Rebar Calculator

Two divisions, then the bars you can buy

A mat is counted twice: across the width for the bars running the length, along the length for those running across. Each count is the clear span over your spacing, rounded up, plus one for the bar at the far end. On the default 12 by 12 ft slab, 144 in over 12 in gives 13 bars one way and 13 the other, 312 ft of steel. Lineal feet are not an order, though: the pieces are packed into the stock length, 26 bars at 20 ft, and a 10 by 6 ft slab needs only 8. For the concrete over the same rectangle, use the concrete calculator.

The spacing is your figure

Both spacings are editable and independent, one along the length and one across the width. They open at 12 in as an example value, not a recommendation: no source we opened publishes a spacing or a bar size for a slab thickness, so none is printed. Bars are counted as the clear span divided by your spacing, rounded up, plus one, and then re-spaced evenly over that span: no gap ends up wider than the spacing you typed, and the actual spacing is printed back to you.

A lap belongs to your placing drawings

The lap field starts blank. CRSI says a lap length varies with the concrete strength and type, the bar grade, size and spacing, the cover and the amount of ties or stirrups, and is always shown on placing drawings — so there is no figure to default to. A bar longer than the stock length is spliced: pieces = (bar − lap) / (stock − lap), rounded up, and every lap adds its own length to the steel. On a 40 by 30 ft slab the 40 ft bars beat a 20 ft stick: blank, they are counted end to end, 2,470 ft and 124 bars, with a caution. Type the 24 in your drawings show and (480 − 24) / (240 − 24) is 2.11: 3 pieces per bar, 103 splices, 2,676 ft, 144 bars — and your lap over the nominal bar diameter, 48 bar diameters at #4.

Stock bars, offcuts back to work

Nucor rolls rebar in 20, 40 and 60 ft mill lengths. That is one producer, and what your yard carries is a different question - ask before you order the 60 ft. Pieces are packed longest-first into whichever you pick: the 40 by 30 ft slab that takes 124 bars of 20 ft takes 72 of 40 ft with 0 splices. Waste starts at zero, no published rebar waste figure having been found; at 10 % the default slab goes from 26 bars to 29.

Cover at the edge, quoted and not applied

Edge clearance is blank and counts as zero, so the bars run to the concrete edge and the plate says so. TxDOT Item 440 asks for a minimum 1-in. clear cover of concrete to the nearest surface of bar unless otherwise shown on the plans: one state DOT's highway specification, quoted, never applied here as a residential minimum. Yours comes from your plans, and it shortens every bar at both ends — 3 in takes the default slab from 312 to 299 ft.

A footing, a wall and a slab on one form

Switch the mode and the length becomes the run, the width the footing width, the longitudinal bars are typed rather than spaced, and a transverse spacing is optional. A 40 ft footing 20 in wide with 3 #5 bars, a 30 in lap, transverse bars at 24 in and 3 in of cover is 21 transverse bars, 158 ft, 6 splices, 8 stock bars, 164.8 lb. A wall mat needs no third mode: type the wall height where the slab wants its width.

Intersections, supports, and the weight

Intersections are counted as geometry. How many of them are tied, with what wire and how much wire a tie takes is not counted: no open source was found for a tie-wire gauge or a length per tie. The crossings are on the takeoff: 169 here, 338 with 2 mats of #3 bar. Type a support spacing and rows are counted both ways; leave it blank and no chair is counted. TxDOT Item 440 places individual bar supports in rows at 4-ft. maximum spacing in each direction on its highway work — quoted, not applied, and no chair height is printed because none was found published. A 24 by 24 ft slab at 48 in each way is 49. Weight comes from a published table: 0.668 lb per foot for a #4 at 0.5 in nominal diameter, TxDOT Item 440, Table 1. That item calls for ASTM A615 bars, but the ASTM text is paid and unopened, so nothing here is cited as ASTM.

What this sheet does that the six pages we could read do not

Of the six pages we could read, none carries a lap length through to the total steel and to the bars ordered — one has a lapping factor of 40 or 60, another a waste or overlap percentage. None packs offcuts into a 20, 40 or 60 ft mill length; the forms we could read take a free bar length. None puts a slab grid and a continuous footing in the same form, none counts intersections and bar supports in the same takeoff, none exports a CSV, and the weight here is worked from a named public table, linked. Independent spacing each way, a bar size select, a scaled drawing, edge clearance, metric units, weight and cost exist on those pages: not claimed here as new.

Bars    n  =  ceil( ( span - 2c ) / spacing ) + 1,  each way
Splice  k  =  ceil( ( bar - lap ) / ( S - lap ) ),  steel = bar + ( k - 1 ) x lap
Order      =  pack the pieces into S, longest first, x ( 1 + waste )
Weight     =  steel x lb per foot,  supports = rows both ways at sp

Frequently asked

How do I calculate how much rebar I need?

Clear span over your spacing, rounded up, plus one, each way, then the pieces packed into stock bars. At 12 in each way, bars to the edge: 13 and 13 bars, 312 ft, 26 bars at 20 ft.

How much rebar for a 12x12 slab?

At 12 in each way, bars to the edge, #4 bar, 20 ft stock: 13 and 13 bars of 12 ft, 26 stock bars, 208.4 lb.

How much rebar for a 10x10 slab?

Same settings: 11 plus 11 bars, 220 ft, 11 stock bars, 147 lb — two 10 ft bars out of one 20 ft stick.

How much rebar for a 30x40 slab?

Same settings: 31 of 40 ft, 41 of 30 ft, 124 bars, 72 splices counted with no lap. With a 24 in lap from your drawings, 144 bars.

How to calculate rebar spacing?

Round the count up, then divide the span again by the gaps you ended up with. A 12.5 ft run at 12 in is 150 in over 12, 12.5, so 14 bars and 13 gaps of 11.54 in.

What size rebar do I need for a 4 inch slab?

From your plans or your local code: no source we opened publishes a bar size for a slab thickness. What it changes here is the weight — the default slab is 208.4 lb in #4, 117.3 in #3, 325.4 in #5, with the same 26 bars.

Limits of this sheet

S-102 counts steel on a rectangle. It designs nothing and is not a bar bending schedule: hooks, bends, stirrups, dowels, edge bands, mesh, tie wire and chair heights are out of scope. Your plans and your local code govern. For the concrete around that steel, the footing takeoff is S-103 footing.

Sheet S-102 prepared by TakeoffMetric Editorial; no author, credential or field measurement claimed.