How the bar list is worked out
A slab grid is two sets of bars crossing at the spacing you set. This estimate subtracts the
entered cover from both ends and places bars at full spacing intervals, starting at one edge.
If the span does not divide evenly, the final bar stops short of the opposite cover line.
Check any additional edge bars or other detailing against the drawings.
bars = floor(clear span ÷ spacing) + 1, each way
Any run longer than a stick has to be spliced, and a splice costs you a lap — forty bar diameters
of steel that does nothing but overlap. The footage on the ticket already carries those laps.
The stick count uses a simple cutting allowance for each direction of each pad. It does not
reuse offcuts between directions or pads, or combine the short end pieces of spliced runs.
An optimized cutting plan can reduce the purchase. The printed order estimate is not a cut list.
Sizes, weights and what they are for
| Bar | Diameter | Weight | Where it goes |
|---|
| #3 | 3/8 in | 0.376 lb/ft | paths, small pads, steps |
| #4 | 1/2 in | 0.668 lb/ft | most residential slabs and footings |
| #5 | 5/8 in | 1.043 lb/ft | driveways with truck traffic, walls |
| #6 | 3/4 in | 1.502 lb/ft | structural work, engineered footings |
Weight matters for two reasons: the yard may price by the pound, and someone has to carry it.
A twenty-foot stick of #5 is twenty-one pounds. Transport weight includes every whole stick
purchased; installed weight includes the grid and laps after cutting. A price per foot is applied
to the purchased footage, including offcuts.
Cover, and why it is not optional
Steel too close to the surface rusts, and rusting steel swells and spalls the concrete off the
top of your slab. Three inches from earth, two from a formed face, and never let a bar sit on the
subgrade — chairs or dobies, not a shovel of stone.
Questions
What spacing should I use?
Eighteen inches on centre suits a domestic
slab in #4. Twelve to sixteen where vehicles park, and follow the drawings on anything engineered.
How long should the laps be?
Forty bar diameters is the common rule —
twenty inches for #4, twenty-five for #5. Some specs allow thirty. It is on the drawings.
Do I need rebar or is mesh enough?
Mesh controls cracking in a thin,
lightly loaded slab. Anything carrying a vehicle, or built on ground that moves, wants bar.
How much tie wire?
About a foot per intersection with the usual
16-gauge annealed wire. The ticket counts the intersections for you.
Other calculators
The same ticket, the same assumptions on screen, for whatever else the job needs.
Concrete slabs, footings, columns and stepsGravel driveways, bases and drainage stoneRiver rock beds, borders and dry creeks, by stone sizeMulch beds and trees, by the yard or the bagAsphalt hot mix by the tonTopsoil beds, lawns and levellingFill dirt holes, grades and backfill, with settlingCompost lawn top-dressing and bed amendingSand paver bedding, play sand and fillCubic yards any volume, in every unit