Rebar Calculator
Calculate how much rebar you need for a concrete slab, patio, driveway or garage floor. Get the number of bars each way, total linear feet, 20 ft sticks to buy, steel weight, chairs, tie wire and cost.
Rebar Calculator: How Much Rebar for a Concrete Slab?
Rebar (reinforcing bar) does not stop concrete from cracking — concrete always shrinks and cracks to some degree — but it holds cracks tightly closed, stops slabs from separating and settling unevenly, and lets a slab bridge small soft spots in the base. For driveways, garage floors, patios that carry hot tubs, and any slab that sees vehicle traffic, a rebar grid is one of the cheapest ways to add years of service life.
A slab grid is laid out with bars running in both directions at a set spacing, measured on center (OC). The bar count in each direction is the run length divided by the spacing, plus one bar to close out the far edge. Bars stop about 3" short of the slab edge so they stay fully covered by concrete. For a 24 × 24 ft slab with 3" edge cover, each bar is 23.5 ft long, and at 18" OC you need 16 bars each way — 32 bars in total.
Bar size and spacing depend on the load. The most common residential choice is #4 (½") rebar at 18"–24" OC for patios and walkways, and #4 at 12"–18" OC for driveways and garage floors. #3 (⅜") bar is sometimes used for light patios and sidewalks, while #5 (⅝") appears in heavier slabs, thickened edges and footings. Rebar weights are fixed by ASTM A615: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft and #5 = 1.043 lb/ft.
When a run is longer than one stick, bars are lapped (overlapped and tied). A common rule of thumb is a lap of 40 bar diameters — 15" for #3, 20" for #4 and 25" for #5 — though engineered plans may require more. Laps add up fast on large slabs, which is why this calculator works out the actual number of 20 ft sticks per run instead of just dividing total footage by 20.
In 2026 US pricing, a 20 ft stick of #4 rebar costs about $11–$15 at home centers and less from a steel or masonry supplier, #3 runs about $7–$10 and #5 about $18–$25. Plastic rebar chairs cost roughly $0.25–$0.60 each and a 3.5 lb roll of 16-gauge tie wire is about $8–$12. For most residential slabs, steel adds roughly $0.75–$1.50 per square foot — small compared with the $6–$12 per square foot cost of a finished concrete slab.
How to Use This Calculator
Understanding Your Result
The highlighted stick count is the number of bars to buy, including laps and 5% extra for cutting mistakes. Installed length and weight help you check against a supplier quote or your vehicle payload. Chair and tie counts are approximate guides for a typical 3 ft support grid.
Key Formulas
Bars in Each Direction
Calculated once across the width (for bars running lengthwise) and once across the length.
Lap Splice Length
#3 = 15", #4 = 20", #5 = 25". Used only when a run is longer than one stick.
Sticks per Long Run
For runs shorter than a stick, multiple pieces are cut from one stick where possible.
Steel Weight
#3 = 0.376, #4 = 0.668, #5 = 1.043 lb/ft.
Worked Example
A 24 × 24 ft garage slab with #4 rebar at 18" OC each way, 3" edge cover and 20 ft sticks.
- 1Bar length each way = 24 ft – 2 × 0.25 ft = 23.5 ft
- 2Bars each way = floor(23.5 ÷ 1.5) + 1 = 15 + 1 = 16
- 3Lap for #4 = 40 × 0.5" = 20" = 1.67 ft
- 4Sticks per run = ceil((23.5 – 1.67) ÷ (20 – 1.67)) = 2; splices per run = 1
- 5Sticks = 32 runs × 2 = 64; +5% = 67.2 → 68 sticks
- 6Installed length = 32 × 23.5 + 32 × 1.67 = 805 ft; weight = 805 × 0.668 = 538 lb
Buy 68 × 20 ft #4 bars (about $750–$1,020 at $11–$15 per stick), roughly 81 chairs and about 172 ft of tie wire for 256 intersections.
Assumptions Behind This Estimate
- Rectangular slab with the same spacing in both directions
- Edge cover applied to all four sides; bars run the full clear dimension
- Lap splices of 40 bar diameters where a run exceeds the stick length
- 5% extra sticks for cutting waste and mistakes
- Chairs on an approximately 3 ft grid; about 8" of tie wire per tied intersection
Rebar Size Reference (ASTM A615)
| Bar Size | Diameter | Weight (lb/ft) | Lap (40d) | Typical Price / 20 ft |
|---|---|---|---|---|
| #3 | ⅜" (0.375) | 0.376 | 15" | $7–$10 |
| #4 | ½" (0.500) | 0.668 | 20" | $11–$15 |
| #5 | ⅝" (0.625) | 1.043 | 25" | $18–$25 |
| #6 | ¾" (0.750) | 1.502 | 30" | $26–$35 |
Prices vary by region and supplier; steel yards are usually cheaper than home centers for large orders.
Before You Buy: Practical Advice
- Stagger lap splices so they do not all line up on the same line across the slab.
- Tie every intersection around the perimeter and every second or third intersection in the field — enough that the mat does not shift when walked on.
- Lay plywood walk boards during the pour so finishers do not push the rebar down off its chairs.
- Add extra #4 bars around slab corners and penetrations, where cracks tend to start.
- Ask your supplier to cut bars to length — many steel yards cut and bend for a small fee and it saves hours with a grinder.
Frequently Asked Questions
How much rebar do I need for a 24x24 slab?
At #4 bar, 18" on center, with 3" edge cover, a 24 × 24 ft slab needs 16 bars each way, each 23.5 ft long. Because 23.5 ft is longer than a 20 ft stick, each run needs two sticks lapped 20", so you need 64 sticks plus about 5% extra — 68 × 20 ft bars, roughly 805 installed feet and 538 lb of steel.
What spacing should rebar be in a slab?
For residential slabs, 18"–24" on center each way is common for patios and walkways, and 12"–18" for driveways and garage floors that carry vehicles. Tighter spacing adds steel but helps control cracking. Follow your plans or local code if they specify spacing.
Is #3 or #4 rebar better for a driveway?
#4 (½") rebar is the usual choice for driveways because it is stiffer and stays in position better when workers walk on it during the pour. #3 rebar is lighter and cheaper and is often fine for patios and sidewalks. For a driveway that will see trucks or RVs, use #4 at 12"–16" OC on a 5–6" slab.
Is rebar or wire mesh better for a concrete slab?
Rebar is stronger and easier to keep at the correct height, which is why many contractors prefer it for driveways and garage floors. Welded wire mesh is cheaper and faster to lay but is often trampled to the bottom of the slab during the pour, where it does little. Fiber-reinforced concrete is another option for crack control but does not replace structural rebar.
How high should rebar be in a slab?
Rebar should sit near the middle of the slab thickness, or slightly above middle — about 2" up in a 4" slab and 2–2½" in a 5–6" slab. Support it on plastic or concrete chairs spaced about every 3–4 ft. Rebar lying on the ground or base gravel provides almost no benefit.
How much does rebar weigh per foot?
Standard ASTM A615 weights are #3 (⅜") 0.376 lb/ft, #4 (½") 0.668 lb/ft, #5 (⅝") 1.043 lb/ft and #6 (¾") 1.502 lb/ft. A 20 ft stick of #4 weighs about 13.4 lb, so 68 sticks weigh roughly 900 lb — check your vehicle payload before picking up a large order.
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