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Concrete Slab Calculator · Free calculator

16x20 Concrete Slab Calculator

A 16' × 20' slab covers 320 sq ft. At 4" thick that's 4.35 cubic yards of concrete with 10% waste — order 4.50 yd³ of ready-mix or 196 × 80 lb bags. Garage and RV pads at 6" need 6.52 yd³. Rebar, forms and base are worked out below.

Quick answer

4" slab (patio / shed)
4.35 yd³
80 lb bags @ 4"
196
6" slab (garage / RV)
6.52 yd³
Rebar #4 @ 18" OC
23 × 20' sticks
2x4 forms
6 × 16' boards
Est. cost @ 4" (ready-mix)
$743

▍Concrete Calculator

ft
in
ft
in
in
Waste Factor
$
L = 20' 0"W = 16' 0"D = 4"
Cubic Yards
4.35yd³
incl. 10% waste · Ready-mix truck: order 4.50 yd³
Cubic Feet
117.33ft³
Area
320.0sq ft
Weight
8.80tons
Weight
17,600lbs
Concrete bags needed
  • 80 lb bag (0.6 ft³ yield)196
  • 60 lb bag (0.45 ft³ yield)261
  • 40 lb bag (0.3 ft³ yield)392

Concrete for a 16x20 slab by thickness

320 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 / Walkway4.35 yd³4.50 yd³1962618.80 tons$743$1,274
5"Driveway / Hot tub pad5.43 yd³5.50 yd³24532611.00 tons$908$1,593
6"Garage / RV Pad6.52 yd³6.75 yd³29439213.20 tons$1,114$1,911

Rebar grid for a 16x20 slab — 18" on center both ways

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

ItemQuantityNotes
Bars running the 20' way11 bars19' 6" each
Bars running the 16' way14 bars15' 6" each
Total rebar432 LFincl. laps
20 ft sticks to buy23+5% cut-off waste
Weight #3 / #4162 / 288 lbs0.376 / 0.668 lb per ft
Wire ties154one per intersection
Rebar chairs (2"–3")42~36" grid
Alt: 6x6 welded wire mesh8 sheets5' × 10' sheets, 6" laps

Forms & site prep for 16x20

ItemQuantityNotes
Form perimeter72 LF2 × (16' + 20')
2x4 form boards (16 ft)6use 2x6 for 5"–6" slabs
Wood stakes (18"–24")30every 3 ft + each corner
Duplex nails602 per stake
Gravel base 4" (compacted)4.54 yd³ / 6.36 tons+15% for compaction
6-mil vapor barrier352 sq ft+10% for laps
Control joints36 LF2 cuts, panels ≤ 10'

How the math works

  1. Area: 20 ft × 16 ft = 320 sq ft
  2. Thickness in feet: 4" ÷ 12 = 0.333 ft
  3. Volume: 320 × 0.333 = 106.67 cu ft
  4. Add 10% waste: 106.67 × 1.10 = 117.33 cu ft
  5. Cubic yards: 117.33 ÷ 27 = 4.35 yd³ → order 4.50 yd³
  6. 80 lb bags: 117.33 ÷ 0.60 = 196 bags (60 lb: ÷ 0.45 = 261)
  7. Rebar: 11 bars × 19' 6" + 14 bars × 15' 6" = 432 LF → 23 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

  • 4.50 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 16x20 slab?

With 10% waste: 4" thick = 4.35 yd³ (order 4.50), 5" = 5.43 yd³ (order 5.50), 6" = 6.52 yd³ (order 6.75). A 16x20 slab is 320 sq ft.

How many 80 lb bags of concrete for a 16x20 slab 4 inches thick?

196 bags of 80 lb concrete (0.60 cu ft each) or 261 bags of 60 lb, including 10% waste. That is about 15,680 lbs of bags to haul.

How much rebar do I need for a 16x20 slab?

A grid at 18" on center both ways uses 11 bars running 20 ft and 14 bars running 16 ft — about 432 linear feet, or 23 twenty-foot sticks of #3 or #4 rebar, 154 ties and ~42 chairs.

How much does a 16x20 concrete slab cost?

Material only at 4": about $743 in ready-mix or $1,274 in 80 lb bags; at 6": about $1,114 ready-mix. Installed, 16x20 slabs typically run $6–$12 per sq ft ($1,920–$3,840) depending on region, access and finish.