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Concrete Slab Calculator · Free calculator

8x8 Concrete Slab Calculator

A 8' × 8' slab covers 64 sq ft. At 4" thick that's 0.87 cubic yards of concrete with 10% waste — order 1.00 yd³ of ready-mix or 40 × 80 lb bags. Garage and RV pads at 6" need 1.30 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
0.87 yd³
80 lb bags @ 4"
40
6" slab (garage / RV)
1.30 yd³
Rebar #4 @ 18" OC
5 × 20' sticks
2x4 forms
4 × 16' boards
Est. cost @ 4" (ready-mix)
$165

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 8' 0"W = 8' 0"D = 4"
Cubic Yards
0.87yd³
incl. 10% waste · Ready-mix truck: order 1.00 yd³
Cubic Feet
23.47ft³
Area
64.0sq ft
Weight
1.76tons
Weight
3,520lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)40
  • 60 lb bag (0.45 ft³ yield)53
  • 40 lb bag (0.3 ft³ yield)79

Concrete for a 8x8 slab by thickness

64 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 / Walkway0.87 yd³1.00 yd³40531.76 tons$165$260
5"Driveway / Hot tub pad1.09 yd³1.25 yd³49662.20 tons$206$319
6"Garage / RV Pad1.30 yd³1.50 yd³59792.64 tons$248$384

Rebar grid for a 8x8 slab — 18" on center both ways

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

ItemQuantityNotes
Bars running the 8' way6 bars7' 6" each
Bars running the 8' way6 bars7' 6" each
Total rebar90 LFincl. laps
20 ft sticks to buy5+5% cut-off waste
Weight #3 / #434 / 60 lbs0.376 / 0.668 lb per ft
Wire ties36one per intersection
Rebar chairs (2"–3")9~36" grid
Alt: 6x6 welded wire mesh2 sheets5' × 10' sheets, 6" laps

Forms & site prep for 8x8

ItemQuantityNotes
Form perimeter32 LF2 × (8' + 8')
2x4 form boards (16 ft)4use 2x6 for 5"–6" slabs
Wood stakes (18"–24")16every 3 ft + each corner
Duplex nails322 per stake
Gravel base 4" (compacted)0.91 yd³ / 1.27 tons+15% for compaction
6-mil vapor barrier70 sq ft+10% for laps
Control jointsnone neededpanel already ≤ 10 ft

How the math works

  1. Area: 8 ft × 8 ft = 64 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 64 × 0.333 = 21.33 cu ft
  4. Add 10% waste: 21.33 × 1.10 = 23.47 cu ft
  5. Cubic yards: 23.47 ÷ 27 = 0.87 yd³ → order 1.00 yd³
  6. 80 lb bags: 23.47 ÷ 0.60 = 40 bags (60 lb: ÷ 0.45 = 53)
  7. Rebar: 6 bars × 7' 6" + 6 bars × 7' 6" = 90 LF → 5 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

  • At 0.87 yd³ this 8x8 pour is bag territory — rent a mixer and have a crew of 2–3 ready.
  • 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 8x8 slab?

With 10% waste: 4" thick = 0.87 yd³ (order 1.00), 5" = 1.09 yd³ (order 1.25), 6" = 1.30 yd³ (order 1.50). A 8x8 slab is 64 sq ft.

How many 80 lb bags of concrete for a 8x8 slab 4 inches thick?

40 bags of 80 lb concrete (0.60 cu ft each) or 53 bags of 60 lb, including 10% waste. That is about 3,200 lbs of bags to haul.

How much rebar do I need for a 8x8 slab?

A grid at 18" on center both ways uses 6 bars running 8 ft and 6 bars running 8 ft — about 90 linear feet, or 5 twenty-foot sticks of #3 or #4 rebar, 36 ties and ~9 chairs.

How much does a 8x8 concrete slab cost?

Material only at 4": about $165 in ready-mix or $260 in 80 lb bags; at 6": about $248 ready-mix. Installed, 8x8 slabs typically run $6–$12 per sq ft ($384–$768) depending on region, access and finish.