How the planner works
Concrete is area × thickness plus your extra. It's shown two ways, cubic yards for ready-mix and 80 lb bags at about 0.60 cubic feet each (Quikrete data sheet). You buy one or the other, not both.
The gravel base uses the same area × the base depth, plus the extra you set. It's shown in cubic yards and in tons at 105 lb per cubic foot (Engineering ToolBox, dry gravel).
Rebar is laid out as a grid at your spacing, 3 in in from the edges and cut from 20 ft sticks. Runs longer than one stick get a lap, meaning the bars overlap by the lap length. Weight comes from the bar's weight per foot. Wire mesh is counted in sheets, with the overlap you enter.
Form boards cover each side of the rectangle. Stakes go at your spacing, plus one at each corner. For an L-shape, enter the real perimeter.
Worked example: the default 12 × 10 ft patio
The slab is 12 × 10 = 120 sq ft. At 4 in thick that's 120 × 4 ÷ 12 = 40 cubic feet, and 10% extra makes 44 cubic feet, or 1.63 cubic yards. In bags, 44 ÷ 0.60 = 73.3, so buy 74 bags of 80 lb.
The 4 in gravel base is also 40 cubic feet, 44 with the default 10% extra, which is 4,620 lb, about 2.3 tons at 105 lb per cubic foot. If you'll compact it, set the extra to 25% the way Quikrete does (5 in loose packs to 4): 50 cubic feet, about 2.6 tons.
#4 rebar on an 18 in grid, 3 in in from the edges, needs 7 bars 11.5 ft long and 8 bars 9.5 ft long. One 20 ft stick gives one 11.5 ft bar or two 9.5 ft bars, so that's 7 + 4 = 11 sticks, about 157 ft and 105 lb of steel.
The edge is 2 × (12 + 10) = 44 ft. With 12 ft boards, that's 4 boards, one per side. A stake every 3 ft is 44 ÷ 3 = 14.7, rounded up to 15, plus 4 corners: 19 stakes.
Only need one part? Use the concrete calculator, the gravel calculator or the rebar calculator.