Rebar calculator

Ten spaces need eleven bars

The counting error that costs a bar at every edge. To cover a clear span at a given spacing you need ceil(span ÷ spacing) spaces, and one more bar than there are spaces:

bars = ceil(clear span ÷ spacing) + 1

A 10 ft wide slab with 3 in cover has a 9.5 ft clear width. At 12 in on centre that is 10 spaces — and 11 bars, not 10. Drop the +1 and the last bar lands a foot inside the edge of the mat, which is where the slab is weakest. The same fencepost applies in the other direction, so on a 20 × 10 ft slab you are short two bars, one each way.

Note that cover comes off both ends of both directions. The mat in a 20 × 10 ft pour is 19.5 × 9.5 ft, not 20 × 10.

A 20 ft stick makes two 9.5 ft bars

This is the expensive direction to get wrong. On that 20 × 10 ft slab you need 21 bars 9.5 ft long. Charged one stick per bar, that is 21 sticks. Cut two out of every 20 ft stick and it is 11. The calculator above does the second one.

The reverse case is worse. A bar longer than a stick has to be spliced, and the overlap is steel you buy and place but that adds no length. IRC Table R608.5.4(1) gives the Grade 60 tension lap as 30 in for #4, 38 in for #5 and 45 in for #6. So each 20 ft stick after the first only extends the bar by 20 − 2.5 = 17.5 ft. Two sticks reach 37.5 ft, so a 40 ft bar is three sticks, not two.

Two honest notes about those lap numbers. First, the laps above are the Grade 60 column, and its lap is 60 bar diameters. The "40 bar diameters" rule of thumb is the Grade 40 column of that same table — 20 / 25 / 30 in for #4 / #5 / #6, against the 30 / 38 / 45 in used here — so a splice laid out from it on Grade 60 bar is a third short. Second, IRC Table R608.5.4(1) has no #3 row. The 22 in used here for #3 comes from the ATC/SEAOC Class B lap splice job aid (Grade 60, uncoated, normal weight, not a top bar, f'c 3000 psi), which agrees with the IRC values for #4 to #6 to within an inch or two. If your drawings specify a lap, use theirs, not ours.

Cover is the number inspectors actually check

Steel that is not at its specified depth may as well not be there, and rust follows cover, not intent. ACI 318-19 Table 20.5.1.3.1 requires 3 in where concrete is cast against and permanently in contact with ground, and for concrete exposed to weather or in contact with ground, 1-1/2 in for #5 and smaller and 2 in for #6 and up.

Cover is measured to the outside of the bar, not its centre. And it has to survive the pour: a mat resting on the subgrade has zero cover underneath, so the bars sit on chairs or dobies. The support count above comes from the spacing you enter — a #3 sags between supports much sooner than a #5, so a heavier bar can be supported further apart.

While you are placing them, tie the intersections. Bars that shift while the truck discharges give you a mat with the right quantity of steel in the wrong place.

Order by the piece, price by the pound

Both numbers are above deliberately. ASTM A615 nominal weights are 0.376, 0.668, 1.043 and 1.502 lb/ft for #3, #4, #5 and #6, so a 20 ft #4 stick weighs 13.4 lb. The yard sells you whole sticks but quotes by the pound or the hundredweight, and the weight you are invoiced for includes every offcut. On the default slab, steel in place is around 277 lb while the invoice covers about 321 lb. Budget from the second one, plan the placement from the first, and never put one number into the other's sentence.

Where this page stops

Two conditions return nothing, and both are real answers rather than failures. Twice the cover meeting the shorter dimension leaves no clear span to space bars across — a 10 in wide grade beam at 3 in cover has 4 in of room, and a mat is the wrong detail for it; that is a job for a single bar or a stirrup cage. And a spacing wider than the clear span leaves the two edge bars and nothing between, which the count will tell you but which usually means the spacing came from the wrong drawing.

What this page will not tell you

Whether this reinforcement is enough. Bar size, spacing and where the steel sits are structural design decisions that depend on the loads, the span, the soil and the concrete strength. They belong to the engineer of record, the structural drawings and your local building code. This calculator takes the spacing you have been given and counts the steel — deciding the spacing is not its job, and it is not the job of any calculator.

The pour this counts, and the ones it does not

What is being counted is one flat rectangular mat of straight bars, in a single layer. Read down this list before you take the stick count to the yard:

• One layer. A slab with top and bottom mats is two runs of this page added together, and the two usually have different cover. Nothing here notices that you only entered one.
• No bends, no hooks, no corner bars. Cut lengths are straight, cover face to cover face. Standard hooks at the ends, L-bars turning a corner, dowels bent into a wall and stirrups or ties in a beam or column all add steel that does not appear in any of these figures — and stirrups in particular can outweigh the longitudinal bars.
• Rectangles only. An L-shaped slab, a pour with an opening in it, or a circular pad has to be split into rectangles and run separately. The bar counts will not be the sum of the pieces, because each rectangle gets its own edge bars.
• The lap is one condition out of many. The figures used are tension laps for Grade 60, uncoated bar in normal-weight concrete, not a top bar, at a stated concrete strength. Epoxy-coated bar, top bars, lightweight concrete or a different f'c all lengthen the splice. If your drawings give a lap, theirs wins.
• Laps are counted, not located. Drawings routinely require splices to be staggered between adjacent bars and kept out of high-stress regions. Obeying that changes the cut plan and can change the stick count; this page splices wherever the arithmetic runs out of stick.
• The cut plan assumes you cut to it. Sticks come from a first-fit cut plan that shares offcuts between the two directions. That is what a careful person with a full cut list achieves. Cutting as you go, or ordering bar pre-cut to length, uses more — and the waste percentage is the only place that shows up.

Bar sizes here stop at #6. Anything heavier is engineered work with a bar schedule, and the schedule is the quantity.