Calculate Your Rebar Materials
Get bars by direction, total linear feet and meters, weight in pounds and tons, lap-aware 20-ft stick counts, and tie-wire and chair estimates for slabs, footings, and circular pours — with ACI spacing checks built in.
Go to Rebar Calculator →For a rectangular slab, the number of bars in one direction is floor((dimension − 2 × cover) ÷ spacing) + 1, applied both ways for a two-way mat. The two pieces people get wrong are the “+1” (there's always one more bar than the number of gaps) and subtracting twice the cover before dividing. Multiply total linear feet by the bar's published weight per foot (a #4 is 0.668 lb/ft) to get pounds for ordering, and add a 40 × bar-diameter lap wherever a run is longer than one 20-ft stick. For a typical residential slab that's #4 bar at 12–18″ on center with 3″ of cover.
Want the exact counts without doing the arithmetic? The free Rebar Calculator returns bars by direction, total linear feet and meters, weight in pounds and tons, the number of 20-ft sticks to buy (lap-aware), plus tie-wire and chair estimates for slabs, footings, and circular pours. This guide explains the math behind it, the ASTM and ACI numbers it's built on, and the decisions it can't make for you: what size, how far apart, and whether you need an engineer.
This is a material estimator, not a structural design tool.
The calculator and this guide tell you how much rebar a chosen layout needs. They do not select bar size or spacing for structural adequacy. For any load-bearing element — a footing carrying a structure, a retaining wall, a beam, a column, or a suspended slab — the bar size, spacing, and layout must come from your project drawings, local building code, or a licensed structural engineer.
📏 Rebar Sizes & Weight Per Foot (ASTM A615)
US rebar is sized in eighths of an inch. A #4 bar is 4/8″ = 0.500″ in diameter; a #5 is 5/8″ = 0.625″, and so on up through #8. The larger sizes (#9–#18) are converted square-bar equivalents, so the simple eighths rule stops at #8. #2, #12, and #13 are not standard A615 sizes — if a chart lists them, treat it with suspicion.
Weight per foot is the number you order by. Use the published nominal value from the table below rather than deriving it on the fly — published ASTM A615 weights are the industry ordering standard and are accurate to within about ±2% of real-world bar (rib deformations and mill tolerances account for the rest).
US Rebar — ASTM A615 Standard Sizes
| Bar | Diameter (in) | Diameter (mm) | Area (in²) | Weight (lb/ft) | Mass (kg/m) |
|---|---|---|---|---|---|
| #3 | 0.375 | 9.5 | 0.11 | 0.376 | 0.560 |
| #4 | 0.500 | 12.7 | 0.20 | 0.668 | 0.994 |
| #5 | 0.625 | 15.9 | 0.31 | 1.043 | 1.552 |
| #6 | 0.750 | 19.1 | 0.44 | 1.502 | 2.235 |
| #7 | 0.875 | 22.2 | 0.60 | 2.044 | 3.042 |
| #8 | 1.000 | 25.4 | 0.79 | 2.670 | 3.973 |
| #9 | 1.128 | 28.7 | 1.00 | 3.400 | 5.060 |
| #10 | 1.270 | 32.3 | 1.27 | 4.303 | 6.404 |
| #11 | 1.410 | 35.8 | 1.56 | 5.313 | 7.907 |
| #14 | 1.693 | 43.0 | 2.25 | 7.650 | 11.384 |
| #18 | 2.257 | 57.3 | 4.00 | 13.600 | 20.240 |
The three green rows (#3–#5) cover nearly all residential flatwork and footings. Rebar ships in 20-ft sticks at most retail yards (40-ft and 60-ft lengths exist for commercial orders). A 20-ft #18 weighs 272 lb and is crane-handled — not a DIY size.
⚖️ Grade Changes Strength, Not Weight
A common ordering mistake is assuming a higher-grade bar weighs more. It doesn't. Grade describes yield strength — Grade 60 is 60,000 psi, Grade 40 is 40,000 psi — and the steel density is identical, so a #4 weighs 0.668 lb/ft whether it's Grade 40, 60, or 80. Grade 60 (A615) is the US structural default.
