq ft
3. Height 8 ft → volume: 0.545 x 8 = 4.36 cubic feet
4. Bags: 4.36 ÷ 0.60 = 7.3 → buy 8 eighty-pound bags

The common error here is using the diameter in place of the radius. Plugging 10 inches directly into pi x r squared multiplies the volume by four — a column that needs 8 bags would be quoted at 30.

Overage Rules

• Slabs on prepared, level ground: add 5-10%.
• Footings in uneven or excavated trenches: add 10-15% — soil absorbs and the trench is never a perfect rectangle.
• Columns and piers: add 10%; a small absolute error in diameter has an outsized effect on volume.
• Always round up to the next quarter yard when ordering ready-mix. Under-ordering means a cold joint or a second truck with its own short-load fee.

A standard ready-mix truck carries 8-10 cubic yards. Loads under 3 yards usually carry a short-load charge, so it is worth calculating the total volume for an entire project — footings, slab and any steps together — before placing a single order. If you are also estimating finishing materials, the same volume-and-waste logic applies to coverage estimates.

Frequently Asked Questions

How much does a cubic yard of concrete weigh? About 4,000 pounds, or roughly 2 tons. That is why you should never attempt to move ready-mix any real distance by wheelbarrow for more than a small patching job.

Can I use 60-pound bags instead of 80? Yes, and they are easier on the back for overhead or awkward pours. Just use the correct yield figure — 0.45 cubic feet per bag, not 0.60 — or you will under-buy by about 25%.

How thick should a slab be? 4 inches for foot traffic, patios and shed floors. 5-6 inches where vehicles drive. Add a compacted 4-inch gravel base underneath either way; the base is not part of the concrete volume.

What if I mix my own from cement, sand and gravel? A standard mix runs about 1 part cement to 2 parts sand to 3 parts gravel by volume, plus water. The total volume shrinks during mixing, so add roughly 25-30% to your calculated volume to account for the reduction.