Concrete Calculator: Bags vs. Bulk

For slabs, footings, pads, and post holes. Enter your project size to get the volume, the exact number of bags to buy, and what the same job costs in ready-mix bulk concrete.

Length (feet)

Width (feet)

Depth / thickness (inches)

4 in is standard for patios/walkways; 5 to 6 in for driveways.

Waste / overage allowance (%)

10% is the usual recommendation: running short mid-pour is far worse than one extra bag.

Bag size

Price per bag ($)

Bulk ready-mix price ($ per cubic yard)

Bulk delivery / short-load fee ($)

Many plants add $50 to $150 for loads under ~3 to 5 yards, or require a minimum order. Check yours.

Concrete volume (before waste)-

Volume with waste allowance-

Bagged concrete

- bags

Bag size-

Estimated bag cost-

Total weight to haul & mix-

Bulk (ready-mix truck)

- cu yd

Concrete cost-

Delivery / short-load fee-

Estimated bulk total-

Estimate only. Quantities assume a level, evenly-graded base. Uneven ground, forms that bow, and spillage all increase real usage. Verify quantities before purchase, and confirm bulk minimums and fees with your local ready-mix plant: a delivery fee can flip which option is cheaper on small jobs.

How the math works

Concrete is sold by volume, so every estimate is the same three steps:

Volume in cubic feet: length (ft) × width (ft) × depth (ft), where depth in feet = inches ÷ 12. A 10 × 10 ft slab at 4 in deep is 10 × 10 × 0.333 = 33.33 cu ft.
Cubic yards (the bulk unit): divide cubic feet by 27. 33.33 ÷ 27 = 1.23 cu yd.
Bags: divide cubic feet by the bag's yield: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb, then round up. 33.33 ÷ 0.60 = 55.6, so 56 bags of 80 lb.

Quick reference: common slab sizes (4 in thick, no waste)

Slab size	Cubic yards	80 lb bags	60 lb bags	40 lb bags
5 × 5 ft	0.31	14	19	28
8 × 8 ft	0.79	36	48	72
10 × 10 ft	1.23	56	75	112
10 × 20 ft	2.47	112	149	223
20 × 20 ft	4.94	223	297	445

Bags or bulk? The practical rule

Under about half a cubic yard (~27 eighty-pound bags), bags are almost always simpler and cheaper once delivery fees and minimums are counted: and you can work at your own pace. Above 1 to 1.5 cubic yards, price the truck: the concrete itself is usually far cheaper per yard, and mixing 100+ bags by hand or with a small mixer is a full day of heavy labor (an 80 lb bag is exactly what it sounds like). The crossover in this calculator already accounts for a delivery fee if you enter one: on small pours, that fee is usually what keeps bags ahead.

Whichever route you choose, order the waste allowance. Concrete is one material where "just enough" is a gamble you lose mid-pour, when the forms are open and the clock is running.

Disclosure: This tool and article were produced with AI-assisted research and writing. Figures are estimates for planning only, not a quote. Bag yields are the standard manufacturer figures for pre-mixed concrete bags. This site may earn advertising revenue and, in future, affiliate commissions; that does not affect any calculation or recommendation shown here.