- ASTM A615 — standard carbon-steel rebar for general construction. Marked with an S. Should not be welded.
- ASTM A706 — low-alloy, weldable, seismic-rated rebar. Marked with a W. Required for welded splices and seismic Design Categories D–F.
- Dual-grade bars marked with both S and W meet A615 and A706 at once.
- Epoxy-coated (A775) and galvanized (A767) bars add roughly 3–9% weight from the coating.
To tie a standard Grade 60 cage together, use 16-gauge tie wire, not welds — welding A615 can embrittle it. If a detail genuinely calls for welded rebar, specify A706.
🧮 How Many Bars? The Grid Formula
For a two-way slab mat, you count bars in each direction independently. The formula is the same both ways:
Two rules carry almost all the accuracy:
The “+1” rule
If a span has 10 gaps between bars, it has 11 bars — the same reason a 10-ft fence at 1-ft spacing needs 11 posts. Tools that drop the “+1” undercount every job. Tools that use ceil on the raw dimension over- or under-count depending on the remainder. floor(clear span ÷ spacing) + 1 is the convention this calculator documents and applies consistently.
Subtract 2 × cover first
Bars stop at the cover line, not the edge of the concrete. Dividing the full dimension by the spacing — instead of the clear span — is the single most common competitor error. For a 10-ft slab with 3″ cover, the clear span is 120 − 6 = 114″, not 120″.
Worked example — 20 × 20 ft driveway, #4 @ 12″, 3″ cover
- Clear span each way = (20 × 12) − (2 × 3) = 234″
- Bars each way = floor(234 ÷ 12) + 1 = 19 + 1 = 20 bars
- Each bar = 234″ ÷ 12 = 19.5 ft
- Total = 20 × 19.5 + 20 × 19.5 = 780 linear feet
- Weight = 780 × 0.668 = ≈ 521 lb (before waste)
For a mat foundation with steel in both the top and bottom faces, multiply the totals by 2. The calculator's “number of mats” input handles this.
🔗 Lap Splices & Ordering Sticks
Rebar comes in 20-ft sticks, so any run longer than 20 ft needs two bars overlapped — never butted — so the load transfers through the concrete from one bar to the next. The field rule of thumb is a lap of 40 × the bar diameter:
- #4 (0.5″) → 40 × 0.5 = 20″ lap
- #5 (0.625″) → 40 × 0.625 = 25″ lap
- #6 (0.75″) → 40 × 0.75 = 30″ lap
The catch most calculators miss: two lapped 20-ft sticks don't reach 40 ft. With a 20″ lap, they reach about 38.3 ft, so a long continuous run can need an extra stick. The calculator accounts for the lap steel both in the linear feet you order and in the stick count — it doesn't hand-wave the overlap away. ACI 318-19 treats this as a Class B tension lap (1.3 × development length, minimum 12″); the 40 db rule is a safe field approximation for Grade 60 in ordinary concrete. Stagger your laps so they don't all land in the same plane, and tie every lap.
🛡️ Concrete Cover — Why & How Much (ACI 318-19 §20.6)
Cover is the concrete between the bar and the nearest surface. It protects the steel from corrosion, fire, and spalling — too little and the bar rusts, expands, and cracks the concrete off. Cover also sets the clear span in the count formula above, so getting it right matters twice.
Minimum Concrete Cover
| Condition | Cover |
|---|---|
| Cast against & permanently exposed to earth (footings) | 3 in (75 mm) |
| Formed, exposed to weather/earth — #6 & larger | 2 in |
| Formed, exposed to weather/earth — #5 & smaller | 1.5 in |
| Not exposed — slabs/walls/joists, #11 & smaller | 0.75 in |
| Beams/columns, not exposed (to ties/stirrups) | 1.5 in |
Cover is what chairs and dobies are for — they hold the bar up off the ground so the concrete flows underneath. Laying rebar straight on the dirt or vapor barrier puts the steel at the bottom of the slab with zero cover, where it does nothing and corrodes fast.
