Rebar Calculator
Plan the rebar grid for a concrete slab - enter the slab size and grid spacing to get the bars each way, total lineal feet with lap splices, the 20 ft sticks to buy and the chairs to support them.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
bars along length = ⌊(width×12 − 6) ÷ spacing⌋ + 1 · bars along width = ⌊(length×12 − 6) ÷ spacing⌋ + 1
lineal ft = bars × (bar run − 6 in of edge clearance)
order lineal ft = lineal ft × 1.10 (lap splices + cut waste)
sticks = order lineal ft ÷ stick length, rounded up · chairs = area ÷ 16, rounded up
weight (lb) = sticks × stick length × lb per ft of the bar size
Assumptions & limits
- Bars are held 3 inches clear of every slab edge, the common residential practice.
- Lap splices are taken at 40 bar diameters (15 in for #3, 20 in for #4); the 10% allowance covers laps and cut waste for typical grids built from 20 ft sticks.
- Chairs (supports) at one per ~4 ft grid — the mesh or bar must sit in the middle of the slab, not on the ground, or it does nothing.
- Bar weights are the ASTM A615 nominal figures, and they are derived rather than looked up: steel at 490 lb per cubic foot is 3.403 lb per square inch of cross-section per foot, so lb/ft = nominal area × 3.403. #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502 lb per foot.
- Stick price defaults to $13 for a 20 ft stick of #4 and follows the bar size when you change it, scaled at the $0.97 per pound that default implies. Set your own quoted price - steel moves with the market.
- This is a QUANTITY takeoff, not a design. Bar size and spacing for footings, walls and anything structural come from the code and the engineer — CRSI publishes placement standards, and no calculator replaces them.
Worked example
A 20 × 16 ft slab on a 12 inch grid, 20 ft sticks at $13
- Bars along length = ⌊(192−6)÷12⌋+1 = 16 · along width = ⌊(240−6)÷12⌋+1 = 20
- Lineal ft = 16 × 19.5 + 20 × 15.5 = 622 ft
- With laps and waste = 622 × 1.10 = 684 ft → 684 ÷ 20 = 34.2 → 35 sticks
- Chairs = 320 ÷ 16 = 20 · weight = 35 × 20 × 0.668 = 468 lb
Result:35 twenty-foot sticks (468 lb of steel), 20 chairs and a roll of tie wire — roughly $415 to $530.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 25, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
How to figure rebar for a slab
A slab grid is two sets of bars: one running the length, spaced across the width, and one running the width, spaced along the length — every bar held 3 inches clear of the edges. The count each way is the dimension divided by the spacing, plus one for the starting bar.
bars each way = ⌊(dimension in inches − 6) ÷ spacing⌋ + 1 · sticks = total lineal ft × 1.10 ÷ 20
The 10% on top covers lap splices — wherever two sticks continue a run, they overlap by 40 bar diameters (20 inches for #4 bar) and get tied with wire — plus normal cut waste. Chairs or dobies every four feet or so hold the grid mid-slab while the concrete goes down, which is the difference between reinforcement and expensive gravel: steel lying on the ground does nothing.
Bar sizes at a glance
| Bar | Diameter | Lap (40d) | Typical use |
|---|---|---|---|
| #3 | 3/8 in | 15 in | Patios, walkways, light pads |
| #4 | 1/2 in | 20 in | Driveways, garage slabs, footings |
| #5 | 5/8 in | 25 in | Heavier footings, engineered work |
Rebar weight chart
Steel is often ordered and priced by weight even when it is delivered as sticks, so the conversion is worth having. The nominal weights below are not a lookup somebody published — they fall straight out of the bar diameter: a square inch of steel one foot long weighs 3.403 lb (steel at 490 lb per cubic foot, divided by 144), so the weight per foot is simply the nominal cross-sectional area times 3.403.
| Bar | Diameter | Weight per ft | 20 ft stick | Sticks per ton |
|---|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | 7.5 lb | 266 |
| #4 | 1/2 in | 0.668 lb | 13.4 lb | 150 |
| #5 | 5/8 in | 1.043 lb | 20.9 lb | 96 |
| #6 | 3/4 in | 1.502 lb | 30.0 lb | 67 |
lb per ft = nominal area (sq in) × 3.403 · #4 = 0.19635 × 3.403 = 0.668
The calculator reports the total weight alongside the stick count, so a takeoff can be handed to a supplier either way. Two practical notes. Weight is what decides how the bundle gets off the truck and around the site — 150 sticks of #4 is a ton, and a ton of steel does not move by hand. And the price per stick tracks weight closely: the $13 default for a 20 ft #4 works out near $0.97 per pound, which is why the calculator moves the price with the bar size when you change it.
Grid spacing by job
Residential slabs run 12 to 18 inch grids: 12 inches for driveways and anything carrying vehicles, 16–18 inches for patios and shed slabs. Tighter grids use more steel for more crack control; the calculator shows the material cost of the choice instantly. For light-duty slabs, 6×6 welded wire mesh is the common alternative — the concrete calculator counts mesh sheets alongside the concrete itself.
Where the calculator stops
This is a quantity takeoff for ground-supported residential slabs. Footings, walls, suspended slabs and anything an engineer would call structural get their bar size, spacing and placement from the code and the design — the CRSI placement standards are the trade reference. Rebar in a footing that an inspector wants to see is not a calculator decision. Pricing the pour itself? The concrete cost calculator handles the ready-mix side.
Stick prices vary with steel markets — the default $13 per 20 ft stick of #4 is a 2026 planning figure. Get current pricing from your supplier, and confirm any structural requirements with your building department.
Frequently asked questions
How much rebar do I need for a concrete slab?
Lay out a grid: bars across the width spaced every 12-18 inches, and the same along the length, held 3 inches in from every edge. Bars each way = (dimension in inches minus 6) / spacing, plus one. A 20 x 16 ft slab on a 12 inch grid needs 16 bars one way and 20 the other - about 620 lineal feet, or 35 twenty-foot sticks once laps and waste are added. The calculator above does the counting.
What rebar spacing should I use?
Residential slabs commonly use a 12 to 18 inch grid: 12 in for driveways and anything that carries vehicles, 16-18 in for patios and shed slabs. Footings and structural work are different - there the size and spacing come from the code and the engineer, not from a rule of thumb.
What size rebar for a driveway or patio?
Number 3 bar (3/8 in) suits patios and walkways; #4 (1/2 in) is the usual choice for driveways, garage slabs and footings. Bigger bar sizes belong to engineered work. For light-duty slabs, 6x6 welded wire mesh is the common alternative - the concrete calculator prices that option.
How much should rebar overlap at a splice?
The standard lap is 40 bar diameters: 15 inches for #3, 20 inches for #4, 25 for #5. Tie the lap with wire in at least two places. This calculator adds 10% to the lineal footage for laps and cut waste, which covers typical residential grids using 20 ft sticks.
Do I need rebar or is wire mesh enough?
For light, ground-supported slabs - patios, walkways, shed floors - 6x6 welded wire mesh controls cracking adequately. Once vehicles are involved, or spans and soils get questionable, rebar on a proper grid is the robust choice. Neither substitutes for engineering where the slab is structural: that is a code and engineer decision, not a calculator output.