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Concrete Slab Calculator · Free calculator

12x12 Concrete Slab Calculator

A 12' × 12' slab covers 144 sq ft. At 4" thick that's 1.96 cubic yards of concrete with 10% waste — order 2.00 yd³ of ready-mix or 88 × 80 lb bags. Garage and RV pads at 6" need 2.93 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
1.96 yd³
80 lb bags @ 4"
88
6" slab (garage / RV)
2.93 yd³
Rebar #4 @ 18" OC
10 × 20' sticks
2x4 forms
4 × 16' boards
Est. cost @ 4" (ready-mix)
$330

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 12' 0"W = 12' 0"D = 4"
Cubic Yards
1.96yd³
incl. 10% waste · Ready-mix truck: order 2.00 yd³
Cubic Feet
52.80ft³
Area
144.0sq ft
Weight
3.96tons
Weight
7,920lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)88
  • 60 lb bag (0.45 ft³ yield)118
  • 40 lb bag (0.3 ft³ yield)176

Concrete for a 12x12 slab by thickness

144 sq ft, 10% waste included. Ready-mix ~$165/yd³ ordered in ¼ yd steps; 80 lb bags ~$6.50.

ThicknessTypical useCubic yardsOrder80 lb bags60 lb bagsWeightReady-mix80 lb bags $
4"Patio / Shed / Walkway1.96 yd³2.00 yd³881183.96 tons$330$572
5"Driveway / Hot tub pad2.44 yd³2.50 yd³1101474.95 tons$413$715
6"Garage / RV Pad2.93 yd³3.00 yd³1321765.94 tons$495$858

Rebar grid for a 12x12 slab — 18" on center both ways

3" edge cover, no lap splices needed, 5% cut-off waste.

ItemQuantityNotes
Bars running the 12' way8 bars11' 6" each
Bars running the 12' way8 bars11' 6" each
Total rebar184 LFincl. laps
20 ft sticks to buy10+5% cut-off waste
Weight #3 / #469 / 123 lbs0.376 / 0.668 lb per ft
Wire ties64one per intersection
Rebar chairs (2"–3")16~36" grid
Alt: 6x6 welded wire mesh4 sheets5' × 10' sheets, 6" laps

Forms & site prep for 12x12

ItemQuantityNotes
Form perimeter48 LF2 × (12' + 12')
2x4 form boards (16 ft)4use 2x6 for 5"–6" slabs
Wood stakes (18"–24")20every 3 ft + each corner
Duplex nails402 per stake
Gravel base 4" (compacted)2.04 yd³ / 2.86 tons+15% for compaction
6-mil vapor barrier158 sq ft+10% for laps
Control joints24 LF2 cuts, panels ≤ 10'

How the math works

  1. Area: 12 ft × 12 ft = 144 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 144 × 0.333 = 48.00 cu ft
  4. Add 10% waste: 48.00 × 1.10 = 52.80 cu ft
  5. Cubic yards: 52.80 ÷ 27 = 1.96 yd³ → order 2.00 yd³
  6. 80 lb bags: 52.80 ÷ 0.60 = 88 bags (60 lb: ÷ 0.45 = 118)
  7. Rebar: 8 bars × 11' 6" + 8 bars × 11' 6" = 184 LF → 10 sticks

Bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft (Quikrete / Sakrete). 1 yd³ = 45 × 80 lb bags.

Trade pro tips

  • 2.00 yd³ is under most plants' 4-yard minimum — ask about the short-load fee before ordering ready-mix.
  • Order 5–10% extra. Subgrade is never perfectly flat, and running short mid-pour leaves a cold joint.
  • Compact 4" of gravel base and slope the forms ¼" per foot away from any structure.
  • Chair the rebar so it sits in the middle third of the slab — rebar lying on the dirt does nothing.
  • Cut control joints ¼ of the slab depth, no more than 2–3 × the thickness (in feet) apart, within 6–18 hours.

Frequently asked questions

How many yards of concrete do I need for a 12x12 slab?

With 10% waste: 4" thick = 1.96 yd³ (order 2.00), 5" = 2.44 yd³ (order 2.50), 6" = 2.93 yd³ (order 3.00). A 12x12 slab is 144 sq ft.

How many 80 lb bags of concrete for a 12x12 slab 4 inches thick?

88 bags of 80 lb concrete (0.60 cu ft each) or 118 bags of 60 lb, including 10% waste. That is about 7,040 lbs of bags to haul.

How much rebar do I need for a 12x12 slab?

A grid at 18" on center both ways uses 8 bars running 12 ft and 8 bars running 12 ft — about 184 linear feet, or 10 twenty-foot sticks of #3 or #4 rebar, 64 ties and ~16 chairs.

How much does a 12x12 concrete slab cost?

Material only at 4": about $330 in ready-mix or $572 in 80 lb bags; at 6": about $495 ready-mix. Installed, 12x12 slabs typically run $6–$12 per sq ft ($864–$1,728) depending on region, access and finish.