🏗️ Real-World Defaults by Project
These are common starting points for non-engineered residential work. They are not a substitute for plans or local code on anything load-bearing — but they tell you what “normal” looks like so you can sanity-check a spec.
Typical Bar, Spacing & Cover
| Project | Bar | Spacing | Cover | Notes |
|---|---|---|---|---|
| Sidewalk / path | #3 (or wire mesh) | 18–24" | 2" | Mesh often sufficient |
| Patio | #3–#4 | 18" | 2" | Small pads may skip with good base + joints |
| Driveway | #4 | 12–16" | 2" | #4 @ 12" for trucks/RVs |
| Garage slab | #4 | 12" | 1.5–2" | Vapor barrier under chairs |
| Basement slab | #3–#4 | 18" | mid-slab | Crack control |
| Residential footing | #4–#5 | 2–4 continuous | 3" | IRC R403; #4 top & bottom min |
| Foundation / stem wall | #4 | 12–18" V & H | 1.5–2" | Near top + near bottom |
| Retaining wall | #4–#6 | 12–16" | 2" | Engineer-designed |
⚠️ The 6 Most Common Rebar Mistakes
1. Forgetting the “+1” bar
Counting gaps instead of bars undercounts every direction of every job. There is always one more bar than the number of spaces between bars.
2. Ignoring edge cover
Dividing the full slab dimension by the spacing instead of the clear span (dimension − 2 × cover) puts a phantom bar past the cover line and overstates both count and length.
3. Welding standard Grade 60 (A615)
A615 isn't formulated for welding and can become brittle at the weld. Tie cages with 16-gauge wire, or specify weldable A706 if the detail truly needs welds.
4. Treating fiber mesh as a structural substitute
Fibers control micro-cracking and plastic shrinkage but are not a replacement for structural rebar. Where a design calls for bars, fiber doesn't carry the tension.
5. Under-lapping or butting splices
Butted bars transfer no load. Laps need ~40 × the bar diameter, staggered and tied. Skimping here is invisible until the slab cracks at the joint.
6. Laying bar on the dirt
Steel at the bottom of the pour has zero cover, does little structural work, and rusts. Set bars on chairs or dobies at the design height — usually mid-slab for crack control.
🛑 When This Is an Engineer's Job
The calculator estimates quantities for a layout you choose. It does not decide whether that layout is strong enough. Get a stamped design from a licensed structural or geotechnical engineer before you order steel if the project involves any of:
- Footings carrying a structure, columns, or load-bearing walls
- Retaining walls — soil pressure governs, and these fail catastrophically (see the retaining wall calculator for the permit/engineer thresholds)
- Beams, grade beams, or suspended (elevated) slabs
- Seismic Design Categories D, E, or F (much of CA, OR, WA, AK, UT)
- Expansive clay soils or groundwater above the footing
- Post-tensioned slabs — these use tendons, not a conventional bar grid; do not estimate them as a mat
- Heavy-equipment pads and anything bearing unusual concentrated loads
For non-structural flatwork — a patio, a shed pad, a sidewalk, a garage slab on good base — the defaults table above and the calculator will get you a clean, code-aware material list.
📚 Authority & References
This guide is built on the same primary sources cited in the calculator's methodology and standards blocks:
Jurisdictions adopt different code editions and local amendments. Verify with your local building department before you build.
Ready to estimate your rebar?
Plug your slab, footing, or circular pour dimensions into the free Rebar Calculator and get bars by direction, total linear feet and meters, weight in pounds and tons, lap-aware 20-ft stick counts, and tie-wire and chair estimates — with ACI spacing checks built in. Pair it with the concrete calculator to size the pour itself.
Open the Rebar Calculator →Calculate Your Rebar Materials
Get bars by direction, total linear feet and meters, weight in pounds and tons, lap-aware 20-ft stick counts, and tie-wire and chair estimates for slabs, footings, and circular pours — with ACI spacing checks built in.
Go to Rebar Calculator →