Slab on Grade Takeoff Calculator: Concrete, Reinforcement and Base
Field note: this tool runs entirely in your browser. Load the page before you head out and it keeps working with no signal, and Print or save PDF gives you a clean copy for the truck.
Slab on Grade Takeoff Calculator: Concrete, Reinforcement and Base
One set of dimensions to the full slab ticket: concrete with waste, rebar grid or mesh, granular base by the ton, vapour barrier, forms and saw cut joints at a spacing set by thickness.
Base tonnage uses 1.5 tons per loose yd^3 of granular; adjust for your pit material. Bar weight is computed from bar diameter and steel density, not looked up. Joint spacing follows the common rule of 2 to 3 ft per inch of slab thickness; your mix and finisher may want it tighter.
The whole slab, not just the concrete
Everyone can multiply length by width by thickness. What makes a slab on grade takeoff useful is everything else on the ticket: the granular base by the ton, the vapour barrier with lap, the reinforcing, the form lumber and the saw cutting. This tool prices out the full assembly from one set of dimensions so nothing gets remembered at the last minute.
A worked example you can check by hand
A 40 by 30 ft slab at 5 in thick holds 500 ft^3 of concrete, which is 18.52 yd^3, and 19.44 yd^3 once five percent waste goes on. Six inches of granular under it is 22.2 yd^3, call it 33 tons at 1.5 tons per loose yard. With number 4 bars at 18 in each way you get 21 bars running the long way and 27 running the short way, 1650 lf in total. A number 4 bar weighs 0.668 lb per ft, computed straight from its half inch diameter and the density of steel, so the grid weighs about 1102 lb.
Joints follow thickness. The common rule puts contraction joints at 2 to 3 ft per inch of slab, so a 5 in slab wants panels around 12.5 ft. On this slab that is two cuts one way, three the other, about 180 lf of saw cutting. The tool defaults to 2.5 and lets you tighten it.
Defaults worth arguing with
Granular density varies by material and moisture; 1.5 tons per yd^3 is a fair planning number for compacted gravel but your pit ticket rules. Mesh sheet coverage depends on sheet size and lap, so the effective area field is editable. And the reinforcing itself, bar size and spacing, comes from the design or the standard detail you are building to, not from this page. The takeoff counts what the design asks for.