Skip to content
BuildMath

Concrete Slab Calculator · Free calculator

20x24 Concrete Slab Calculator

A 20' × 24' slab covers 480 sq ft. At 4" thick that's 6.52 cubic yards of concrete with 10% waste — order 6.75 yd³ of ready-mix or 294 × 80 lb bags. Garage and RV pads at 6" need 9.78 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
6.52 yd³
80 lb bags @ 4"
294
6" slab (garage / RV)
9.78 yd³
Rebar #4 @ 18" OC
35 × 20' sticks
2x4 forms
8 × 16' boards
Est. cost @ 4" (ready-mix)
$1,114

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 24' 0"W = 20' 0"D = 4"
Cubic Yards
6.52yd³
incl. 10% waste · Ready-mix truck: order 6.75 yd³
Cubic Feet
176.00ft³
Area
480.0sq ft
Weight
13.20tons
Weight
26,400lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)294
  • 60 lb bag (0.45 ft³ yield)392
  • 40 lb bag (0.3 ft³ yield)587

Concrete for a 20x24 slab by thickness

480 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 / Walkway6.52 yd³6.75 yd³29439213.20 tons$1,114$1,911
5"Driveway / Hot tub pad8.15 yd³8.25 yd³36748916.50 tons$1,361$2,386
6"Garage / RV Pad9.78 yd³10.00 yd³44058719.80 tons$1,650$2,860

Rebar grid for a 20x24 slab — 18" on center both ways

3" edge cover, 14 lap splices at 20" (40 bar diameters for #4), 5% cut-off waste.

ItemQuantityNotes
Bars running the 24' way14 bars23' 6" each
Bars running the 20' way16 bars19' 6" each
Total rebar664 LFincl. laps
20 ft sticks to buy35+5% cut-off waste
Weight #3 / #4250 / 444 lbs0.376 / 0.668 lb per ft
Wire ties224one per intersection
Rebar chairs (2"–3")56~36" grid
Alt: 6x6 welded wire mesh12 sheets5' × 10' sheets, 6" laps

Forms & site prep for 20x24

ItemQuantityNotes
Form perimeter88 LF2 × (20' + 24')
2x4 form boards (16 ft)8use 2x6 for 5"–6" slabs
Wood stakes (18"–24")34every 3 ft + each corner
Duplex nails682 per stake
Gravel base 4" (compacted)6.81 yd³ / 9.54 tons+15% for compaction
6-mil vapor barrier528 sq ft+10% for laps
Control joints64 LF3 cuts, panels ≤ 10'

How the math works

  1. Area: 24 ft × 20 ft = 480 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 480 × 0.333 = 160.00 cu ft
  4. Add 10% waste: 160.00 × 1.10 = 176.00 cu ft
  5. Cubic yards: 176.00 ÷ 27 = 6.52 yd³ → order 6.75 yd³
  6. 80 lb bags: 176.00 ÷ 0.60 = 294 bags (60 lb: ÷ 0.45 = 392)
  7. Rebar: 14 bars × 23' 6" + 16 bars × 19' 6" = 664 LF → 35 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

  • 6.75 yd³ fits in one ready-mix truck — order by the ¼ yard and have the chute reach planned.
  • 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 20x24 slab?

With 10% waste: 4" thick = 6.52 yd³ (order 6.75), 5" = 8.15 yd³ (order 8.25), 6" = 9.78 yd³ (order 10.00). A 20x24 slab is 480 sq ft.

How many 80 lb bags of concrete for a 20x24 slab 4 inches thick?

294 bags of 80 lb concrete (0.60 cu ft each) or 392 bags of 60 lb, including 10% waste. That is about 23,520 lbs of bags to haul.

How much rebar do I need for a 20x24 slab?

A grid at 18" on center both ways uses 14 bars running 24 ft and 16 bars running 20 ft — about 664 linear feet, or 35 twenty-foot sticks of #3 or #4 rebar, 224 ties and ~56 chairs.

How much does a 20x24 concrete slab cost?

Material only at 4": about $1,114 in ready-mix or $1,911 in 80 lb bags; at 6": about $1,650 ready-mix. Installed, 20x24 slabs typically run $6–$12 per sq ft ($2,880–$5,760) depending on region, access and finish.