Concrete side of the calculation
Slab volume is simply length × width × thickness, converted to dry volume using the standard × 1.54 factor and split across a fixed 1:2:4 mix, the same method used in the main Concrete calculator.
Reinforcement side of the calculation
For the steel mat, the tool works out how many bars fit across the width and along the length at your given centre-to-centre (c/c) spacing, sums the total running length of bar, then converts that to weight using the standard rebar formula:
This gives you one mesh layer — bars running both directions in a single plane. A typical two-way slab has top and bottom reinforcement mats, so for full reinforcement, double the steel figure this tool returns, or run it twice with your top and bottom mat's respective bar sizes and spacing if they differ.
Frequently asked questions
Does this replace a structural design?
No. This gives quick material quantities for a slab whose thickness, bar size, and spacing have already been decided — by a structural drawing, code minimum, or span-to-depth rule of thumb. It doesn't check deflection, punching shear, or moment capacity.
Why does it only show one layer of reinforcement?
Most slabs need top and bottom steel mats (and sometimes different spacing in each direction). Showing a single-layer result keeps the tool flexible — run it once per mat if your top and bottom differ, or double the figure if they're identical.
What cement bag size should I use?
Use whichever is standard in your market — 50kg is the common metric bag size in most countries; ~42.6kg (94lb) is the standard US bag.