Reinforcing steel is bought by the bar, but a slab needs it laid out as a grid. Counting the bars takes a little care: you need one at each edge, the spacing sets how many fit between, and bars longer than the stock length have to be spliced with an overlap. This calculator handles all of that for a rectangular slab and reports the stock bars to buy, the total length and weight, and the tie points and chairs for placing them.
How to use the rebar calculator
- Choose US or Metric units.
- Enter the slab length and width.
- Enter the bar spacing (the same in both directions) and the edge clearance.
- Pick the bar size and the stock length you can buy.
- Set the lap splice in bar diameters and the number of layers.
- Add an extra allowance and, optionally, the price per bar.
Rebar formulas
When a run is shorter than a stock bar, as many runs as fit are cut from each bar. When it is longer, each run uses full bars plus one shorter end piece, and short end pieces are nested on shared stock bars. Weight is the length placed (laps included) times the bar’s weight per foot.
Worked example: a 24 × 24 ft garage slab
#4 bars at 18 in each way, 3 in from the edges: ⌈ 282 ÷ 18 ⌉ + 1 = 17 bars each way, each 23 ft 6 in long.
Lap: 40 × ½ in = 20 in. Each run takes one 20 ft bar plus a 5 ft 2 in end piece; three end pieces come from one bar.
Per direction: 17 + 6 = 23 stock bars; both directions 46; with 5% extra → 49 bars of 20 ft.
Length placed: 34 runs × 25.17 ft = 856 ft, weighing 572 lb. Intersections to tie: 17 × 17 = 289.
Rebar sizes
| Size | Diameter | Weight | 40-diameter lap |
|---|---|---|---|
| #3 (#10) | ⅜ in (9.5 mm) | 0.376 lb/ft | 15 in |
| #4 (#13) | ½ in (12.7 mm) | 0.668 lb/ft | 20 in |
| #5 (#16) | ⅝ in (15.9 mm) | 1.043 lb/ft | 25 in |
| #6 (#19) | ¾ in (19.1 mm) | 1.502 lb/ft | 30 in |
| #8 (#25) | 1 in (25.4 mm) | 2.670 lb/ft | 40 in |
Grade 60 steel is standard for most building work. Bars are commonly stocked in 10- and 20-foot lengths at home centers and in 20-, 40- and 60-foot lengths at steel suppliers.
Placing and tying
- Chairs or dobies hold the grid at the right height; space them about every 3 to 4 feet, closer for light bars.
- Tie wire holds intersections together while concrete is placed. Light slabs are often tied at every other intersection; tie every one where the grid will be walked on.
- Stagger laps so splices in neighboring bars do not line up across the slab.
- Keep the cover. Bars too close to the surface or the soil rust and spall the concrete.
Many residential slabs on ground are poured with welded wire reinforcement, fibers or no steel at all, and footings and walls have their own bar patterns. The quantities here are for estimating only; bar size, spacing, laps and cover in structural work come from the design and the building code.
The concrete calculator gives the cubic yards for the same slab, and the concrete block calculator counts vertical bars and grout for block walls.
Frequently asked questions
How much rebar do I need for a 10 × 10 slab?
With bars 12 inches apart each way and 3 inches of edge clearance, the slab takes 11 bars in each direction, each 9 ft 6 in long — 22 bars and about 209 feet in all. Cut two per 20-foot stock bar, that is 12 stock bars, or 13 with 5% extra.
How much does rebar weigh per foot?
Under ASTM A615 the nominal weights are 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5, 1.502 lb/ft for #6, 2.044 lb/ft for #7 and 2.670 lb/ft for #8. A 20-foot #4 bar weighs about 13.4 lb.
How long should rebar laps be?
A common rule of thumb for slabs on ground is 40 bar diameters: 15 inches for #3, 20 inches for #4 and 25 inches for #5. Structural work follows the lap lengths in the design, which under ACI 318 are often longer, especially for larger bars and higher-strength steel.
What does #4 rebar mean?
US rebar sizes are numbered in eighths of an inch of nominal diameter, so #4 is 4/8 = ½ inch and #5 is ⅝ inch. In soft-metric labeling the same bars are #13 and #16, their diameters in millimeters.
Where should rebar sit in a slab?
Supported on chairs or concrete blocks so it ends up inside the concrete, commonly near mid-depth or in the upper half of a slab on ground, with about 3 inches of cover where concrete is cast against soil. Bars lying on the ground do almost nothing.