Rebar Calculator

What this calculator does

Slab dimensions, bar spacing, bar size, edge cover and stock length go in. Out comes the bar count in each direction, the net linear feet of steel, the lap length, the extra steel that lap splices consume, total linear feet, how many pieces of stock that is, total weight, the number of grid intersections to tie and the tie wire that takes, plus a cost at your price per pound.

The lap steel is the part most quick estimates miss, and on a large slab it is five to ten percent of the order.

Counting the mat

A reinforcing mat is two sets of parallel bars at right angles. Each set is counted from the dimension it spreads across, not the dimension it runs along.

Take a 40 by 24 foot slab with 18-inch spacing and 3 inches of edge cover. Cover pulls the steel in from every edge, so the bars actually run 39.5 feet and 23.5 feet.

• Bars running lengthwise are spread across the 23.5-foot width: 23.5 × 12 ÷ 18 = 15.7, floor to 15, plus one is 16 bars, each 39.5 feet.
• Bars running widthwise spread across the 39.5-foot length: 39.5 × 12 ÷ 18 = 26.3, floor to 26, plus one is 27 bars, each 23.5 feet.

That is 632 + 634.5 = 1,266.5 linear feet before splices.

The plus-one is the same fencepost rule that governs stud layout and floor joists. It appears everywhere in construction and it is wrong by exactly one every time it is missed.

Cover: why the steel stops short of the edge

Concrete protects steel from corrosion, and it only does so where there is concrete around the steel. Bar too close to a face rusts, rust expands to several times the volume of the metal it came from, and the expansion spalls the concrete off — which exposes more steel, which rusts faster. It is a self-accelerating failure and it is the reason concrete structures have a service life at all.

Required cover varies by exposure: more for concrete cast against earth, less for interior surfaces, more still in marine environments. Three inches is a common figure for a slab cast on grade. Take yours from the drawing or the code, and hold it at every edge, which is exactly what this calculator does before it counts anything.

Laps, and the steel they eat

Rebar comes in stock lengths — 20 feet is the common yard length, 40 and 60 are available. Any run longer than the stock needs a splice, and a splice is not a butt joint.

Two bars laid end to end transfer no force between them. The bars must overlap far enough that the bond between steel and concrete can develop the full strength of the bar across the splice. The rule of thumb is 40 bar diameters: 15 inches for a #3, 20 for a #4, 25 for a #5, 30 for a #6.

Now the cost. On our example slab with 20-foot stock, the 39.5-foot lengthwise bars need one splice each — 16 bars × 20 inches = 26.7 extra feet. The 23.5-foot widthwise bars also need one splice each — 27 × 20 inches = 45 feet. That is nearly 72 linear feet of steel that exists only to be overlapped, about 6 percent of the order, and it is invisible if you estimate by dividing the slab area by the spacing.

The real required lap depends on concrete strength, whether the bar is epoxy coated, bar spacing, and whether the splice is in tension or compression. A structural drawing will state it. Forty diameters is a serviceable default when nothing else is specified, and it is conservative for most compression splices.

Weight, because that is how it is sold

Rebar is priced by weight almost everywhere, even when it is delivered in counted lengths. The nominal ASTM weights are fixed and worth knowing:

Bar	Diameter	Weight
#3	3/8 in	0.376 lb/ft
#4	1/2 in	0.668 lb/ft
#5	5/8 in	1.043 lb/ft
#6	3/4 in	1.502 lb/ft

Our 1,338 total linear feet of #4 comes to about 894 pounds — a useful sanity check when a quote arrives, and a useful thing to know before you decide to carry it all yourself.

Placement beats quantity

The most important thing about rebar is not how much of it you buy.

Reinforcement only works where the concrete is in tension. For a slab on grade that generally means the middle third to lower third of the thickness — get it wrong and the steel sits in the compression zone, where concrete was already strong and the bar contributes essentially nothing.

Which means: set the bar on chairs before the pour. The alternative — the hook-and-lift, where someone drags the mat upward through wet concrete as the pour proceeds — puts steel at an unknown, inconsistent, and usually insufficient depth. It is fast, it is common, and it substantially wastes the money you just spent on steel.

What this leaves out

• Design. Bar size, spacing, cover and lap length come from an engineer for anything structural. This counts what the drawing calls for.
• Chairs, dobies and supports. Not counted, and not optional.
• Bends, hooks, corner bars and dowels. Fabricated shapes are quoted per piece by the fabricator.
• Welded wire mesh. A different product with different rules, common in light slabs.
• Fibre reinforcement. Sometimes a partial substitute for temperature steel; never a substitute for structural steel.

The concrete calculator covers the volume this steel goes into, and the footing calculator handles the strip and pier pours that usually come first.