A slab volume, its ingredients and the bags behind them, from three dimensions.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Concrete Slab Takeoff tool.
A slab takeoff is a volume with a waste allowance and a mix, and the arithmetic is trivial until the thickness matters. Thickness is the input that moves everything: a slab at a hundred and fifty millimetres is half the concrete of one at three hundred on the same plan, and every tonne of sand and aggregate follows it. The takeoff measures the slab you describe, including thickenings under load bearing walls where you enter them, and converts the volume into cement, sand and aggregate at the mix you specify. Reinforcement is not included, because it is measured from a bar schedule rather than from a slab area, and the tool says so rather than adding a percentage of steel that nobody could check.
A slab of eighty square metres at a hundred and fifty millimetres is twelve cubic metres. Two thickenings of nine metres at three hundred wide by four hundred deep add two point one six, so fourteen point one six in total, and fifteen with waste. At a one to two to four mix that is roughly four and a third cubic metres of cement, eight point two of sand and sixteen of aggregate, converted to bags and tonnes for the market you are building in. The reinforcement for that slab is a separate schedule, and the takeoff does not attempt it.
Area multiplied by thickness, plus any thickenings and poured edges you describe, plus a waste allowance. The takeoff does that from the dimensions you enter and shows the volume before and after waste, because a checking engineer wants to see both. The volume then converts into cement, sand and aggregate at the mix you select, in the units your market sells, so the order is in bags and tonnes rather than cubic metres of concrete that nobody delivers.
Because volume is area times depth, so the depth multiplies the entire order rather than adding to it. Going from a hundred to a hundred and fifty millimetres on the same eighty square metre slab adds four cubic metres of concrete, and with it a tonne and a half of cement and several tonnes of aggregate. That is why a slab depth is a design decision with a material consequence, and why the page will not assume one for you.
Yes, where you enter them. A thickening under a load bearing wall or a poured edge at the perimeter is a run with a width and a depth, and the takeoff adds its volume to the slab rather than ignoring it. Most slab estimates that come out short are short on exactly these details, because the area was measured and the edges were not.
No. Reinforcement is measured from a bar schedule, by diameter, length and count, which is a different calculation with a different unit. A percentage of concrete volume is a rule of thumb that produces a figure nobody can check against the design. The takeoff says plainly that the steel is separate, and the reinforcement tool measures it properly.
No. It measures materials for a slab whose thickness and reinforcement have been designed. Span to depth ratios, loading, restraint and crack control all belong to a structural engineer, and a slab that is under designed does not fail politely. The takeoff's job is to turn a decided design into concrete, sand and aggregate that can be ordered and poured.