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 32 chairs on a 4 ft support grid, and 437 intersections eat about 3.3 lb of tie wire. Order weight is 842 lb on 30 ft stock, not the 580.5 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. Development length starts from a single engineering conflict: a bar needs concrete around it to bond, so cover must be at least what the exposure demands — but that same cover is what gives the bar its confinement, so more cover shortens the development length. Cover is set by ACI 318-19 Table 20.6.1.3.1, not by preference:
| 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.
The spacing enters squared, which is why a tighter grid costs so much more: drop the support grid from 4 ft to 3 ft and the same 480 sq ft slab goes from 32 chairs to 56. Buy chairs by height, not just count — a 1 in chair and a 3 in chair are different products.
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 437 ties; tie every other one and it takes 219, and the roll lasts twice as long.
| Item | Working | Result |
|---|---|---|
| Bars across | floor(286.5 ÷ 12) + 1 | 23 |
| Bars along | floor(238.5 ÷ 12) + 1 | 19 |
| Net length | (23 × 24) + (19 × 20) − covers | 869 ft |
| Net weight | 869 × 0.668 | 580.5 lb |
| Chair height | 3/4 in cover + 0.25 in (half of #4) | 1 in |
| Chairs | 480 ÷ 16 × 1.05 | 32 |
| Intersections | 23 × 19 | 437 |
| Tie wire | 437 × 8 in × 1.10 ÷ 12 × 0.0104 | 3.3 lb |
| Order weight, 20 ft stock | 84 bars × 20 ft × 0.668 | 1,122 lb |
| Order weight, 30 ft stock | 42 bars × 30 ft × 0.668 | 842 lb |
Swipe the table sideways for more columns →
The last two rows are the point every first-half calculator misses. The same 580.5 lb of steel is bought as 1,122 lb or 842 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 is doing two jobs at once, and they pull in opposite directions for the material takeoff. Push the bar in — bigger chairs, more cover — and the bar gets shorter because the usable span shrinks, so you save a little steel. But the same cover enters the confinement term (cb + Ktr)/db in the development length formula:
More cover raises (cb + Ktr)/db, up to the ACI cap of 2.5, so the required embedment falls. For a #8 bar at 12 in spacing, 1.0 in cover needs 26 in of embedment and 2.0 in cover needs only 16 in — the same bar, the same concrete, a 38 per cent difference caused entirely by the chair. Or turn it the other way: going from 3/4 in to 3 in cover on our 24 ft × 20 ft slab removes 27 ft of bar and 18 lb of steel from the grid, then adds embedment back into the lap. The net steel is close to a wash; the chair and lap detail is not.
The development length page works the full six-step calculation for a #8 bar 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 23 bars one way and 19 the other, 869 ft and 580.5 lb.
None of its own — cover is a code value, not a choice. ACI 318-19 Table 20.6.1.3.1 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, and in both directions at once. More cover shortens each bar and shifts the bar count — going from 3/4 in to 3 in on our 24 ft by 20 ft slab removes 27 ft of bar and about 18 lb of steel. But more cover also shortens the required development length, so the lap splices inside that bar get shorter. The grid and the lap pull opposite ways; the accessory count does not change at all.
Divide the slab area by the support spacing squared and add a small waste allowance. A 480 sq ft slab on a 4 ft grid is 480 ÷ 16 = 30 chairs, or 32 with 5 per cent waste. Tighten the grid to 3 ft and the same slab needs 56, because the spacing enters the formula squared. 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 437-intersection mat needs 3.3 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 — 580.5 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 84 bars, or 1,122 lb; on 30 ft stock it takes 42 bars, or 842 lb. Same slab, two order weights, and the heavier one leaves you with more cut ends to handle.