Bar counts, weight, and the half of the takeoff the other calculators leave out: cover, chairs and tie wire
← Back to the calculatorThen the part that gets forgotten: that 3/4 in cover demands a 1 in chair, the slab needs 42 chairs on a 4 ft support grid, and 480 intersections eat about 3.66 lb of tie wire. Order weight is 882 lb on 30 ft stock, not the 648.3 lb net — the difference is the offcut.
Open the calculator for your own dimensions, or read the three steps below.
Most tools under this word do the same first half: enter a slab, get bars and weight. That half is geometry, and it is solved. The second half is the accessories — and it is where the money and the callbacks are. Get the cover wrong and the inspection fails; forget the chairs and the top mat sinks into the slab during the pour; skip the tie wire and the crew stops work to go get a roll.
This page does the whole takeoff in three steps and shows every number, so you can check it against your own drawings.
Work in clear distances, not overall dimensions. The first bar sits one cover in from the edge, so the usable span in each direction is the slab dimension less two covers. Convert to inches, and:
For a 24 ft × 20 ft slab with #4 at 12 in on centre and 3/4 in cover:
Here is the step no simple calculator shows. A bar needs concrete around it to bond, so cover has to be at least what the exposure demands, and that number is not a preference: it is ACI 318-19. Get it wrong in either direction and the slab is wrong — thin cover lets the bar corrode, thick cover costs chair height and eats the usable depth.
| Condition | Bar size | Min. cover |
|---|---|---|
| Cast against & permanently in contact with earth | All | 3 in |
| Exposed to weather or soil | #6 to #18 | 2 in |
| Exposed to weather or soil | #5 and smaller | 1 1/2 in |
| Interior, beams & columns | All | 1 1/2 in |
| Interior, slabs & walls | #11 and smaller | 3/4 in |
Swipe the table sideways for more columns →
The reinforcement sits inside the cover, on chairs. On a slab on grade the bottom mat is held up by chairs of a given height, and the chair height is what the drawings specify: the clear cover plus half a bar diameter. Buy the wrong chair and the cover is wrong, and the inspector sees it immediately.
| Condition | Cover | Chair for #4 | Chair for #5 | Chair for #6 | Chair for #8 |
|---|---|---|---|---|---|
| Earth contact (footing) | 3 in | 3 1/4 in | 3 5/16 in | 3 3/8 in | 3 1/2 in |
| Weather / soil, #6+ | 2 in | 2 1/4 in | 2 5/16 in | 2 3/8 in | 2 1/2 in |
| Weather / soil, #5− | 1 1/2 in | 1 3/4 in | 1 13/16 in | 1 7/8 in | 2 in |
| Interior beam / column | 1 1/2 in | 1 3/4 in | 1 13/16 in | 1 7/8 in | 2 in |
| Interior slab / wall | 3/4 in | 1 in | 1 1/16 in | 1 1/8 in | 1 1/4 in |
Swipe the table sideways for more columns →
Once you know the intersections and the slab area, the accessories are arithmetic.
Count grid points, not area. The shortcut of one chair per 4 sq ft gives 480 ÷ 4 = 120 chairs on this slab, nearly three times the 42 you will place, because one chair supports a patch of mat rather than a single point. Tighten the support grid from 4 ft to 3 ft and the count goes 42 → 63, so the support spacing matters far more than any area rule. Buy chairs by height first — a 1 in chair and a 3 in chair are different products, and the height is what sets the cover. The rebar chair page works the height, the tier gap and this count in full.
The wire per tie depends on the knot, not the bar: a snap tie eats 4 to 6 in, a wrap and twist 6 to 9 in, a saddle tie 8 to 10 in, and a figure eight on heavy bars 10 to 14 in. Tie pattern matters more than anything else — tie every crossing and a 480 sq ft mat takes 480 ties; tie every other one and it takes 240, and the roll lasts twice as long.
| Item | Working | Result |
|---|---|---|
| Bars across | floor(286.5 ÷ 12) + 1 | 24 |
| Bars along | floor(238.5 ÷ 12) + 1 | 20 |
| Net length | (24 × 23.875) + (20 × 19.875) | 970.5 ft |
| Net weight | 970.5 × 0.668 | 648.3 lb |
| Chair height | 3/4 in cover + 0.25 in (half of #4) | 1 in |
| Chairs | (floor(24 ÷ 4) + 1) × (floor(20 ÷ 4) + 1) | 42 |
| Intersections | 24 × 20 | 480 |
| Tie wire | 480 × 8 in × 1.10 ÷ 12 × 0.0104 | 3.66 lb |
| Order weight, 20 ft stock | 68 bars × 20 ft × 0.668 | 908.5 lb |
| Order weight, 30 ft stock | 44 bars × 30 ft × 0.668 | 882 lb |
Swipe the table sideways for more columns →
The last two rows are the point every first-half calculator misses. The same 648.3 lb of steel is bought as 908.5 lb or 882 lb depending on which stock bar you order, because the offcut is a remainder, not a percentage. The estimator page works that arithmetic in full, and the slab page carries it through the lap splices.
