Rebar Calculator
Work out how many 20 ft rebar sticks a slab needs: grid bar count, total length, lap splices, weight, chairs and tie wire.
Your estimate
30 bars
Stock bars to buy (incl. 5% extra)
- Total length of rebar
- 546 ft
- Grid
- 14 bars × 19.5 ft + 14 bars × 19.5 ft
- Weight
- 365 lb
- Crossings to tie
- 196
- Lap splice
- none — bars fit in one length
Show the math
- Bars running the length = ⌊(width − 2 × cover) ÷ spacing⌋ + 1 = 14, each 19.5 ft
- Bars running the width = ⌊(length − 2 × cover) ÷ spacing⌋ + 1 = 14, each 19.5 ft
- Total = 546 ft; weight = length × 0.67 lb/ft = 365 lb
- Stock bars: short runs are cut several per bar, long runs are spliced; + 5% → 30
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Rebar | #4 (1/2 in), 20 ft sticks | 30 bars | — |
| Rebar chairs / dobies | about one per 3 × 3 ft | ≈ 45 | — |
| Tie wire | 16 ga (1.6 mm) annealed, one tie per crossing | 196 ties | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Quick answer A 20 × 20 ft slab with #4 rebar at 18 in on center and 3 in edge cover needs 546 ft of bar — 14 bars × 19.5 ft + 14 bars × 19.5 ft — which is 30 bars of 20 ft stock with 5% extra, about 365 lb and 196 ties. A 6 × 6 m slab with 12 mm bar at 400 mm needs 32 bars of 6 m stock. Bars each way = ⌊(side − 2 × cover) ÷ spacing⌋ + 1.
Method & assumptions
Bars running the length = ⌊(Width − 2 × Cover) ÷ Spacing⌋ + 1 Bars running the width = ⌊(Length − 2 × Cover) ÷ Spacing⌋ + 1 Bar length = Side − 2 × Cover (+ lap at each splice) Weight = Total length × unit weight (#4 = 0.668 lb/ft) Ties = bars one way × bars the other way
- Spacing
- center-to-center distance between parallel bars
- Cover
- concrete between the bar and the edge of the slab
- Lap splice
- overlap where two bars join, often 40 bar diameters (20 in for #4)
Assumptions & limits
- A single mat of bars running both ways at the same spacing.
- Bars are cut from stock lengths; short pieces are cut several per stick, long runs are spliced with a lap.
- Thickened edges, dowels into existing concrete and corner bars are not included.
- This counts material only — bar size, spacing and placement for structural slabs come from your plans or local code.
Rounding: purchase quantities are always rounded up to whole units as sold. See our methodology.
Diagram
How to use this calculator
- Enter the slab length and width (the inside of the forms).
- Set the grid spacing from your plans. 12–18 in is typical for driveways and garage slabs; 18–24 in for patios.
- Set the edge cover — the distance from the form to the first bar. 3 in (75 mm) is common.
- Pick the bar size and the stock length you will buy. Big-box stores sell 10 and 20 ft #3–#5 bars.
- If bars must be spliced, check the lap length under Advanced options against your plans.
Worked examples
Imperial: 24 × 24 ft garage slab, #4 at 16 in, 20 ft sticks
- Bars each way = ⌊(288 − 6) ÷ 16⌋ + 1 = 18; each run = 23.5 ft
- Each run needs 2 pieces with one 20 in lap → 25.17 ft
- Total = 36 × 25.17 = 906 ft = 45.3 → 46 sticks; +5% = 49; weight ≈ 605 lb
Metric: 4 × 3 m shed base, 10 mm bar at 300 mm, 6 m stock
- Bars along 4 m = ⌊(3.0 − 0.15) ÷ 0.3⌋ + 1 = 10 × 3.85 m
- Bars along 3 m = ⌊(4.0 − 0.15) ÷ 0.3⌋ + 1 = 13 × 2.85 m
- 10 × 3.85 + 13 × 2.85 = 75.6 m ≈ 47 kg
In depth: rebar planning notes
Why bar count, not area, drives the order
Rebar for a slab is a grid: bars in one direction set at the chosen spacing, crossed by bars in the other direction. The number of bars in each direction is the clear width divided by the spacing, plus one for the starting bar — a 20 ft slab with 3 in of edge cover and 18 in spacing gets 14 bars, not 13.3. Each bar is the slab dimension minus the cover at both ends. Multiply out both directions and you have the linear footage.
The second step is turning footage into sticks. If each run fits inside a stock length, the leftover can only be reused if it is long enough for another run, so a 19½ ft bar consumes a whole 20 ft stick. Where runs are longer than the stock, bars are joined with a lap splice — commonly 40 bar diameters, or 20 in for #4 — and every splice adds that overlap to the order. Staggering laps so they don't all fall on one line across the slab is good practice.
