How many bags of concrete for a 12×12 slab, 6 inches thick?

Data checked October 2026

Short answer

A 12 × 12 ft slab 6 inches thick needs 2.67 cubic yards of concrete — 132 bags of 80-lb mix (or 176 × 60-lb), including 10% extra.

Volume: 144 sq ft × 6 in = 72 cu ft (2.67 yd³) before waste.
Bags at 10% extra: 132 × 80 lb, 176 × 60 lb or 264 × 40 lb — that is 10,560 lb to haul (3.1 pallets).
Cost: about $783 in 80-lb bags vs. about $825 for 3 yd³ of ready-mix delivered — bags are cheaper here, but mean more mixing.
Base: about 3.9 tons of crusher run for a 4-in compacted gravel base extending 6 in past the edges.
Water: roughly 99–149 gallons to mix 132 bags.
12×12 is the go-to patio size for a table and grill, and a common shed base.

Adjust it to your exact job

What are you pouring?

Project

Most spa makers ask for 4–6 in; a filled 6-person tub can top 5,000 lb.

Size

Shape

Length

= 12' · 3.66 m

Width

= 12' · 3.66 m

Thickness

Slab thickness

Form lumber is smaller than its name: a 2×4 is 3.5 in tall, a 2×6 is 5.5 in.

Exact thickness

You need

132× 80-lb bags

2.93 yd³ of concrete including 10% extra · 10,560 lb of bags to lift · 3.1 pallets

Concrete volume: 2.67yd³before waste
Area: 144sq ft
Mixing water: 99–149galstart low; max weakens the slab
Finished weight: 5.2tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	264	—
50 lb · 0.375 ft³	212	—
60 lb · 0.45 ft³	176	$854
80 lb · 0.6 ft³	132	$783

Bags or a ready-mix truck?

132 bags

$783

Cheaper ✓ · materials only, before tax

3 yd³ delivered

$825

incl. short-load & delivery fees

At these prices a truck becomes cheaper from about 3.25 yd³ (147 bags). Over 1–2 yd³, a truck also saves hours of mixing: plan on 3–4 minutes per bag in a mixer.

That's 5.3 tons of bags — more than most pickups can carry in one trip. Ask about store delivery.

Timeline for standard 4000 psi mix

Walk on: 24 hours
Light loads / posts: 24 hours
Keep moist (cure): 5 days ≥70°F · 7 days at 50–70°F
Protect from freezing: 48 hours

Shopping list

132 bags Standard 4000 psi mix — 80-lb bags$783
3.9 tons Crusher run / road base (4 in compacted)$149
6 boards 2×8 form boards, 8 ft
20 stakes Wood form stakes
149 gal Clean water for mixing99–149 gal; start low, never exceed the max

Estimated total$932

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
132 bags × $Pallet price applied (42+ bags)
$783
Gravel base (bulk)
3.9 tons × $
$148

Materials total$931

Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.

Did this answer your question?

Compare

The same 12×12 footprint at other thicknesses

Thickness	Cubic yards	80-lb bags	60-lb bags	Bag cost	Ready-mix
3.5 in (2×4)	1.56	77	103	$457	$613
4 in	1.78	88	118	$522	$655
5 in	2.22	110	147	$652	$740
6 in	2.67	132	176	$783	$825

How this was worked out

12 ft × 12 ft = 144 sq ft. Multiply by the thickness in feet (6 ÷ 12 = 0.5 ft) to get 72 cubic feet, and divide by 27 for 2.67 cubic yards.

Adding 10% for uneven ground and spillage gives 79.2 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 79.2 ÷ 0.6 = 132, rounded up to 132 bags.

Ready-mix is priced at about $170 per yard for 4,000 psi plus delivery, fuel and — under 4 yards — a short-load fee. Plants sell in quarter-yard steps, so the order is 3 yd³.

The full method, material data and tips are on the main concrete slab calculator page.

Similar jobs

Other sizes people check

How many bags of concrete for a 10×10 slab, 6 inches thick?

