Why the bar count is one more than the divisions
Spacing is measured centre to centre, so a field 138 inches wide at 18 inches on centre does not hold 138 divided by 18 bars. It holds seven full spaces with a bar at each end of every space, which is eight bars. Miss the plus one and you are a bar short in each direction, which on a small pad is two bars and on a large one is enough to stop work.
The field is also not the slab. Bar stops short of the edge by the cover distance on all four sides, so a 20 by 12 foot slab with 3 inches of cover has a steel field of 19.5 by 11.5 feet. That is why the bar lengths in the results are shorter than the slab dimensions, and it is also why the field figure is worth checking against the drawing before you start cutting.
Laps are the part that gets underbought
Rebar comes in fixed stock lengths, commonly 20 feet from a yard and 10 or 12 feet from a big box store because that is what fits in a car. Any run longer than the stock needs a splice, and a splice is not a butt joint. The two bars overlap and are tied, and the overlap is dead length that does no work at either end.
Lap length is usually specified as a multiple of the bar diameter. Forty diameters is a figure that appears very often in residential drawings as a design target: for a number 4 bar that is 40 times half an inch, or 20 inches. That means a 40 foot run in 20 foot stock is not two sticks. Two sticks lapped 20 inches cover 38 feet 4 inches, so you need three, and the third is mostly offcut. This is the single biggest source of surprise on a rebar order.
| Bar | Diameter | Weight | 40 db lap |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft | 15 in |
| #4 | 1/2 in | 0.668 lb/ft | 20 in |
| #5 | 5/8 in | 1.043 lb/ft | 25 in |
| #6 | 3/4 in | 1.502 lb/ft | 30 in |
| #7 | 7/8 in | 2.044 lb/ft | 35 in |
| #8 | 1 in | 2.670 lb/ft | 40 in |
The bar number is the diameter in eighths of an inch, which is the one piece of rebar trivia that is actually useful on site: a number 5 is five eighths across. Lap length in the table is arithmetic from that diameter, not a requirement, and the real figure on your job depends on concrete strength, bar coating, spacing and what the designer specified.
Chairs, and why the bar ends up on the ground anyway
Steel that lies on the subgrade does nothing. It has to sit at the height the design put it, which for a single mat is usually somewhere in the middle to lower third of the slab, and that means supports. Chairs on a four foot grid is a common flat-work spacing and gives roughly one support per 16 square feet, but the spacing you actually need depends on the bar stiffness. Number 3 bar sags noticeably between supports that a number 5 spans without moving.
The failure mode everybody has seen is hooking the mesh or bar up with a rake as the concrete goes in. It works when someone is genuinely doing it and stops working the moment the pour gets busy, which is the moment it matters. Chairs cost very little compared to a slab with the steel in the wrong place, and unlike the pull-up method they are still doing their job in the corner nobody was watching.
Tie wire and the honest count
The intersection count in the results is every crossing in the mat. Nobody ties every one on a slab. Alternate intersections, tied in a chequerboard, holds the grid square and is the common practice; every intersection is for cages, walls and anywhere the mat has to survive being walked on before the pour. At roughly nine inches of wire per tie including the twist, a 112 intersection grid tied at half is about 42 feet of wire, so a single roll covers a lot of slabs.
Once the mat is laid out, the concrete itself is a separate calculation. Volume for the slab comes off the concrete slab calculator, and if this steel is going into a strip footing rather than a flat slab the concrete footing calculator handles the continuous bar case where every bar runs the full perimeter.
Questions people ask
What spacing should the rebar be?
That is a design decision and this page does not make it. Spacing depends on the slab thickness, the load, the soil, the concrete strength and the reinforcement ratio the designer chose, and reasonable slabs exist at everything from 12 to 24 inches on centre with different bar sizes. What you will see in the field is that residential flat work often runs number 4 bar somewhere in the 16 to 24 inch range, but citing that as a rule for your slab would be wrong. Use the figure on your drawing, or get one from whoever is responsible for the design.
Can I use welded wire mesh instead of bar?
They are different products with different behaviour and the substitution is a design question rather than a shopping one. Mesh is quicker to place and comes in rolls or flat sheets, but it is thinner steel at a fixed spacing, it wants overlapping by a full grid square at the joints, and rolled mesh fights you the whole time it is trying to become a roll again. Flat sheet mesh is far easier to keep at the right height. If your drawing specifies bar, do not swap it for mesh because it is easier to carry.
How much does rebar weigh?
Per foot: 0.376 lb for number 3, 0.668 for number 4, 1.043 for number 5, 1.502 for number 6, 2.044 for number 7 and 2.670 for number 8. A 20 foot stick of number 4 is therefore about 13.4 pounds, which one person carries easily, and a 20 foot stick of number 8 is 53 pounds, which is a two-person lift over any distance. The calculator gives both the placed weight and the as-purchased weight, and the second is the one that matters when you are working out how many trips to the yard this is.
Does the calculator include hooks, corner bars or dowels?
No. It counts a flat rectangular mat: straight bars in two directions, spliced where the run exceeds the stock length. Hooked ends, standees, corner bars that turn the reinforcement round a corner, dowels tying a slab to a footing or a wall, and any bent shape are all extra and are usually detailed on the drawing with a bar bending schedule. If your job has them, price them separately and add them to what this page gives you.
Why is the offcut figure so large on some layouts?
Because bar is bought in whole sticks and cut to length, and the leftovers only get reused if another piece on the job happens to fit. A 11.5 foot bar cut from a 20 foot stick leaves 8.5 feet, which is useless for another 11.5 foot bar, so the calculator counts one stick per bar. When the run is short enough to get two or three pieces out of a stick the offcut collapses. If you can order 10 or 12 foot stock for a job with short bars, do, because the drop is what you are paying for.