How many bags of concrete for a 12×12 slab, 3.5 inches thick?

Data checked October 2026

Short answer

A 12 × 12 ft slab 3.5 inches thick needs 1.56 cubic yards of concrete — 77 bags of 80-lb mix (or 103 × 60-lb), including 10% extra.

Volume: 144 sq ft × 3.5 in = 42 cu ft (1.56 yd³) before waste.
Bags at 10% extra: 77 × 80 lb, 103 × 60 lb or 154 × 40 lb — that is 6,160 lb to haul (1.8 pallets).
Cost: about $457 in 80-lb bags vs. about $613 for 1.75 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 58–87 gallons to mix 77 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

4 in over 4 in of compacted gravel; slope 1/8 in per foot away from the house.

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

77× 80-lb bags

1.71 yd³ of concrete including 10% extra · 6,160 lb of bags to lift · 1.8 pallets

Concrete volume: 1.56yd³before waste
Area: 144sq ft
Mixing water: 58–87galstart low; max weakens the slab
Finished weight: 3tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	154	—
50 lb · 0.375 ft³	124	—
60 lb · 0.45 ft³	103	$500
80 lb · 0.6 ft³	77	$457

Bags or a ready-mix truck?

77 bags

$457

Cheaper ✓ · materials only, before tax

1.75 yd³ delivered

$613

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 3.1 tons of bags — more than most pickups can carry in one trip. Ask about store delivery.

Freeze–thaw climate?

Regular bagged mixes are not air-entrained. For exposed slabs where winters freeze, Crack-Resistant mix (4–8% air) or air-entrained ready-mix holds up better against scaling.

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

77 bags Standard 4000 psi mix — 80-lb bags$457
3.9 tons Crusher run / road base (4 in compacted)$149
6 boards 2×4 form boards, 8 ft
20 stakes Wood form stakes
87 gal Clean water for mixing58–87 gal; start low, never exceed the max

Estimated total$606

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
77 bags × $Pallet price applied (42+ bags)
$457
Gravel base (bulk)
3.9 tons × $
$148

Materials total$605

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 (3.5 ÷ 12 = 0.292 ft) to get 42 cubic feet, and divide by 27 for 1.56 cubic yards.

Adding 10% for uneven ground and spillage gives 46.2 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 46.2 ÷ 0.6 = 77, rounded up to 77 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 1.75 yd³.

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

Similar jobs

Other sizes people check

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 3.5 inches thick needs 1.56 cubic yards of concrete — 77 bags of 80-lb mix (or 103 × 60-lb), including 10% extra.

Volume: 144 sq ft × 3.5 in = 42 cu ft (1.56 yd³) before waste.

Bags at 10% extra: 77 × 80 lb, 103 × 60 lb or 154 × 40 lb — that is 6,160 lb to haul (1.8 pallets).

Cost: about $457 in 80-lb bags vs. about $613 for 1.75 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 58–87 gallons to mix 77 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

4 in over 4 in of compacted gravel; slope 1/8 in per foot away from the house.

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

77× 80-lb bags

1.71 yd³ of concrete including 10% extra · 6,160 lb of bags to lift · 1.8 pallets

Concrete volume: 1.56yd³before waste
Area: 144sq ft
Mixing water: 58–87galstart low; max weakens the slab
Finished weight: 3tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	154	—
50 lb · 0.375 ft³	124	—
60 lb · 0.45 ft³	103	$500
80 lb · 0.6 ft³	77	$457

Bags or a ready-mix truck?

77 bags

$457

Cheaper ✓ · materials only, before tax

1.75 yd³ delivered

$613

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 3.1 tons of bags — more than most pickups can carry in one trip. Ask about store delivery.

Freeze–thaw climate?

Regular bagged mixes are not air-entrained. For exposed slabs where winters freeze, Crack-Resistant mix (4–8% air) or air-entrained ready-mix holds up better against scaling.

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

77 bags Standard 4000 psi mix — 80-lb bags$457
3.9 tons Crusher run / road base (4 in compacted)$149
6 boards 2×4 form boards, 8 ft
20 stakes Wood form stakes
87 gal Clean water for mixing58–87 gal; start low, never exceed the max

Estimated total$606

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
77 bags × $Pallet price applied (42+ bags)
$457
Gravel base (bulk)
3.9 tons × $
$148

Materials total$605

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 (3.5 ÷ 12 = 0.292 ft) to get 42 cubic feet, and divide by 27 for 1.56 cubic yards.

Adding 10% for uneven ground and spillage gives 46.2 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 46.2 ÷ 0.6 = 77, rounded up to 77 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 1.75 yd³.

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