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Concrete Slab Calculator · Free calculator

12x20 Concrete Slab Calculator

A 12' × 20' slab covers 240 sq ft. At 4" thick that's 3.26 cubic yards of concrete with 10% waste — order 3.50 yd³ of ready-mix or 147 × 80 lb bags. Garage and RV pads at 6" need 4.89 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
3.26 yd³
80 lb bags @ 4"
147
6" slab (garage / RV)
4.89 yd³
Rebar #4 @ 18" OC
17 × 20' sticks
2x4 forms
6 × 16' boards
Est. cost @ 4" (ready-mix)
$578

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 20' 0"W = 12' 0"D = 4"
Cubic Yards
3.26yd³
incl. 10% waste · Ready-mix truck: order 3.50 yd³
Cubic Feet
88.00ft³
Area
240.0sq ft
Weight
6.60tons
Weight
13,200lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)147
  • 60 lb bag (0.45 ft³ yield)196
  • 40 lb bag (0.3 ft³ yield)294

Concrete for a 12x20 slab by thickness

240 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 / Walkway3.26 yd³3.50 yd³1471966.60 tons$578$956
5"Driveway / Hot tub pad4.07 yd³4.25 yd³1842458.25 tons$701$1,196
6"Garage / RV Pad4.89 yd³5.00 yd³2202949.90 tons$825$1,430

Rebar grid for a 12x20 slab — 18" on center both ways

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

ItemQuantityNotes
Bars running the 20' way8 bars19' 6" each
Bars running the 12' way14 bars11' 6" each
Total rebar317 LFincl. laps
20 ft sticks to buy17+5% cut-off waste
Weight #3 / #4119 / 212 lbs0.376 / 0.668 lb per ft
Wire ties112one per intersection
Rebar chairs (2"–3")28~36" grid
Alt: 6x6 welded wire mesh6 sheets5' × 10' sheets, 6" laps

Forms & site prep for 12x20

ItemQuantityNotes
Form perimeter64 LF2 × (12' + 20')
2x4 form boards (16 ft)6use 2x6 for 5"–6" slabs
Wood stakes (18"–24")26every 3 ft + each corner
Duplex nails522 per stake
Gravel base 4" (compacted)3.41 yd³ / 4.77 tons+15% for compaction
6-mil vapor barrier264 sq ft+10% for laps
Control joints32 LF2 cuts, panels ≤ 10'

How the math works

  1. Area: 20 ft × 12 ft = 240 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 240 × 0.333 = 80.00 cu ft
  4. Add 10% waste: 80.00 × 1.10 = 88.00 cu ft
  5. Cubic yards: 88.00 ÷ 27 = 3.26 yd³ → order 3.50 yd³
  6. 80 lb bags: 88.00 ÷ 0.60 = 147 bags (60 lb: ÷ 0.45 = 196)
  7. Rebar: 8 bars × 19' 6" + 14 bars × 11' 6" = 317 LF → 17 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

  • 3.50 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 12x20 slab?

With 10% waste: 4" thick = 3.26 yd³ (order 3.50), 5" = 4.07 yd³ (order 4.25), 6" = 4.89 yd³ (order 5.00). A 12x20 slab is 240 sq ft.

How many 80 lb bags of concrete for a 12x20 slab 4 inches thick?

147 bags of 80 lb concrete (0.60 cu ft each) or 196 bags of 60 lb, including 10% waste. That is about 11,760 lbs of bags to haul.

How much rebar do I need for a 12x20 slab?

A grid at 18" on center both ways uses 8 bars running 20 ft and 14 bars running 12 ft — about 317 linear feet, or 17 twenty-foot sticks of #3 or #4 rebar, 112 ties and ~28 chairs.

How much does a 12x20 concrete slab cost?

Material only at 4": about $578 in ready-mix or $956 in 80 lb bags; at 6": about $825 ready-mix. Installed, 12x20 slabs typically run $6–$12 per sq ft ($1,440–$2,880) depending on region, access and finish.