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Concrete Slab Calculator · Free calculator

24x30 Concrete Slab Calculator

A 24' × 30' slab covers 720 sq ft. At 4" thick that's 9.78 cubic yards of concrete with 10% waste — order 10.00 yd³ of ready-mix or 440 × 80 lb bags. Garage and RV pads at 6" need 14.67 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
9.78 yd³
80 lb bags @ 4"
440
6" slab (garage / RV)
14.67 yd³
Rebar #4 @ 18" OC
53 × 20' sticks
2x4 forms
8 × 16' boards
Est. cost @ 4" (ready-mix)
$1,650

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 30' 0"W = 24' 0"D = 4"
Cubic Yards
9.78yd³
incl. 10% waste · Ready-mix truck: order 10.00 yd³
Cubic Feet
264.00ft³
Area
720.0sq ft
Weight
19.80tons
Weight
39,600lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)440
  • 60 lb bag (0.45 ft³ yield)587
  • 40 lb bag (0.3 ft³ yield)880

Concrete for a 24x30 slab by thickness

720 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 / Walkway9.78 yd³10.00 yd³44058719.80 tons$1,650$2,860
5"Driveway / Hot tub pad12.22 yd³12.25 yd³55073424.75 tons$2,021$3,575
6"Garage / RV Pad14.67 yd³14.75 yd³66088029.70 tons$2,434$4,290

Rebar grid for a 24x30 slab — 18" on center both ways

3" edge cover, 36 lap splices at 20" (40 bar diameters for #4), 5% cut-off waste.

ItemQuantityNotes
Bars running the 30' way16 bars29' 6" each
Bars running the 24' way20 bars23' 6" each
Total rebar1,002 LFincl. laps
20 ft sticks to buy53+5% cut-off waste
Weight #3 / #4377 / 669 lbs0.376 / 0.668 lb per ft
Wire ties320one per intersection
Rebar chairs (2"–3")80~36" grid
Alt: 6x6 welded wire mesh18 sheets5' × 10' sheets, 6" laps

Forms & site prep for 24x30

ItemQuantityNotes
Form perimeter108 LF2 × (24' + 30')
2x4 form boards (16 ft)8use 2x6 for 5"–6" slabs
Wood stakes (18"–24")40every 3 ft + each corner
Duplex nails802 per stake
Gravel base 4" (compacted)10.22 yd³ / 14.31 tons+15% for compaction
6-mil vapor barrier792 sq ft+10% for laps
Control joints108 LF4 cuts, panels ≤ 10'

How the math works

  1. Area: 30 ft × 24 ft = 720 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 720 × 0.333 = 240.00 cu ft
  4. Add 10% waste: 240.00 × 1.10 = 264.00 cu ft
  5. Cubic yards: 264.00 ÷ 27 = 9.78 yd³ → order 10.00 yd³
  6. 80 lb bags: 264.00 ÷ 0.60 = 440 bags (60 lb: ÷ 0.45 = 587)
  7. Rebar: 16 bars × 29' 6" + 20 bars × 23' 6" = 1,002 LF → 53 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

  • 10.00 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 24x30 slab?

With 10% waste: 4" thick = 9.78 yd³ (order 10.00), 5" = 12.22 yd³ (order 12.25), 6" = 14.67 yd³ (order 14.75). A 24x30 slab is 720 sq ft.

How many 80 lb bags of concrete for a 24x30 slab 4 inches thick?

440 bags of 80 lb concrete (0.60 cu ft each) or 587 bags of 60 lb, including 10% waste. That is about 35,200 lbs of bags to haul.

How much rebar do I need for a 24x30 slab?

A grid at 18" on center both ways uses 16 bars running 30 ft and 20 bars running 24 ft — about 1,002 linear feet, or 53 twenty-foot sticks of #3 or #4 rebar, 320 ties and ~80 chairs.

How much does a 24x30 concrete slab cost?

Material only at 4": about $1,650 in ready-mix or $2,860 in 80 lb bags; at 6": about $2,434 ready-mix. Installed, 24x30 slabs typically run $6–$12 per sq ft ($4,320–$8,640) depending on region, access and finish.