How much concrete do I need for a 12×12 slab at 4 inches?

A 12×12 slab at 4 inches thick needs 4.94 cubic yards of concrete, which is about 223 bags of 80lb concrete or 74 bags of 60lb concrete.

How much concrete do I need for a 12×12 slab at 6 inches?

A 12×12 slab at 6 inches thick needs 7.41 cubic yards of concrete, which is about 334 bags of 80lb concrete or 445 bags of 60lb concrete.

Should I use bags or ready-mix for a 12×12 slab?

For a 12×12 slab at 4 inches (4.94 cubic yards), ready-mix is more practical. Order 1.5 yards to account for waste. If using bags, you need 223+ bags of 80lb concrete.

How Much Concrete for a 20×20 Slab?

Q: How much does a 12×12 concrete slab cost?

A 12×12 slab costs approximately $170-$260 in concrete alone (at $140/yd³), depending on thickness. With labor, forming, and finishing, expect $1,200-$2,400 total.

Get exact concrete amounts, bag counts, and cost estimates for your 12×12 project.

Decision Guide

20x20 slab truck-order decision

A 20x20 slab is firmly in ready-mix territory. The question is not whether bags are possible, but how to order enough volume with waste and coordinate the pour without getting caught by delivery constraints.

Best default

Ready-mix is the safer default

Budget check

Plan around 5.43 yd³ with waste and treat bags as a sanity check, not a buying plan.

Logistics

At this size, delivery timing, placement speed, and finishing help matter as much as raw concrete price.

Before you order

Waste-adjusted yards are the real supplier number.
Thickness changes truck scheduling and finishing pace.
Bag math is useful only as a cost sanity check here.

Next steps

Lock thickness and reinforcement assumptions first.
Use total yards with waste when you call suppliers.
Treat bag calculations as a sanity check only.

Common mistakes

Pretending a 20x20 slab is still a realistic bag job.
Ignoring how extra thickness changes ordering and crew needs.
Using a copied small-slab budget for a much larger pour.

Use this page to plan the order. Delivery fees, access, and finishing help can still move the total cost.

Price a 20×20 pour Run the full 20×20 calc

Quick Answer: Concrete for a 20×20 Slab

4" Thick (Patio/Walkway)

4.94 yd³

223 bags (80lb)

6" Thick (Driveway)

7.41 yd³

334 bags (80lb)

8" Thick (Heavy-duty)

9.88 yd³

445 bags (80lb)

Adjustable Calculator

Length and width are locked to 12×12. Adjust thickness and cost.

Length (ft)

Width (ft)

Thickness (inches)

Cost per yd³ ($)

Results

Cubic feet:0

Cubic yards:0

With 10% waste:0

80lb bags:0

Ready-mix cost:$0

Bag cost (80lb @ $6):$0

20×20 Slab Cost Estimate

Thickness	Yards	80lb Bags	Concrete Cost	Total w/ Labor*
4 inches	4.94	80	$691	$1,200-$1,800
6 inches	7.41	120	$1,037	$1,500-$2,200
8 inches	9.88	160	$1,383	$1,800-$2,600

*Total includes forming, rebar, pouring, and finishing labor. Prices vary by region.

About a 12×12 Concrete Slab

A 12×12 slab (400 square feet) is one of the most common DIY concrete projects. It's the right size for a small patio, shed base, hot tub pad, or walkway section.

Common Uses for a 20×20 Slab

Patio: Large patio, two-car garage, driveway, BBQ pad
Shed Foundation: Standard 12×12 shed base
Hot Tub Pad: Requires 6"+ thickness for weight
Walkway/Porch: Entryway landing

How to Calculate

Volume = 12 ft × 12 ft × (4 in ÷ 12) = 33.3 cu ft ÷ 27 = 4.94 cubic yards

Tips for Your 20×20 Slab

Order 1.5 yards to account for waste (10% extra)
Use 6" thickness for driveways or anything bearing weight
Add wire mesh or fiber reinforcement for crack prevention
For a shed, 4" is sufficient; for a hot tub, use 6-8"

FAQ

How many 80lb bags for a 12×12 slab at 4 inches?

You need about 223 bags of 80lb concrete (4.94 cubic yards). Add 10% extra for waste, so buy 245 bags.

How much does a 12×12 concrete slab cost?

Concrete alone costs $170-$260 depending on thickness. With labor, forming, and finishing, expect $1,200-$2,400 total for a professional installation.

Should I use ready-mix or bags?

For 4.94 cubic yards (at 4"), ready-mix is easier and often cheaper. Order 1.5 yards. If doing it yourself and can't get a truck, 223+ bags of 80lb concrete will work but is labor-intensive.