How the math works
Area: 20 ft × 20 ft = 400 sq ftThickness in feet: 4" ÷ 12 = 0.333 ftVolume: 400 × 0.333 = 133.33 cu ftAdd 10% waste: 133.33 × 1.10 = 146.67 cu ftCubic yards: 146.67 ÷ 27 = 5.43 yd³ → order 5.50 yd³80 lb bags: 146.67 ÷ 0.60 = 245 bags (60 lb: ÷ 0.45 = 326)Rebar: 14 bars × 19' 6" + 14 bars × 19' 6" = 546 LF → 29 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
- 5.50 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 20x20 slab?
With 10% waste: 4" thick = 5.43 yd³ (order 5.50), 5" = 6.79 yd³ (order 7.00), 6" = 8.15 yd³ (order 8.25). A 20x20 slab is 400 sq ft.
How many 80 lb bags of concrete for a 20x20 slab 4 inches thick?
245 bags of 80 lb concrete (0.60 cu ft each) or 326 bags of 60 lb, including 10% waste. That is about 19,600 lbs of bags to haul.
How much rebar do I need for a 20x20 slab?
A grid at 18" on center both ways uses 14 bars running 20 ft and 14 bars running 20 ft — about 546 linear feet, or 29 twenty-foot sticks of #3 or #4 rebar, 196 ties and ~49 chairs.
How much does a 20x20 concrete slab cost?
Material only at 4": about $908 in ready-mix or $1,593 in 80 lb bags; at 6": about $1,361 ready-mix. Installed, 20x20 slabs typically run $6–$12 per sq ft ($2,400–$4,800) depending on region, access and finish.