Sources

Data behind this answer

Quikrete Concrete Mix No. 1101 — technical data sheet — bag yields (80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30), water per bag, curing times; post method: 6 in of dry mix in the hole bottom, 24 h before strain · checked October 2026
Quikrete 5000 No. 1007 — technical data sheet — 5,000 psi strength curve, 10–12 h walk-on · checked October 2026
Quikrete Fast-Setting No. 1004 — technical data sheet (rev. Nov 2025) — 50 lb = 0.375 cu ft, 20–40 min set, hole 3× post width and 1/3 of post length, ~1 gal water per bag, 30 in lifts, 4 h before loading a post · checked October 2026
Quikrete Crack-Resistant No. 1006 — technical data sheet — 4–8% entrained air for freeze–thaw exposure; same yields per bag weight · checked October 2026
Sakrete High-Strength Concrete Mix — TDS — same yields per bag weight; Sakrete recommends +10% overage · checked October 2026
Home Depot & Lowe's listings — Quikrete 80 lb Concrete Mix — $5.98–$6.98 per 80-lb bag; $5.93 bulk tier at 42+ bags; 42 bags per pallet; Fast-Setting 50 lb $7.97 · checked October 2026
Ernest Maier — #3 and #4 steel rebar listings — #4 $0.51/ft and #3 $0.31/ft, sold in 20 ft lengths (2026) · checked October 2026
Published 2026 ready-mix plant price sheets (e.g. Gehring Concrete) — $146–$285 per yd³ before fees; short-load fees $50–$200 · checked October 2026
BLS Producer Price Index — ready-mix concrete (via FRED) — +2.3% Sept 2025 → Sept 2026 · checked October 2026
International Residential Code 2021 — R506 & Table R402.2 (model text) — 3.5 in minimum slab, 4 in base course, 6-mil poly vapor retarder under heated slabs (not patios, walks, driveways or garages), minimum strength and air by exposure · checked October 2026

Short answer

A 12 × 12 ft slab 6 inches thick needs 2.67 cubic yards of concrete — 132 bags of 80-lb mix (or 176 × 60-lb), including 10% extra.

Volume: 144 sq ft × 6 in = 72 cu ft (2.67 yd³) before waste.

Bags at 10% extra: 132 × 80 lb, 176 × 60 lb or 264 × 40 lb — that is 10,560 lb to haul (3.1 pallets).

Cost: about $783 in 80-lb bags vs. about $825 for 3 yd³ of ready-mix delivered — bags are cheaper here, but mean more mixing.

Base: about 3.9 tons of crusher run for a 4-in compacted gravel base extending 6 in past the edges.

Water: roughly 99–149 gallons to mix 132 bags.

12×12 is the go-to patio size for a table and grill, and a common shed base.

Adjust it to your exact job

What are you pouring?

Project

Most spa makers ask for 4–6 in; a filled 6-person tub can top 5,000 lb.

Size

Shape

Length

= 12' · 3.66 m

Width

= 12' · 3.66 m

Thickness

Slab thickness

Form lumber is smaller than its name: a 2×4 is 3.5 in tall, a 2×6 is 5.5 in.

Exact thickness

You need

132× 80-lb bags

2.93 yd³ of concrete including 10% extra · 10,560 lb of bags to lift · 3.1 pallets

Concrete volume: 2.67yd³before waste
Area: 144sq ft
Mixing water: 99–149galstart low; max weakens the slab
Finished weight: 5.2tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	264	—
50 lb · 0.375 ft³	212	—
60 lb · 0.45 ft³	176	$854
80 lb · 0.6 ft³	132	$783

Bags or a ready-mix truck?

132 bags

$783

Cheaper ✓ · materials only, before tax

3 yd³ delivered

$825

incl. short-load & delivery fees

At these prices a truck becomes cheaper from about 3.25 yd³ (147 bags). Over 1–2 yd³, a truck also saves hours of mixing: plan on 3–4 minutes per bag in a mixer.

That's 5.3 tons of bags — more than most pickups can carry in one trip. Ask about store delivery.

Timeline for standard 4000 psi mix

Walk on: 24 hours
Light loads / posts: 24 hours
Keep moist (cure): 5 days ≥70°F · 7 days at 50–70°F
Protect from freezing: 48 hours

Shopping list

132 bags Standard 4000 psi mix — 80-lb bags$783
3.9 tons Crusher run / road base (4 in compacted)$149
6 boards 2×8 form boards, 8 ft
20 stakes Wood form stakes
149 gal Clean water for mixing99–149 gal; start low, never exceed the max

Estimated total$932

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
132 bags × $Pallet price applied (42+ bags)
$783
Gravel base (bulk)
3.9 tons × $
$148

Materials total$931

Typical U.S. big-box prices checked October 2026. Tap any price to enter your store's.

Thickness

Cubic yards

80-lb bags

60-lb bags

Bag cost

Ready-mix

3.5 in (2×4)

1.56

103

$457

$613

4 in

1.78

118

$522

$655

5 in

2.22

110

147

$652

$740

6 in

2.67

132

176

$783

$825

How this was worked out

12 ft × 12 ft = 144 sq ft. Multiply by the thickness in feet (6 ÷ 12 = 0.5 ft) to get 72 cubic feet, and divide by 27 for 2.67 cubic yards.

Adding 10% for uneven ground and spillage gives 79.2 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 79.2 ÷ 0.6 = 132, rounded up to 132 bags.

Ready-mix is priced at about $170 per yard for 4,000 psi plus delivery, fuel and — under 4 yards — a short-load fee. Plants sell in quarter-yard steps, so the order is 3 yd³.

The full method, material data and tips are on the main concrete slab calculator page.