A 12 x 12 ft slab 4 in thick is 48 cu ft, 1.78 cu yd. Add 10% for uneven subgrade and leftovers and it is 1.96, so the order is 2 yd of ready-mix, because plants sell by the quarter yard, or 88 bags of 80 lb mix at the 0.6 cu ft label yield.
The concrete calculator does that for a slab, a footing strip or round piers, in the browser, with nothing sent anywhere. It sits beside the drywall calculator and the whole-house takeoff, which already prices footing, stem wall, slab, rebar and forms together.
Limits are on the page: bag yields are label figures that vary by brand, the short-load threshold (about 4 yd) is a rough guide that differs by plant, and nothing checks thickness, reinforcement or frost depth. A test recomputes the 12 x 12 case and the 45-bags-per-yard figure in the FAQ from the code, and the page was clicked in Chromium at phone width.