How Many Bags of Concrete Do I Need? (2026 Guide)

Whether you are pouring a small patio, a shed foundation, or fence post footings, knowing how much concrete to order prevents waste and costly short-loads. Use our concrete calculator for instant results, or follow the steps below.

The Formula

Concrete is measured in cubic yards (cu yd) for large pours and 80lb / 60lb / 50lb / 40lb bags for small projects. The basic formula is:

Volume (cubic feet) = Length × Width × Thickness

Cubic yards = Cubic feet ÷ 27

Always convert thickness from inches to feet before multiplying (4 inches = 0.333 ft, 6 inches = 0.5 ft).

Step 1: Calculate Volume

Measure the area. For a slab or patio, multiply length × width in feet. For a footing, multiply width × depth × length. For a round post hole, use π × radius² × depth.
Determine thickness. Standard slabs are 4" (0.333 ft) for walkways and 6" (0.5 ft) for driveways. Footings are typically 8–12" deep.
Multiply. Length × Width × Thickness = cubic feet. Divide by 27 for cubic yards.
Add waste. Add 5–10% for spillage, uneven excavation, and over-excavation. Round up to the nearest whole bag or quarter yard.

Bag Yield Reference

Bag Size	Yield per Bag	Bags per Cubic Yard	Typical Use
40 lb	0.30 cu ft	90 bags	Small repairs, post holes
50 lb	0.375 cu ft	72 bags	Patches, small footings
60 lb	0.45 cu ft	60 bags	Patios, small slabs
80 lb	0.60 cu ft	45 bags	General purpose, best value

An 80lb bag is the most cost-effective for DIY projects. At $4–$6 per bag, one cubic yard costs $180–$270 in bags. Ready-mix concrete delivered by truck costs $120–$180 per cubic yard (with a 1-yard minimum). Bags may be practical under ~1 cubic yard; over ~2 cubic yards, ready-mix is usually cheaper, while 1–2 cubic yards depends on access and labor.

Quick Reference: Slab Concrete

Slab Size	Thickness	Cubic Yards	80lb Bags	Est. Cost (bags)
10×10 ft	4"	1.23 cu yd	~56	$224–$336
10×10 ft	6"	1.85 cu yd	~84	$336–$504
12×12 ft	4"	1.78 cu yd	~80	$320–$480
12×16 ft	4"	2.37 cu yd	~107	$428–$642
16×16 ft	6"	4.74 cu yd	~214	$856–$1,284
20×20 ft	6"	7.41 cu yd	~334	Order ready-mix

For projects over 2 cubic yards, ready-mix delivery is almost always cheaper and far less labor-intensive than mixing bags.

Fence Post Footings

For a 4×4 fence post, the standard footing is 12" diameter × 36" deep (below frost line). The hole volume is approximately 2.36 cubic feet, or 3.93 80lb bags, before subtracting the space occupied by the post. Add 5–10% for waste and uneven excavation.

Hole Diameter	Depth	Concrete per Hole	80lb Bags
8"	24"	0.70 cu ft	1.17 bags
10"	30"	1.36 cu ft	2.27 bags
12"	36"	2.36 cu ft	3.93 bags
12"	48"	3.14 cu ft	5.24 bags

For 20 holes at 12"×36", plan approximately 79 bags of 80lb concrete before waste, or about 87 bags with 10% waste. Subtract the post displacement when appropriate, then use our fence calculator to estimate posts, rails, and concrete for your full fence run.

Bags vs. Ready-Mix: When to Choose Which

Use bags when: Project is under 1–2 cubic yards, access is limited (wheelbarrow distance), or you need to pour in stages over multiple days.
Use ready-mix when: Project is over 2 cubic yards, you need a consistent mix, or you want to finish in one pour. Minimum order is usually 1 yard; short-load fees apply under 5 yards.

Common Mistakes

Too-thin slabs. A 4" slab is fine for walkways and patios but driveways need 5–6" with rebar or wire mesh.
Forgetting excavation. You need 4–6" of compacted gravel base under slabs, plus the slab thickness. Excavate accordingly.
Adding too much water. Extra water makes concrete easier to pour but drastically reduces strength. Follow the bag's water instruction exactly.
Not accounting for cure time. Concrete reaches ~75% strength in 7 days and full strength in 28 days. Keep it moist for the first 7 days for proper curing.
Underestimating footings. Footings must extend below the frost line (36–48" in most climates). Shallow footings heave in winter.

FAQ

How many 80lb bags make a yard?

45 bags of 80lb concrete make one cubic yard. Each 80lb bag yields 0.60 cubic feet, and 27 cubic feet = 1 cubic yard (27 ÷ 0.60 = 45).

How much concrete for a 10×10 slab?

A 10×10 slab at 4" thick needs 1.23 cubic yards, or about 56 bags of 80lb concrete. At 6" thick, it needs 1.85 cubic yards or ~84 bags.

How thick should a concrete patio be?

4 inches is standard for pedestrian patios and walkways. For driveways or heavy equipment, use 5–6 inches with reinforcement (rebar or fiber mesh).

Bags or ready-mix, which is cheaper?

For under 2 cubic yards, bags are usually cheaper (no delivery fee). For over 2 yards, ready-mix delivery costs less per yard and saves hours of mixing labor.

How Many Bags of Concrete Do I Need?