Reinforce strongly.
Rebar lineal feet and pieces for any concrete slab. Standard grid spacing, with overlap for continuous runs.
How we calculated this
The calculator assumes a grid layout: rebar running both directions at the specified on-center spacing, creating a mesh that reinforces concrete against tension forces. Longitudinal bars run along the length of the slab; transverse bars run across the width. Bar count in each direction is the slab dimension divided by spacing, plus one for the closing bar at the far edge.
For slabs longer than 20 feet (the standard rebar stick length), bars must be overlapped where joined. The calculator adds 1.5 feet of overlap per joint. The technically correct overlap is 40 × bar diameter (which is 20 inches for #4 rebar, 25 inches for #5), but 1.5 feet is a safe simplification covering most cases.
Perimeter ring: an additional rebar loop around the edge of the slab, tied to the grid. Recommended for driveways and structural slabs, optional for simple patios. The ring strengthens edges which tend to crack first. Skip it for shallow decorative slabs.
Bar size selection: #4 (1/2 inch diameter) is the residential default for 4-inch slabs. #3 (3/8") is used in lighter applications like walkways. #5 (5/8") and #6 (3/4") are for structural slabs, retaining walls, and any pour over 6 inches thick. Larger bars don't just add strength: they add significant weight and cost.
Weight is calculated from standard lb-per-linear-foot values. This matters for ordering: steel is typically sold by weight. A full 20-foot stick of #4 rebar weighs 13.4 lb. A large slab might need 300-500+ pounds of rebar, enough to require multiple truck trips or careful loading.
Not included: chairs or dobie blocks (used to hold rebar up from the subgrade to the middle of the slab depth, one every 4 feet is typical), tie wire (for fastening bars at intersections, about 1 lb per 200 feet of rebar), saddles/corners for turn connections. Budget those separately.
The number on the bar is eighths of an inch
Rebar sizing is one of the few honest naming systems in construction: a #4 bar is 4/8 of an inch in diameter, a #5 is 5/8, and so on up the ladder. That single fact unlocks the rest of the math, because lap lengths, weights, and spacing rules all key off the diameter. Residential concrete lives almost entirely on two sizes: #3 for slabs and driveways, #4 for footings, thicker slabs, and anywhere a plan note says "or better."
The calculator above turns a slab or footing dimension into a bar count, total linear feet, and weight, which is how suppliers quote. This page carries the size chart, the grid spacing that residential slabs actually use, the 40-diameter lap rule inspectors measure with a tape, and the mesh-versus-rebar question that starts arguments on every jobsite.
How we calculated these numbers▾
Bar dimensions and weights follow ASTM A615, the standard covering common Grade 60 deformed bar. Lap, cover, and development rules come from ACI 318; the 40-diameter lap cited here is the common field simplification for Grade 60 bar in residential work, and engineered drawings override it wherever they specify a length. Placement practice follows CRSI guidance.
The rebar size chart
Diameter | Weight per ft | Lap at 40d | |
|---|---|---|---|
| #3 | 3/8" | 0.376 lb | 15" |
| #4 | 1/2" | 0.668 lb | 20" |
| #5 | 5/8" | 1.043 lb | 25" |
| #6 | 3/4" | 1.502 lb | 30" |
| #7 | 7/8" | 2.044 lb | 35" |
| #8 | 1" | 2.670 lb | 40" |
Common bar sizes per ASTM A615. A 20 foot stick of each: #3 is 7.5 lb, #4 is 13.4 lb, #5 is 20.9 lb, #6 is 30 lb.
The lap column answers the overlap question directly: where two bars continue a run, overlap them 40 bar diameters and tie the lap in at least two places. For the #4 bar in a typical footing that is 20 inches of overlap. Laps in adjacent runs should also be staggered rather than lined up in one row, so the splice zone does not become the weak plane of the pour.
Slab-on-grade spacing that actually gets used
For a residential 4 inch slab, the standard reinforcement is #3 or #4 bar in a grid at 12 to 18 inches on center each way, set at mid-depth of the slab on chairs. Driveways and anything carrying vehicles want the tighter 12 inch grid in #4. Footings run bars the long way: two #4 continuous for most one-story walls, three for two-story, per the typical prescriptive tables, with the engineered plan winning any disagreement. The chairs are not optional equipment. A bar lying on the vapor barrier does nothing except cost money; concrete cracks from the bottom and the steel has to be up in the section to catch it.
Rebar or wire mesh?
Rebar grid | Welded wire mesh | |
|---|---|---|
| Holds position during pour | Yes, on chairs | Rarely; sheets get walked down |
| Crack control | Strong, both directions | Adequate if actually mid-depth |
| Cost for 400 ft² slab | $180-280 | $110-170 |
| Best use | Driveways, garages, anything loaded | Light-duty patios and walks |
For a 4 inch residential slab. Fiber is a supplement for surface shrinkage cracking, not a replacement for steel where loads or spans matter.
