How this rebar calculator works
A slab or footing mat is a grid of bars running both ways, and the count in each direction is the same one-line rule — the clear run divided by the spacing, plus one closing bar:
linear feet = Σ (bars × their length) → sticks = lf × (1 + waste) ÷ stock length
At the defaults — a 20 × 20 ft slab, #4 bar at 16 in on center, 3 in cover — the mat is 16 bars each way, about 624 linear feet, which is 33 twenty-foot sticks with waste, roughly 417 lb of steel and $396; the full chain is in the worked example below.
Rebar sizes and grades
Rebar is deformed steel bar sized by eighths of an inch — a “#4” is 4/8, or ½ in, across. The number sets both the strength and the weight you’ll haul:
- #3 (⅜ in) — 0.376 lb/ft: light-duty — patios, sidewalks, thin slabs and stirrups.
- #4 (½ in) — 0.668 lb/ft: the residential workhorse — slabs on grade, driveways, footings. The default here.
- #5 (⅝ in) — 1.043 lb/ft: driveways carrying trucks, garage slabs, wall footings.
- #6 and up (¾ in+): structural — grade beams, columns and heavy footings, usually engineer-specified.
- Grade 60: the standard 60,000 psi yield bar for building work; sold in 20 ft sticks, tied into mats with wire and held up on chairs.
Rebar spacing, cover, and depth in the slab
Getting the steel in the right place matters as much as buying enough of it. The working rules for a slab on grade:
- Spacing: 12–18 in on center both ways; a rule of thumb is no more than three times the slab thickness, and never over 18 in.
- Cover: 3 in of concrete over bar cast against the earth, 2 in against formed edges — the calculator’s cover input keeps bar back from the edge.
- Height in the slab: chair the mat up into the middle third of the thickness; rebar lying on the ground does nothing.
- Laps: where bars splice, overlap them about 40 bar diameters (20 in for #4) — the waste % covers the extra length.
- Ties: wire-tie intersections every one or two crossings so the mat holds its grid during the pour.
Rebar weight per foot, and steel by slab size
Table 1 is the unit weight of each bar size; Table 2 runs the #4-at-16-in default out to common slab sizes — bars each way, twenty-foot sticks, and pounds of steel.
| Bar | Diameter | lb / ft |
|---|---|---|
| #3 | ⅜″ | 0.376 |
| #4 | ½″ | 0.668 |
| #5 | ⅝″ | 1.043 |
| #6 | ¾″ | 1.502 |
| #7 | ⅞″ | 2.044 |
| #8 | 1″ | 2.670 |
| Slab | Bars ea. way | 20-ft sticks | Weight |
|---|---|---|---|
| 10 × 10 | 9 | 9 | 114 lb |
| 12 × 12 | 10 | 13 | 154 lb |
| 20 × 20 | 16 | 33 | 417 lb |
| 24 × 24 | 19 | 47 | 597 lb |
How to lay out a rebar mat
- Measure the slab length and width; total an odd footprint on the square footage calculator first.
- Pick the bar size and spacing for the job — #4 at 16 in is the residential default — from the layout guide.
- Set the cover so bar sits back from the edge — 3 in against earth.
- Leave 5–10% waste for lap splices and offcuts, more on a slab with lots of cuts.
- Read the sticks, ties and weight, and add chairs and tie wire from the exclusions below.
Worked example: rebar for a 20 × 20 slab
The default, worked line by line — a 20 × 20 ft slab, #4 bar at 16 in on center, 3 in cover, 5% waste, 20 ft sticks at $12:
pieces = 16 + 16 = 32, each 19.5 ft = 624 lf
order = 624 × 1.05 = 655 lf ÷ 20 = 33 sticks
weight = 624 × 0.668 = 417 lb · ties = 16 × 16 = 256
cost = 33 × $12 = $396
The mat is symmetric here because the slab is square; a 12 × 20 slab would take 10 bars one way and 16 the other. Note the weight climbs fast with bar size — the same grid in #5 instead of #4 is 651 lb, not 417, which is why residential slabs stay on #4 unless the load calls for more.
What this rebar estimate doesn’t include
The grid is the steel; a poured slab needs a handful of things the bar count leaves out:
- Chairs and dobies. Supports that hold the mat up in the slab’s middle third — priced per piece.
- Tie wire. A roll of 16-gauge wire to tie the intersections; the count is in the results.
- Extra lap length. Splices add about 40 bar diameters each — the waste % covers a normal amount, not a heavily spliced job.
- Edge and dowel bars. Perimeter bars, thickened-edge bars and dowels into footings, figured separately.
- The concrete. The pour itself — yards, bags and cost on the slab calculator.
- Engineering. Structural slabs and footings are designed; this sizes a standard mat, it isn’t a substitute for a spec.
Rebar FAQ
How much rebar do I need for a slab?
Lay a grid both ways: bars each direction = the run ÷ spacing, plus one. A 20 × 20 ft slab at 16 in on center of #4 bar is 16 bars each way — 32 pieces, about 624 linear feet, or 33 twenty-foot sticks with 5% waste, roughly 417 lb of steel and $396 at $12 a bar.
How far apart should rebar be in a slab?
Residential slabs on grade run 12 to 18 in on center, both ways; 16 in is the common default. A working rule is spacing no more than three times the slab thickness (18 in for a 6 in slab) and never over 18 in. Tighter spacing or bigger bar for driveways and anything carrying vehicles.
What size rebar for a concrete slab?
#3 (⅜ in) or #4 (½ in) for patios, shed floors and sidewalks; #4 to #5 for driveways and garage slabs; #5 and up for footings and structural work. #4 at 16 in on center is the everyday residential slab default — the size the calculator opens on.
How much rebar for a 24x24 garage slab?
A 24 × 24 ft garage slab at 16 in on center of #4 bar is 19 bars each way — 38 pieces, about 893 linear feet, or 47 twenty-foot sticks with 5% waste. That is roughly 597 lb of steel and 361 tie intersections. Step up to #5 for a slab that will see a lift or heavy trucks.
How much does rebar weigh?
By the foot: #3 is 0.376 lb, #4 is 0.668, #5 is 1.043, #6 is 1.502, up to #8 at 2.67 lb/ft. So a 20 ft stick of #4 weighs about 13.4 lb. The calculator totals the grid and multiplies by the bar’s unit weight for the job’s pounds of steel.
How much rebar do I need per square foot?
At 16 in on center both ways, a grid works out to about 1.5 linear feet of bar per square foot of slab — so a 400 ft² slab takes roughly 600 ft of rebar before waste. Drop to 12 in spacing and it climbs to about 2 lf per ft²; open up to 24 in and it falls to about 1.
Do I need rebar or wire mesh for a slab?
Welded wire mesh handles thin, lightly loaded slabs — sidewalks and shed floors; rebar is for anything thicker or load-bearing — driveways, garages, footings — because it can be sized, spaced and tied to carry real load. Fiber mix controls shrinkage cracking but does not replace structural steel.
Sources and standards
The bar weights are a physical standard; the layout rules are code and common practice:
- ASTM A615: the standard for deformed carbon-steel bar — the #3–#8 sizes and their unit weights.
- ACI 318 / 332: concrete cover (3 in against earth), lap-splice lengths and spacing limits.
- Grade 60: the 60,000 psi yield bar standard for residential and light commercial work.
- The engineer of record: structural slabs, footings and grade beams are designed — this is a planning tool, and the spec governs.
Embed this rebar calculator on your site
Run the rebar calculator on your own site, free. Paste the snippet where you want it; the visible credit link is the only condition.