Cover does two jobs at once, and for the takeoff they pull in opposite directions. Push the bar in — bigger chairs, more cover — and the usable span shrinks, so the grid buys slightly less bar. Push it out and the grid grows. Development length does not enter that trade on this site, because the expression behind every number here has no cover term:
fy, f'c, the bar diameter, the divisor C and three factors. Cover is not one of the factors, but it decides which C applies: the 19 in for a #4 and the 48 in for a #8 hold at 3/4 in of cover and at 3 in alike, and drop to 29 and 72 in the moment the cover falls below one bar diameter. That is the honest limit of any table: it answers one row exactly and leaves the rest to the clause. Above that limit, what cover moves is the grid. On our 24 ft × 20 ft slab, going from 3/4 in to 3 in of cover at the same spacing removes 16.5 ft of bar and 11 lb of steel — about 1.7 per cent of the 648.3 lb total, which is smaller than the difference between ordering 20 ft and 40 ft stock.
The development length page works the calculation for a #8 bar symbol by symbol and gives the tables for #3 to #11.
Work the grid in clear distances: subtract the clear cover from both ends of each direction to get the usable span, divide by the centre-to-centre spacing, round down, and add one bar line. Multiply each direction's bar count by the bar length in that direction, and the total is the net length. For a 24 ft by 20 ft slab with #4 at 12 in o.c. and 3/4 in cover, that is 24 bars one way and 20 the other, 970.5 ft and 648.3 lb.
None of its own — cover is a code value, not a choice. ACI 318-19 sets it: 3/4 in for interior slabs and walls with #11 bars and smaller, 1.5 in for interior beams and columns, 1.5 in for weather-exposed work with #5 bars and smaller, 2 in for weather-exposed work with #6 to #18 bars, and 3 in for concrete cast against and permanently in contact with earth. Use whichever line matches your exposure condition.
Yes, but only through the grid. More cover shortens each bar: going from 3/4 in to 3 in on our 24 ft by 20 ft slab removes 16.5 ft of bar and about 11 lb of steel, 1.7 per cent of the 648.3 lb total. Development length does not move with cover — the expression behind it has no cover term — so the lap splices inside that bar stay the length they were. The chair count does not change at all; only the chair height does.
Count grid points, not area. A 480 sq ft slab on a 4 ft support grid is (24 ÷ 4) + 1 = 7 across and (20 ÷ 4) + 1 = 6 along, so 42 chairs; tighten the grid to 3 ft and it becomes 9 × 7 = 63. The per-area rule of one chair per 4 sq ft gives 120 on the same slab, nearly three times too many, because it treats a chair as supporting a point rather than a patch. Chairs come in fixed heights, so match the height to the cover table first and the count second.
4 to 6 in for a snap tie, 6 to 9 in for a wrap and twist, 8 to 10 in for a saddle tie, and 10 to 14 in for a figure eight on heavy bars. At 8 in per tie with 10 per cent waste, each intersection uses about 0.0076 lb of 16 gauge black annealed wire — so a 480-intersection mat needs 3.66 lb, and a 3.5 lb roll covers it with a little to spare.
No, and on a crack-control mat on grade you normally should not. Tie every crossing around the perimeter and at the lap splices, then about half the interior crossings in a staggered pattern. Structural reinforcement, walls, columns and seismic work are tied fully. Tie wire holds the bars in position during the pour; it adds nothing to the finished slab's strength.
Net weight is the steel in the finished grid — 648.3 lb on our slab. Order weight is what you buy, because bars come in fixed lengths and the offcut is a remainder, not a percentage. On 20 ft stock the job takes 68 bars, or 908.5 lb; on 30 ft stock it takes 44 bars, or 882 lb. Same slab, two order weights, and the heavier one leaves you with more cut ends to handle.
Budget 7 to 12 kg of 16 gauge black annealed wire per tonne of steel, taking the top of that range for small bars and dense mats and the bottom for heavy bars and light tying. Our 480 sq ft slab is the low end of that: 3.66 lb of wire against 648.3 lb of steel is 5.6 kg per tonne, because only the perimeter, the laps and about half the interior crossings are tied. Carry the rate to a bigger job and a 20 tonne slab lands between 310 and 530 lb of wire, which at 0.0104 lb/ft is 30,000 to 51,000 ft, or 85 to 145 of the 3.5 lb rolls. The wire never scales with the steel alone — it scales with the intersections, which is why the rate has a range and not a number.