A 20 by 20 foot driveway slab, 4 inches thick, #4 bar at 12 inches on center both ways. Bars per direction: 20 ft at 12 inch spacing is 19 interior bars plus edges, call it 20 runs each way at 19 feet after 3 inch cover, so 40 bars x 19 ft = 760 linear feet.
In 20 foot sticks that is 38 sticks. At 0.668 lb/ft the order weighs about 508 pounds; Denver yard pricing near $0.85 per foot puts the steel at roughly $645, plus a bag of ties and $60 of chairs. Every splice where sticks meet gets the 20 inch lap. Freeze-thaw country rewards the tighter grid: the slab will crack somewhere, and the steel is what keeps the crack hairline instead of a trip edge.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Where rebar jobs go wrong
Bars on the dirt. The most common failure on DIY pours. Steel in the bottom half inch of a slab is not reinforcement, it is buried scrap. Chairs cost about fifteen cents apiece.
Short laps. A 6 inch overlap where 20 is required fails inspection in jurisdictions that look, and fails the slab in the ones that do not. The 40d numbers are in the table above; measure them, do not eyeball them.
Rusty is fine, flaky is not. Light surface rust actually improves bond and is nothing to sand off. Loose, flaking scale is a different matter and that steel goes back.
Cutting cover to fit the form. Concrete protects the steel from corrosion only if there is enough of it: 3 inches against earth, 1-1/2 inches to a formed face. Bars pushed against the form leave a rust line telegraphing through the surface within a few winters.
Steel is half the slab math
The other half is the pour itself: the concrete calculator converts the same slab dimensions into cubic yards, and a driveway pour usually sits on a compacted base the gravel calculator quantifies. For a footing supporting masonry, the brick calculator counts what goes on top.
Run the slab through the calculator above, buy the chairs with the steel, and photograph the grid before the truck arrives. That photo settles every future argument about what is in the slab.
Frequently asked
What size rebar do I need for a 4-inch slab?
#4 rebar (1/2 inch diameter) at 16-inch spacing is the standard for a 4-inch residential slab (patio, driveway, garage floor). For heavier use (commercial or truck-loaded driveways), go to 12" spacing or #5 bar. Sidewalks can use #3 at 18" spacing for light residential.
Do I actually need rebar in a slab?
For thin decorative slabs under 3 inches: no, wire mesh is sufficient. For 4"+ slabs that bear any load: yes: rebar is cheap insurance against cracks. For anything over 6 inches or bearing vehicles: always. The cost difference ($50-150 for a typical slab) is trivial vs. the cost of cracking and replacing.
What's the spacing for a driveway?
12-inch grid minimum for passenger car driveways; 16" for a simple residential path. Use #4 bar. For driveways bearing trucks or RVs, 12" spacing with #5 bar is advisable. In freeze-thaw regions, add extra bars near the edges where frost heave concentrates.
How much overlap between rebar sticks?
The code minimum is 40 times the bar diameter. For #4 (1/2" bar): 20 inches minimum overlap. For #5 (5/8" bar): 25 inches. The calculator uses 1.5 feet (18") for simplicity, which is conservative for #3 and just over the minimum for #4. Tie the overlap with wire at 2-3 points.
What's a perimeter ring?
An additional bar loop running around the outside edge of the slab, tied to the grid. Slab edges are the weak spot where frost heave, tree roots, and vehicle loads concentrate stress. A perimeter ring strengthens that edge. Recommended for driveways; optional for light-duty patios.
Can I use wire mesh instead?
Welded wire mesh (WWF) is an alternative for thin slabs up to 4 inches. Pro: easier to install (roll it out). Con: harder to position at mid-slab depth (tends to sink to the bottom during the pour, providing minimal structural value). For any slab you care about, stick to rebar on chairs.
What position in the slab should rebar sit?
Middle-to-upper third of the slab thickness. For a 4-inch slab, the rebar grid should sit 1.5-2 inches from the top. Use dobie blocks or chairs to hold the rebar at this height, never let it sit on the ground, where it provides no structural benefit and may rust out from below.
How much does rebar weigh?
Per linear foot: #3 = 0.376 lb, #4 = 0.668 lb, #5 = 1.043 lb, #6 = 1.502 lb. A typical 20-foot stick of #4 weighs 13.4 lb. A slab with 500 feet of rebar weighs 334 lb. The calculator shows the total weight: check if your vehicle can carry it.
Sources
- ASTM A615: Deformed Carbon-Steel Bars - The specification behind the size chart: diameters and weights per foot
- ACI 318: Development and Lap Splice Provisions - The code behind lap lengths and concrete cover; engineered drawings compute from this
- CRSI: Placing Reinforcing Bars - Industry placement practice: chairs, ties, and lap staggering
- ACI 332: Residential Concrete - Prescriptive residential footing and slab reinforcement
- ASTM A1064: Welded Wire Reinforcement - The standard for the mesh alternative compared on this page
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