The formula
bars = length ÷ spacing + 1, in each direction
Where it comes from
A mat is counted one direction at a time. In each one, the number of bars is the opposite side divided by the spacing, plus one: if the span is a metre and the bars sit every twenty centimetres, that is six bars and not five, because one goes at each end. That "plus one" is the most repeated mistake on site and it shows up in the order. The weight comes from the mass per metre of the diameter, which is the area of the section times the density of steel: 7,850 kg per cubic metre.
How to work it out by hand
- Take the cover off both edges in each direction
- Divide by the spacing and add one: those are the bars
- Multiply each group by its length and add the metres
- Add the lap at each splice, forty diameters each
- Multiply the metres by the kilos per metre of the diameter
What is worth knowing
Cover is not a safety margin in the calculation: it is what stops the steel from rusting. A bar showing through, or sitting a centimetre from the surface, starts corroding within a few years, and rust takes up more volume than steel, so it bursts the concrete from the inside. It is the most common defect in structures and almost always comes from spacers badly placed or missing altogether. The forty-diameter lap is the general rule on site: for a 10 mm bar that is 40 cm of overlap. And a warning that bears repeating: the reinforcement of a slab carrying load is defined by the structural design of the project, not by a calculator. This page is for ordering material and checking a quotation, not for deciding how anything is reinforced.