Calculate weight, total length, and quantity for various rebar sizes and grades.
Two way runs bars in both directions, like a grid. One way runs them one direction only.
Pick from your own standard — metric is millimetres, US is eighths of an inch.
Yield strength. It goes on the order but does not change the weight.
Measured bar centre to bar centre, not the gap between them.
Leave blank if your merchant cuts to length. Fill it in only if you buy fixed lengths and cut them yourself.
Extra to order for laps and offcuts. 5% is typical.
Nothing to work out yet
Enter values for Slab Length, Slab Width, Bar Spacing (centres).
Give it the slab and the spec'd bar spacing and it counts the mat for you: bars running the length of the slab are spaced across its width, and a two-way mat adds a second set running the other way. Both edges carry a bar, so a 4 m width at 200 mm centres takes 21 bars, not 20 — that missing end bar is the usual off-by-one in a hand take-off. Weight uses the standard mass per metre for the bar size, a #4 being 0.994 kg/m and a #5 1.552 kg/m. Waste starts at zero, so the figures are the exact take-off until you set an allowance. Your waste factor adds spare pieces for laps and miscuts, and the order is then worked back to whole stock bars: reinforcement arrives in fixed lengths, so the page tells you how many to buy and what to cut them into rather than naming a slab-length bar no merchant sells. Where a run is longer than one stock bar it is spliced, and each splice adds a lap of 40 bar diameters — ACI 318's Class B figure, never under 300 mm — to the steel you buy, because the overlap is bar that gets ordered and never seen. Tie wire is quoted at 6 kg per tonne of steel and chairs at roughly one per 0.83 m² of mat: both are small, cheap and the reason a delivery sits unusable when they are forgotten. The lap here is an estimating allowance; where a drawing specifies a lap, that figure wins. If your schedule already lists a count, switch the first dropdown and enter it directly — though a bar count alone carries no slab area, so no chair figure comes with it.
A two-way mat in a 6 by 4 metre slab, 12 mm bar at 200 mm centres, Grade 60. The calculator returns 52 bars, 250 metres of steel, 222 kg in weight, plus 1.3 kg of tie wire and 29 chairs.
The cut list is the part worth reading: 21 bars of 6 m running the length, and 31 bars of 4 m running across the width. Note that a 4 metre width at 200 mm centres gives 21 bars, not 20 — both ends carry a bar, so it is the number of gaps plus one. Over a whole mat that fencepost rule is the difference between a complete grid and one short bar at the edge in each direction.
Weight comes from the bar size alone: 12 mm steel is 0.888 kg per metre, so 250 m is 222 kg. Grade does not enter the arithmetic at all. Grade 40 and Grade 60 of the same diameter weigh exactly the same; the grade is a yield strength, and it belongs on the order rather than in the weight.
Ordering the cut list instead of the stock. Nobody sells a 4 metre bar. Reinforcement arrives in stock lengths — 12 m is the common metric one — and is cut on site or by the merchant. If you hand a supplier "31 bars of 4 m" as an order, you will either be refused or quietly charged for 31 full lengths.
That is why this page separates Bars to Buy from the Cut List. Leave Stock Length blank if your merchant cuts to length and the two are the same thing. Fill it in if you buy fixed lengths and cut them yourself, and the count changes to whole stock bars with the laps added, because a bar that has to be spliced needs an overlap and that overlap is steel you have to buy.
Size, grade, and either a number of stock lengths or a cut schedule they will produce. The 12 mm · Grade 60 line under Bars to Buy is exactly what goes on the order.
Do not forget the tie wire and the chairs. Steel that arrives with nothing to tie it and nothing to hold it off the ground cannot be fixed — the mat ends up lying on the membrane, which puts the reinforcement in the wrong half of the slab and makes it close to useless. Both are estimated here rather than left to be remembered on the day: about 6 kg of wire per tonne of steel, and roughly one chair per square metre of mat.
The working shows the bar count, the total length, and the kg-per-metre figure used for your size. Bar weights are standard and published; if yours differ, it is because you are on a different standard, and the Rebar Size dropdown is grouped by standard for exactly that reason.
Watch that grouping. A #10 US bar is 32.3 mm; a 10M Canadian bar is 11.3 mm. They are nearly three times apart in diameter and roughly eight times apart in weight, and they are one line from each other in any ungrouped list. Pick from your own standard.