Rebar Calculator: Estimate Rebar Quantity, Spacing & Linear Feet for a Slab
This rebar calculator figures the number of bars, grid spacing, and total linear feet needed to reinforce a slab, footing, or driveway. Enter your slab dimensions and preferred spacing to size a #3, #4, or #5 rebar grid before you order.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How to Use the Rebar Calculator
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
total linear ft = (bars_across_W × length_L) + (bars_across_L × length_W); sticks = total ÷ 20
- ·bars_across_W = number of bars spaced across the width
- ·bars_across_L = number of bars spaced across the length
- ·length_L, length_W = span length of each bar (ft)
- ·spacing = center-to-center bar spacing (in), converted to ft
- ·20 = length of a standard #4 rebar stick (ft)
Bar count each way is (side length ÷ spacing) + 1 to include both edge bars. Total linear feet is divided by 20 to get the number of standard sticks, then rounded up.
Worked examples
Example 1: 10 ft × 10 ft slab, rebar on a 12-inch grid
- Spacing 12 in = 1 ft, so bars each way = (10 ÷ 1) + 1 = 11
- One direction: 11 bars × 10 ft = 110 ft
- Other direction: 11 bars × 10 ft = 110 ft
- Total = 220 ft; sticks = 220 ÷ 20 = 11
Result: 220 linear feet — 11 sticks of 20 ft #4 rebar
Example 2: 20 ft × 24 ft slab, rebar at 18-inch spacing
- Spacing 18 in = 1.5 ft
- Across 20 ft: (20 ÷ 1.5) + 1 ≈ 15 bars × 24 ft = 360 ft
- Across 24 ft: (24 ÷ 1.5) + 1 = 17 bars × 20 ft = 340 ft
- Total = 700 ft; sticks = 700 ÷ 20 = 35
Result: 700 linear feet — 35 sticks of 20 ft #4 rebar
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Rebar Calculator FAQs
How do I calculate how much rebar I need for a slab?
Divide each side length by your bar spacing to find the number of bars in each direction, then multiply by the opposite side length to get total linear feet. For a typical slab, most builders use #4 rebar on a 12- to 18-inch grid.
What spacing should I use for rebar in a concrete slab?
Residential slabs and driveways are commonly reinforced with rebar spaced 12 to 18 inches on center in both directions. Tighter 12-inch spacing adds strength for heavier loads, while 18 inches is common for light-duty pads.
What size rebar is best for a driveway or patio?
#4 rebar (1/2 inch) is the most common choice for residential driveways, patios, and slabs. Lighter pads sometimes use #3, and heavy-load slabs may step up to #5.
How much rebar overlap do I need at splices?
A standard lap splice is about 40 times the bar diameter, so #4 rebar overlaps roughly 20 inches. Always add extra linear footage to your order to cover splices and waste.
How deep should rebar be placed in concrete?
Rebar should sit in the middle third of the slab thickness with at least 2 to 3 inches of concrete cover from the ground and edges. Use chairs or dobies to hold the grid at the correct height during the pour.
Do I need rebar or is wire mesh enough?
Wire mesh controls cracking in thin, lightly loaded slabs, while rebar provides real structural strength for driveways, footings, and load-bearing slabs. Many driveways use both, or a rebar grid on chairs.
What does the number in #4 rebar mean?
The number is the bar diameter in eighths of an inch, so #4 rebar is 4/8, or 1/2 inch, in diameter. Likewise #5 is 5/8 inch and #3 is 3/8 inch. Residential slabs, patios, and driveways most commonly use #3 or #4 bar.
How long is a standard stick of rebar?
The most common stock length is 20 feet, which is what this calculator uses to count sticks. Rebar is also sold in 10-, 40-, and 60-foot lengths at many suppliers. Twenty-foot bars are the practical choice for most DIY and residential jobs.
How many pieces of rebar are in a bundle?
Bundle counts vary by bar size because bundles are sold by weight, roughly 2 tons each from the mill. A bundle of #4 rebar often contains a few hundred 20-foot sticks, but home centers usually sell single sticks. For a slab, just order the number of sticks the calculator returns plus a couple extra.
Should rebar sit in a grid or be placed continuously?
For slabs, rebar is laid as a grid of bars running both directions and tied at each intersection. This spreads tensile stress across the whole slab and controls cracking. Continuous single runs are used mainly in footings and beams, not flat slabs.
How do I tie rebar intersections together?
Use 16-gauge tie wire and a wire twister or pliers to secure each crossing point with a simple saddle or wrap tie. Tying holds the grid in position during the pour so bars do not shift. Not every intersection must be tied; every other one is often enough for slab mats.
Can I use fiber mesh instead of rebar?
Fiber mesh helps control fine surface shrinkage cracks but does not replace rebar for structural strength. For driveways, footings, and load-bearing slabs, rebar or welded wire mesh is still needed. Fiber is best thought of as a supplement, not a substitute.
How much does rebar cost per foot?
#4 rebar typically runs about $0.75 to $1.25 per linear foot, so a 20-foot stick costs roughly $15 to $25. Prices track the steel market and rise for larger bar sizes. Buying full sticks rather than pre-cut lengths keeps the cost down.