How the math works
Area: 40 ft × 30 ft = 1200 sq ftThickness in feet: 4" ÷ 12 = 0.333 ftVolume: 1200 × 0.333 = 400.00 cu ftAdd 10% waste: 400.00 × 1.10 = 440.00 cu ftCubic yards: 440.00 ÷ 27 = 16.30 yd³ → order 16.50 yd³80 lb bags: 440.00 ÷ 0.60 = 734 bags (60 lb: ÷ 0.45 = 978)Rebar: 20 bars × 39' 6" + 27 bars × 29' 6" = 1,665 LF → 88 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
- 16.50 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 30x40 slab?
With 10% waste: 4" thick = 16.30 yd³ (order 16.50), 5" = 20.37 yd³ (order 20.50), 6" = 24.44 yd³ (order 24.50). A 30x40 slab is 1200 sq ft.
How many 80 lb bags of concrete for a 30x40 slab 4 inches thick?
734 bags of 80 lb concrete (0.60 cu ft each) or 978 bags of 60 lb, including 10% waste. That is about 58,720 lbs of bags to haul.
How much rebar do I need for a 30x40 slab?
A grid at 18" on center both ways uses 20 bars running 40 ft and 27 bars running 30 ft — about 1,665 linear feet, or 88 twenty-foot sticks of #3 or #4 rebar, 540 ties and ~140 chairs.
How much does a 30x40 concrete slab cost?
Material only at 4": about $2,723 in ready-mix or $4,771 in 80 lb bags; at 6": about $4,043 ready-mix. Installed, 30x40 slabs typically run $6–$12 per sq ft ($7,200–$14,400) depending on region, access and finish.