Placement: cover, chairs and ties
Rebar only works when it ends up inside the concrete at the right depth. In a 4 in slab on ground it is usually placed near mid-depth, supported on plastic or wire chairs (or concrete dobies) about every 3–4 ft in each direction. Bars lying on the gravel and "pulled up" during the pour almost always end up at the bottom. ACI 318 asks for 3 in of cover where concrete is cast against and permanently exposed to earth, which is why the edge cover here defaults to 3 in.
Ties hold the grid together while it is walked on during the pour. A tie at every crossing is the safest; many crews tie every other crossing on large slabs. Annealed 16 gauge wire and a twisting tool make the job quick. Mesh and fiber are common alternatives for crack control in light patios, but where your plans, permit or local code call for rebar, the bar size and spacing on those documents govern — use this calculator to turn them into a materials list.
Reference table
| Bar | Diameter · weight |
|---|---|
| #3 | 0.375 in · 0.376 lb/ft |
| #4 | 0.500 in · 0.668 lb/ft |
| #5 | 0.625 in · 1.043 lb/ft |
| #6 | 0.750 in · 1.502 lb/ft |
| 10 mm / 12 mm / 16 mm | 0.617 / 0.888 / 1.578 kg/m |
Key facts
- Bar number
- diameter in eighths of an inch (#4 = 4/8 in)
- 20 ft #4 stick weighs
- ≈ 13.4 lb
- Chair spacing
- about every 3–4 ft each way
- Common lap
- 40 bar diameters (check plans)
Tips & common mistakes
- Keep bars off the ground on chairs or dobies — rebar lying in the gravel does nothing once it is cast at the bottom of the slab.
- Stagger lap splices so they don't all line up across the slab.
- Bend-and-cut is easier with a rebar cutter/bender than an angle grinder for #3 and #4.
- Wire mesh and fiber are alternatives for crack control in patios; they are not a substitute for rebar where plans call for it.
Frequently asked questions
How much rebar do I need for a 20 × 20 slab?
At 18 in on center each way with 3 in cover, 14 bars each way of 19.5 ft — about 546 ft or 28 sticks of 20 ft #4, plus a few spares.
What size rebar for a concrete slab?
#3 or #4 is typical for residential slabs on ground, often at 12–18 in on center; driveways and garage slabs usually use #4. Follow your plans or local code.
How far apart should rebar be in a slab?
Commonly 12, 16 or 18 in on center for driveways and garages, and up to 24 in for light patios. Bars should sit near the middle of the slab on chairs.
How much does a 20 ft piece of #4 rebar weigh?
About 13.4 lb (0.668 lb per foot). A 20 ft #3 is about 7.5 lb and a 20 ft #5 about 20.9 lb.
How long should rebar overlap?
A common rule is 40 bar diameters — 15 in for #3, 20 in for #4, 25 in for #5. Structural plans may specify more.
Sources
- South Dakota DOT – Construction Manual ch. 7, Table 7.1 Properties of standard reinforcing bars (PDF) — accessed : “#3 0.376 lb/ft (0.560 kg/m) · #4 0.668 (0.994) · #5 1.043 (1.552) · #6 1.502 (2.235); diameters 0.375 / 0.500 / 0.625 / 0.750 in”
- CRSI – Product Catalog 2025 (ASTM standard reinforcing bars, PDF) — accessed : “ASTM STANDARD REINFORCING BARS – Weight (lb/ft): #3 0.376, #4 0.668, #5 1.043, #6 1.502”
- ASTM A615 – deformed carbon-steel bars for concrete reinforcement — general reference — not cited for a specific figure
- American Concrete Institute – ACI 318 building code resources — general reference — not cited for a specific figure
Default values (yields, coverage rates and densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Concrete Slab CalculatorEstimate cubic yards and bags for a slab or patio, including a 12×12 ft pad at 4 in, with waste, the formula and a shopping list.
- Concrete Block CalculatorCount 8×8×16 cinder blocks (CMU) for a wall, plus bags of mortar and the grout or concrete to fill the cores — with the math shown.
- Concrete Step CalculatorEstimate concrete volume and bags for steps and stoops. Riser and tread notes cite model IRC limits only — not a code approval.
Learn more in our guides
Method, waste and worked examples: Concrete volume: formulas, waste and examples.
Embed this calculator
Run a blog, forum or contractor site? Add this free rebar calculator to your page. It resizes itself, has no ads and sets no tracking cookies.
Free for any site. Keep the small “Calculator by DIYCalcLab” credit visible; it is rel="nofollow" by design. No ads or tracking cookies inside the widget. Widget terms & all calculators.
Cite this calculator
DIYCalcLab. “Rebar Calculator.” Oct 19, 2026. https://diycalclab.com/concrete/rebar-calculator/
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.