How the math works
Area: 60 ft × 40 ft = 2400 sq ftThickness in feet: 4" ÷ 12 = 0.333 ftVolume: 2400 × 0.333 = 800.00 cu ftAdd 10% waste: 800.00 × 1.10 = 880.00 cu ftCubic yards: 880.00 ÷ 27 = 32.59 yd³ → order 32.75 yd³80 lb bags: 880.00 ÷ 0.60 = 1467 bags (60 lb: ÷ 0.45 = 1956)Rebar: 27 bars × 59' 6" + 40 bars × 39' 6" = 3,343 LF → 176 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
- 32.75 yd³ is more than one 10-yd truck — schedule trucks 30–45 minutes apart to avoid cold joints.
- 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 40x60 slab?
With 10% waste: 4" thick = 32.59 yd³ (order 32.75), 5" = 40.74 yd³ (order 40.75), 6" = 48.89 yd³ (order 49.00). A 40x60 slab is 2400 sq ft.
How many 80 lb bags of concrete for a 40x60 slab 4 inches thick?
1467 bags of 80 lb concrete (0.60 cu ft each) or 1956 bags of 60 lb, including 10% waste. That is about 117,360 lbs of bags to haul.
How much rebar do I need for a 40x60 slab?
A grid at 18" on center both ways uses 27 bars running 60 ft and 40 bars running 40 ft — about 3,343 linear feet, or 176 twenty-foot sticks of #3 or #4 rebar, 1,080 ties and ~280 chairs.
How much does a 40x60 concrete slab cost?
Material only at 4": about $5,404 in ready-mix or $9,536 in 80 lb bags; at 6": about $8,085 ready-mix. Installed, 40x60 slabs typically run $6–$12 per sq ft ($14,400–$28,800) depending on region, access and finish.