How much concrete,
down to the bag.

Field guide

The concrete buying guide.

Concrete is sold three different ways and the right choice depends almost entirely on how much of it you need. Get the quantity wrong by 5% and you can usually still finish the pour; get it wrong by 20% and you've either left a cold joint in the slab or paid for half a yard of waste. The maths is short; the judgement is in the rounding.

How concrete volume is calculated

Every shape reduces to length × width × depth (with all three dimensions in the same unit) for a slab, or π × radius² × height for a circular column. The cubic feet, cubic yards, and cubic metres are all the same number expressed in different units:

One cubic yard is what you get when a ready-mix truck dumps 27 cubic feet of concrete in your form. The smallest delivery most ready-mix companies will run a truck for is one yard, and they bill for short loads under that.

How many bags of concrete for a slab?

Bag yields are printed on every major-brand bag. They're consistent across Quikrete, Sakrete, and the store-brand equivalents:

• 80-lb bag ≈ 0.60 ft³ ≈ 0.017 m³, about 45 bags per cubic yard.
• 60-lb bag ≈ 0.45 ft³ ≈ 0.0127 m³, about 60 bags per cubic yard.
• 40-lb bag ≈ 0.30 ft³ ≈ 0.0085 m³, about 90 bags per cubic yard.

Worked example: a 10 ft × 10 ft slab poured 4 in thick is 10 × 10 × (4 ÷ 12) = 33.3 ft³ ≈ 1.23 yd³. With a 10% waste factor that becomes ~1.36 yd³, about 62 80-pound bags or 82 60-pound bags.

Bags vs. ready-mix: the break-even

The crossover happens around ½ yard:

• Under ½ yd³: bags win on price, mixing effort is manageable, and you control the timing.
• ½ to 1 yd³: judgement call. Bags are still cheaper but mixing 30+ bags by hand on a hot day is brutal, and a power mixer rental closes the gap.
• Over 1 yd³: order ready-mix. The truck arrives premixed at consistent strength, you pour in one session (no cold joints), and the per-yard cost is dramatically lower than bagged once you cross the short-load fee.

Why you always over-order

A real pour is never the textbook volume. Sub-grade dips a half-inch where you didn't notice, the form flexes, the mixer leaves a tail, and trowel-finishing pulls a thin sacrificial layer from the top. The standard rule:

• 5% for tight, well-prepped slabs on compacted gravel.
• 10% for typical residential pours; this is the default this calculator uses.
• 15–20% for hand-dug footings, irregular pier holes, or any sub-grade you can't trust.

Running short is the failure mode you're protecting against. If the pour stops while you mix more, you've cast a cold joint: a planar weakness that cracks first and admits water for the life of the slab.

Slab thickness rules of thumb

• 4 in (10 cm): patios, sidewalks, residential driveways for cars.
• 5 in (12 cm): driveways that see light trucks or RVs.
• 6 in (15 cm): garage floors, shed foundations, anything supporting heavy machinery.
• 8 in (20 cm)+: workshops, commercial floors, foundations that take point loads.

Footings and frost depth

A footing must extend below the local frost line, otherwise freeze/thaw cycles will heave it and crack whatever it supports. In the U.S. this ranges from 12 inches in the South to 48+ inches across the upper Midwest and New England. Always check your local code.

Round columns and post holes

A 12-inch sonotube poured 4 ft deep is π × 0.5² × 4 = 3.14 ft³, or about 5 80-pound bags after a 10% over-order. The calculator handles the cylinder maths for any diameter and height; for a fence-line of identical posts, set the quantity field.

What this calculator does not do

It estimates pour volume and bag count, not concrete mix design. PSI rating (3000, 3500, 4000…), aggregate size, slump, fibre or rebar reinforcement, and air-entrainment for freeze-thaw climates are separate decisions you make at the supply yard. For anything structural (a foundation, a retaining wall, a load-bearing column), defer to the engineered drawings.

Disclaimer

Numbers are estimates intended for ordering. Always confirm against the bag's printed yield and your supplier's posted yardage; both can vary slightly by manufacturer and product line. For structural pours, follow the engineer's specification.