How many bags of concrete for a 3×3 slab, 4 inches thick?

Data checked October 2026

Short answer

A 3 × 3 ft slab 4 inches thick needs 0.11 cubic yards of concrete — 6 bags of 80-lb mix (or 8 × 60-lb), including 10% extra.

Volume: 9 sq ft × 4 in = 3 cu ft (0.11 yd³) before waste.
Bags at 10% extra: 6 × 80 lb, 8 × 60 lb or 11 × 40 lb — that is 480 lb to haul.
Cost: about $38.82 in 80-lb bags vs. about $485 for 1 yd³ of ready-mix delivered — bags are cheaper here, but mean more mixing.
Base: about 0.4 tons of crusher run for a 4-in compacted gravel base extending 6 in past the edges.
Water: roughly 5–7 gallons to mix 6 bags.
A 3×3 pad is the usual base for a central-AC condenser or a generator.

Adjust it to your exact job

What are you pouring?

Project

4 in for sheds; go 5–6 in with thickened edges for heavy workshops.

Size

Shape

Length

= 3' · 0.91 m

Width

= 3' · 0.91 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

6× 80-lb bags

0.12 yd³ of concrete including 10% extra · 480 lb of bags to lift

Concrete volume: 0.11yd³before waste
Area: 9sq ft
Mixing water: 5–7galstart low; max weakens the slab
Finished weight: 0.2tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	11	—
50 lb · 0.375 ft³	9	—
60 lb · 0.45 ft³	8	$38.80
80 lb · 0.6 ft³	6	$38.82

Bags or a ready-mix truck?

6 bags

$38.82

Cheaper ✓ · materials only, before tax

1 yd³ delivered

$485

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.

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

6 bags Standard 4000 psi mix — 80-lb bags$38.82
0.4 tons Crusher run / road base (4 in compacted)$14.15
2 boards 2×6 form boards, 8 ft
8 stakes Wood form stakes
7 gal Clean water for mixing5–7 gal; start low, never exceed the max

Estimated total$52.97

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
6 bags × $
$38.82
Gravel base (bulk)
0.4 tons × $
$15.20

Materials total$54.02

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 3×3 footprint at other thicknesses

Thickness	Cubic yards	80-lb bags	60-lb bags	Bag cost	Ready-mix
3.5 in (2×4)	0.1	5	7	$32.35	$485
4 in	0.11	6	8	$38.82	$485
5 in	0.14	7	10	$45.29	$485
6 in	0.17	9	11	$58.23	$485

How this was worked out

3 ft × 3 ft = 9 sq ft. Multiply by the thickness in feet (4 ÷ 12 = 0.333 ft) to get 3 cubic feet, and divide by 27 for 0.11 cubic yards.

Adding 10% for uneven ground and spillage gives 3.3 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 3.3 ÷ 0.6 = 5.5, rounded up to 6 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 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 3 × 3 ft slab 4 inches thick needs 0.11 cubic yards of concrete — 6 bags of 80-lb mix (or 8 × 60-lb), including 10% extra.

Volume: 9 sq ft × 4 in = 3 cu ft (0.11 yd³) before waste.

Bags at 10% extra: 6 × 80 lb, 8 × 60 lb or 11 × 40 lb — that is 480 lb to haul.

Cost: about $38.82 in 80-lb bags vs. about $485 for 1 yd³ of ready-mix delivered — bags are cheaper here, but mean more mixing.

Base: about 0.4 tons of crusher run for a 4-in compacted gravel base extending 6 in past the edges.

Water: roughly 5–7 gallons to mix 6 bags.

A 3×3 pad is the usual base for a central-AC condenser or a generator.

Adjust it to your exact job

What are you pouring?

Project

4 in for sheds; go 5–6 in with thickened edges for heavy workshops.

Size

Shape

Length

= 3' · 0.91 m

Width

= 3' · 0.91 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

6× 80-lb bags

0.12 yd³ of concrete including 10% extra · 480 lb of bags to lift

Concrete volume: 0.11yd³before waste
Area: 9sq ft
Mixing water: 5–7galstart low; max weakens the slab
Finished weight: 0.2tons

Other bag sizes

Bags needed by size
Bag	Bags	Est. cost
40 lb · 0.3 ft³	11	—
50 lb · 0.375 ft³	9	—
60 lb · 0.45 ft³	8	$38.80
80 lb · 0.6 ft³	6	$38.82

Bags or a ready-mix truck?

6 bags

$38.82

Cheaper ✓ · materials only, before tax

1 yd³ delivered

$485

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.

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

6 bags Standard 4000 psi mix — 80-lb bags$38.82
0.4 tons Crusher run / road base (4 in compacted)$14.15
2 boards 2×6 form boards, 8 ft
8 stakes Wood form stakes
7 gal Clean water for mixing5–7 gal; start low, never exceed the max

Estimated total$52.97

Cost estimate

Tap a price to use your store's

80-lb Standard 4000 psi mix
6 bags × $
$38.82
Gravel base (bulk)
0.4 tons × $
$15.20

Materials total$54.02

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)

0.1

$32.35

$485

4 in

0.11

$38.82

$485

5 in

0.14

$45.29

$485

6 in

0.17

$58.23

$485

How this was worked out

3 ft × 3 ft = 9 sq ft. Multiply by the thickness in feet (4 ÷ 12 = 0.333 ft) to get 3 cubic feet, and divide by 27 for 0.11 cubic yards.

Adding 10% for uneven ground and spillage gives 3.3 cu ft. Quikrete and Sakrete both rate an 80-lb bag at 0.6 cu ft, so 3.3 ÷ 0.6 = 5.5, rounded up to 6 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 yd³.

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