Rebar Calculator
A rebar calculator counts bars each way as ⌈usable length ÷ spacing⌉ + 1. A 20 × 12 ft slab on a 12 in grid needs 512 linear ft, or 26 twenty-foot bars.
Rebar calculator: bars, linear feet, and stock lengths
Rebar is steel bar set inside concrete to carry the pulling forces that concrete cannot hold. In a residential slab, #3 or #4 bar on a grid about 12 to 18 inches apart each way is common, set on chairs in the middle third of the slab, but your plan or engineer sets the final spacing.
A rebar calculator lays out a grid of steel bars inside a slab and totals the length. For each direction, count the bars as ⌈usable length ÷ spacing⌉ + 1, where usable length removes the edge clearance from both sides. A run that is not an exact multiple of the spacing still gets a bar at the far edge. A 20 × 12 ft slab on a 12-inch grid with 3-inch clearance needs 512 linear feet, or 26 stock bars of 20 ft.
Structural reinforcement is code-governed. For load-bearing slabs, footings, and driveways, confirm bar size, spacing, cover, and laps with a licensed engineer.
The formula in plain words
Picture a tic-tac-toe board drawn inside the slab. Some bars run the long way. Others run the short way. Where they cross, the grid forms squares.
- Usable width = slab width in inches − 2 × clearance
- Usable length = slab length in inches − 2 × clearance
- Lengthwise bars = ⌈usable width ÷ spacing⌉ + 1, each as long as the slab
- Widthwise bars = ⌈usable length ÷ spacing⌉ + 1, each as wide as the slab
- Linear feet = (lengthwise bars × length) + (widthwise bars × width)
- Bars to buy = linear feet ÷ stock length, rounded up
The “+ 1” counts the bar at the start of each row, because 10 gaps need 11 posts. Rounding up means a short last gap still gets its own bar at the far edge.
Worked examples
- Patio slab, 20 ft × 12 ft, 12 in grid, 3 in clearance, 20 ft bars
- Usable width: 144 − 6 = 138 in, so ⌈138 ÷ 12⌉ + 1 = 12 + 1 = 13 lengthwise bars
- Usable length: 240 − 6 = 234 in, so ⌈234 ÷ 12⌉ + 1 = 20 + 1 = 21 widthwise bars
- Linear feet: (13 × 20) + (21 × 12) = 512 ft
- Stock bars: 512 ÷ 20 = 25.6, rounded up to 26 bars
- Garage apron, 16 ft × 10 ft, 16 in grid, 3 in clearance, 20 ft bars
- Lengthwise: ⌈114 ÷ 16⌉ + 1 = 8 + 1 = 9 bars
- Widthwise: ⌈186 ÷ 16⌉ + 1 = 12 + 1 = 13 bars
- Linear feet: (9 × 16) + (13 × 10) = 274 ft
- Stock bars: 274 ÷ 20 = 13.7, rounded up to 14 bars
Bars per 10 ft dimension (3 in clearance)
| Grid spacing | Bars across 10 ft | Approx. linear ft per sq ft of slab |
|---|---|---|
| 12 in | 11 | about 2.2 |
| 16 in | 9 | about 1.8 |
| 18 in | 8 | about 1.6 |
| 24 in | 6 | about 1.2 |
Each count includes the bar at the far edge. Linear feet per square foot is the rule-of-thumb view, worked out for a 10 × 10 ft slab. Tighter grids multiply cost quickly, so compare to the spacing on your plan.
How to measure and plan
- Slab size: Use the finished length and width from the form boards.
- Cover: Take the clearance from your plan. A typical figure is 2 to 3 inches, but follow the drawing.
- Chairs: Order bar supports or chairs so the grid sits mid-depth, not on the dirt.
- Laps: Add lap length wherever two bars meet end to end. Longer slabs than the stock bar make this unavoidable.
- Tie wire: Plan some tie wire for the crossings, though it is not counted here.
Common mistakes
- Starting the first bar at the form. Without clearance, the steel touches the edge and rusts.
- Forgetting the “+ 1”. Ten spaces take eleven bars, not ten.
- Ordering exact footage. Laps, offcuts, and bent ends all consume more steel than the grid length.
- Letting the grid sink. Steel on the ground does no good once the concrete is poured around it.
- Mixing up bar size and spacing. A thicker bar on a wider spacing is not interchangeable with a thin bar on a tight one.
What a lap splice adds
The count assumes unbroken bars. Say a slab run is 24 ft and your stock is 20 ft. You need two pieces that overlap. With a 20 inch lap (1.67 ft) the second piece must be 24 − 20 + 1.67 = 5.67 ft long.
Total steel for that run is 25.67 ft, not 24 ft, so each splice adds the lap length to your order. Lap length depends on bar size and code, so take the figure from your plan.
Related calculators
Size the concrete with the concrete slab calculator and the concrete footing calculator. To check what the bars will weigh on the truck, use the rebar weight calculator.
Frequently asked questions
What spacing should I use for slab rebar?
Residential slabs commonly use #4 (½-inch) bar on a 12- to 18-inch grid, but your plan, local code, or engineer sets the requirement. Tighter spacing means more steel. This calculator lets you enter whatever spacing your plan calls for.
Why subtract edge clearance?
Rebar must sit inside the slab with concrete cover on every side so the steel does not rust. The calculator removes the clearance from each edge, so a 3-inch clearance trims 6 inches from the width and 6 from the length before bars are spaced.
How does the calculator count bars in two directions?
A slab grid has bars both lengthwise and widthwise. It counts how many fit across the width, each running the full length, and how many fit along the length, each running the full width. It adds the two spans to get total linear feet.
Does this include lap splices?
No. The stock-bar count is total linear feet divided by your bar length, with no overlap added. Where bars must be spliced end to end, lap length is set by the engineer or code. As a common rule of thumb it is about 40 bar diameters, roughly 20 inches for #4 bar, so buy extra.
What is rebar and how far apart should it be in a slab?
Rebar is steel bar set inside concrete to carry tension the concrete cannot. Residential slabs often use #3 or #4 bar on a grid about 12 to 18 inches apart each way, but your plan or engineer sets the spacing. Tighter spacing means more steel.
What size rebar is standard for a driveway?
Driveways often use #3 or #4 bar on a roughly 16- to 18-inch grid, or welded wire mesh as a lighter alternative. Heavier loads and thicker slabs move up in bar size or tighten the spacing. Confirm with your engineer or local code.
Estimates only. Verify quantities with your supplier before purchasing.