Concrete slab calculator

1. Area
   A = L × W
   20' 0" × 12' 0" = 240.00 SF
2. Slab volume
   V₁ = A × T
   240.00 SF × 4" = 80.00 ft³
3. 80 lb bags
   Bags = ⌈V ÷ bag yield⌉
   80.00 ft³ ÷ 0.60 ft³ = 134
4. Weight
   W = V × 140 lb/ft³
   80.00 ft³ × 140 lb/ft³ = 11,200 lb

Where the numbers come from

• Bagged concrete yield: 50 lb = 0.375 ft³ · 60 lb = 0.45 ft³ · 80 lb = 0.60 ft³QUIKRETE 5000 High Early Strength Concrete Mix data sheet, "Yield". Regular QUIKRETE Concrete Mix 1101 gives the same 0.45 and 0.60 ft³. All three are 0.0075 ft³ per lb, so a bag weight you type is figured at that rate (e.g. 40 lb = 0.30 ft³). High-yield mixes (e.g. Sakrete Maximizer, 0.92 ft³ per 80 lb) differ — check the bag.Source
• Weight of normal-weight concrete: 140 lb/ft³QUIKRETE 5000 data sheet, Table 1: unit weight (ASTM C138) approximately 140 lb/ft³. Ready-mix is often 145–150 lb/ft³ — check your mix ticket.Source
• Weight of lightweight concrete: 105–120 lb/ft³Sakrete Maximizer Concrete Mix technical data sheet (04-24): 105–120 lb per cu. ft. The weight is shown for both ends of the range. Bag counts still use the QUIKRETE 5000 yields; Maximizer itself yields more (40 lb = 0.46 ft³, 80 lb = 0.92 ft³).Source
• Cubic yard: 1 CY = 27 ft³ = 0.764555 m³Exact by definition: 1 ft = 0.3048 m.
• Dowels into the slab below (when added): #4 @ 12", 3" in from the edge of the pad, drilled 4" into the slab in epoxy, bent with a 90° standard hook at the middle of the padFor a pad poured on an existing slab. The dowels sit on a line the cover in from the side of the pad (the outline moved in on every side; for a circle, the radius less the cover), counted ⌈line ÷ spacing⌉. Each is a bent bar: drilled depth, straight up, then a 90° standard hook lying at the middle of the pad. Hook per ACI 318 (318-11 §7.1.2 and Table 7.2; Table 25.3.1 in 318-19): inside bend 6 bar diameters, straight tail 12 bar diameters (#4: 3" bend, 6" tail). Detailed the way rebar shops do (ACI 315 §3.7, CRSI Manual of Standard Practice): every dimension out to out of the bar, and the bar length is the sum of them — leg (drilled depth + half the pad + half the bar) + hook G (12 db + D ÷ 2 + db). G checks with the CRSI table of recommended 90° end hooks: #3 6", #4 8", #5 10", #6 1'-0", #7 1'-2", #8 1'-4". Epoxy: DeWalt AC100+ Gold 27.9 fl oz, worked out the way the epoxy anchor page does it. A typical structural detail for equipment pads uses #3 pins @ 12" drilled 3" — the page starts at #4 @ 12" drilled 4"; use your drawings.
• Gravel base (when added): V = slab area × gravel depth · CY = ft³ ÷ 27The volume in place, after it is compacted. Gravel is delivered loose and settles when it is rolled, so order more than this — ask your supplier how much more for their stone. Thickened edges and footings are not counted.