Material calculator
Rebar calculator
Enter the slab dimensions, bar size and grid spacing to get the number of bars each way, total linear feet including laps, tie wire count, steel weight and a cost estimate.
Your project
#3 and #4 cover most residential slabs. #5 and up are for structural work.
Residential slabs are usually 18" or 24" on center; driveways and structural slabs 12"-16".
Concrete cover from the slab edge to the first bar. 3 inches is standard for slabs on grade.
Total rebar
664
linear feet of #4 (1/2") · 34 × 20 ft sticks
- Slab area
- 480 sq ft
- Bars running 23.5 ft (long way)
- 14 bars
- Bars running 19.5 ft (short way)
- 16 bars
- Grid intersections (ties)
- 224 ties
- Tie wire
- 1 × 3.5 lb roll
- Lap splice length
- 20 in per splice
- Steel weight
- 444 lb (0.22 tons)
- Estimated cost
- $498
Bar counts include the edge cover setback and lap splices at every stock-length joint. Chairs or dobies are still needed to hold the mat at mid-slab depth — steel laid on the ground does nothing.
How rebar quantities are calculated
Rebar in a slab is a grid, so it is counted twice — once for each direction. Take the slab dimension perpendicular to the bar run, subtract the edge cover from both sides, divide by the spacing, and add one for the closing bar.
A 24 ft x 20 ft slab with 3 inch cover and 18 inch spacing has a 19.5 ft clear width, giving 14 bars running the long way, and a 23.5 ft clear length, giving 16 bars running the short way. That is 14 × 23.5 + 16 × 19.5 = 641 linear feet before splices.
Splices matter on anything longer than a stock bar. Grade 60 slab steel laps at roughly 40 bar diameters, so #4 bar laps 20 inches and #5 laps 25 inches. Every joint in a run adds that much extra steel.
Choosing bar size and spacing
For slabs on grade, #3 bar at 18 inches on center is the common minimum for patios and sheds. #4 at 18 or 16 inches on center is the residential driveway and garage standard. #5 and larger belong in footings, structural slabs and anything carrying a wall load.
Spacing is set by the engineer or the local code, not by preference. Tighter spacing spreads load better but adds steel fast — going from 18 to 12 inches on center adds about 50% more linear feet.
Position is as important as quantity. Slab steel must sit in the middle third of the slab thickness, held on chairs. Rebar dragged up during the pour ends up in the wrong plane and provides almost no benefit.
Rebar size, weight and lap length
Grade 60 deformed bar. Lap shown at 40 bar diameters.
| Size | Diameter | lb / ft | lb per 20 ft stick | Lap splice |
|---|---|---|---|---|
| #3 (3/8") | 0.375" | 0.376 | 7.5 | 15" |
| #4 (1/2") | 0.5" | 0.668 | 13.4 | 20" |
| #5 (5/8") | 0.625" | 1.043 | 20.9 | 25" |
| #6 (3/4") | 0.75" | 1.502 | 30 | 30" |
| #7 (7/8") | 0.875" | 2.044 | 40.9 | 35" |
| #8 (1") | 1" | 2.670 | 53.4 | 40" |
Linear feet of rebar per 100 square feet of slab
Both directions combined, before laps and edge cover.
| Spacing | Linear feet / 100 sq ft | #4 weight |
|---|---|---|
| 12" on center | 200 | 134 lb |
| 16" on center | 150 | 100 lb |
| 18" on center | 133 | 89 lb |
| 24" on center | 100 | 67 lb |
Frequently asked questions
+How much rebar do I need for a concrete slab?
Divide each slab dimension by the grid spacing, add one bar per direction, then multiply by the run length. At 18 inches on center you need roughly 133 linear feet of rebar per 100 square feet of slab, counting both directions.
+What size rebar is used in a driveway?
#4 bar (1/2 inch) at 16 to 18 inches on center is the residential driveway standard for a 4 to 5 inch slab. #3 bar is acceptable for patios and light-duty pads; #5 is used where trucks or RVs park.
+How much does rebar weigh per foot?
#3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft, #6 is 1.502 lb/ft and #8 is 2.67 lb/ft. Multiply your total linear feet by the figure for your bar size to get shipping weight.
+How long should a rebar lap splice be?
A common rule of thumb for Grade 60 slab steel is 40 bar diameters: 15 inches for #3, 20 inches for #4 and 25 inches for #5. Structural work follows the engineer's drawings, which may require more.
+How far apart should rebar be spaced?
18 to 24 inches on center for light residential slabs, 16 to 18 inches for driveways and garages, and 12 inches or tighter for structural slabs and footings. Follow local code or the stamped drawings when one exists.
+How many rebar ties do I need?
One tie per grid intersection, which equals bars one way multiplied by bars the other way. A 3.5 lb roll of 16 gauge tie wire is good for roughly 350 ties, and pre-cut bag ties come 500 to a bag.