One direction of your slab — the longer dimension.Please enter a valid length greater than 0.
The other direction of your slab.Please enter a valid width greater than 0.
#4 is standard for residential slabs. #5–#6 for driveways and structural floors.
12 in O.C. is the most common residential spacing. 6–8 in for heavily loaded slabs.Please enter a valid spacing greater than 0.
Distance from slab edge to the first bar. ACI minimum: 1.5 in for #5 and smaller, 2 in for #6 and larger.
Add 5–10% for cut waste on standard grids. Add 10–15% for complex shapes.
$
Leave blank to skip cost estimate. Typical range: $0.45–$1.10/LF depending on rebar size and market.
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Your Rebar Estimate
Total Rebar (with waste)
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Linear Feet
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Total Bar Count
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Weight (lbs)
Bar Count by Direction
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Bars running length-wise
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Bars running width-wise
—Slab Area (ft²)
—Spacing
—Rebar Size
—Waste Factor
Estimated Material Cost
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Rebar material cost only. Add labor ($0.50–$1.50/ft² for placing and tying), tie wire, chairs/spacers, and delivery for a complete reinforcement budget. Use our Full Project Estimator for a complete breakdown.
Step 1: Convert all inputs to inches
Step 2: Bars in length direction = FLOOR((Width_in − 2 × Cover_in) / Spacing_in) + 1
Step 3: Bars in width direction = FLOOR((Length_in − 2 × Cover_in) / Spacing_in) + 1
Step 4: LF per length-direction bar = Length_in / 12
Step 5: LF per width-direction bar = Width_in / 12
Step 6: Total LF (no waste) = (Bars_length × LF_each_length) + (Bars_width × LF_each_width)
Step 7: Total LF (with waste) = Total LF × (1 + waste% / 100)
Step 8: Weight (lbs) = Total LF × lb/LF for selected rebar size
Measure your slab dimensions.
You need the length and width of the area to be reinforced — not the overall job site, just the concrete pour area inside the forms. Use the actual interior form dimensions, not the outside of the lumber. Enter the longer dimension as Length; it doesn't affect the total result, but it determines which bars run which direction in the breakdown.
Select your rebar size and spacing.
Pick the rebar designation (#3 through #8) from the dropdown — your structural drawings or local code will specify this. Enter your center-to-center spacing, or use the quick-select buttons for the four most common residential and commercial spacings. If your drawings say "12 in O.C." or "@ 12" it means 12 inches center-to-center — enter 12 in the spacing field.
Set edge cover and waste factor.
Edge cover is the distance from the edge of the slab to the first bar. ACI 318 requires a minimum of 1.5 inches for #5 bar and smaller in non-exposed conditions, and 2 inches for #6 and larger — the default of 2 inches covers both cases. The waste factor accounts for off-cuts when bars don't divide evenly into your slab; 10% is appropriate for most rectangular slabs with a standard 20-foot stock length.
Use your results to order materials.
The total linear feet figure, divided by your stock bar length (typically 20 ft or 40 ft), tells you the number of full bars to order. Round up to the nearest whole bar — never round down. The weight output lets you cross-check against the supplier's delivery ticket. If you entered a price per linear foot, the cost estimate reflects material only.
⚠ Pro Tip: The most common rebar layout mistake is forgetting that every bar in each direction needs to span the full perpendicular dimension, not just reach from one end to a midpoint. On a 20 ft × 20 ft slab with #4 at 12 in O.C. and 2 in cover, you get 20 bars each way — and every one of those 40 bars is 20 feet long. That's 800 linear feet. Contractors who eyeball this without a calculator routinely underorder by 20–30%.
Rebar Spacing Formula
The bar count formula is derived from standard CRSI (Concrete Reinforcing Steel Institute) layout practice. Each direction is calculated independently: the number of bars spanning a dimension equals the floor of the clear span divided by spacing, plus one — because the first bar is placed at one cover distance from the edge, not at the center of the first space.
Step
Formula
Example (20 × 20 ft, #4 @ 12 in, 2 in cover)
1. Clear span each direction
Dim_in − 2 × Cover_in
240 − 4 = 236 in
2. Bars each direction
FLOOR(clear span / spacing) + 1
FLOOR(236 / 12) + 1 = 20 bars
3. LF per bar (lengthwise bars)
Width_in / 12
240 / 12 = 20 LF
4. Total LF (no waste)
(Bars_L × LF_L) + (Bars_W × LF_W)
(20 × 20) + (20 × 20) = 800 LF
5. Add waste factor (10%)
Total LF × 1.10
800 × 1.10 = 880 LF
6. Weight
Total LF × 0.668 lb/ft
880 × 0.668 = 588 lbs
Common Slab Rebar Reference Table
Bar counts and linear feet for standard slab sizes — #4 rebar, 12 in O.C., 2 in edge cover. No waste factor applied.
Slab Size
Bars Each Way
Total Bars
Total LF
Weight (lbs)
10 × 10 ft
10
20
200 LF
134 lbs
12 × 12 ft
12
24
288 LF
193 lbs
16 × 16 ft
16
32
512 LF
342 lbs
20 × 20 ft
20
40
800 LF
534 lbs
20 × 40 ft
20 / 40
60
1,600 LF
1,069 lbs
24 × 24 ft
24
48
1,152 LF
770 lbs
30 × 30 ft
30
60
1,800 LF
1,203 lbs
40 × 60 ft
40 / 60
100
4,800 LF
3,206 lbs
Based on #4 rebar, 12 in O.C. spacing, 2 in edge cover on all sides. Add 10% for real-world ordering. Weight uses 0.668 lb/LF for #4 rebar.
Which Rebar Size Should You Use?
Rebar size is the single most consequential choice in a reinforcement layout — and the one most often underspecified by DIYers. Picking #3 when the application demands #5 doesn't save money; it creates a slab that will crack under load within years. The table below reflects standard industry practice for common applications.
Recommended rebar size and spacing by concrete application type.
Application
Min Rebar Size
Typical Spacing
Notes
Residential patio / sidewalk
#3
18 in O.C.
Light foot traffic only; wire mesh is a common substitute
Residential garage floor
#4
12–18 in O.C.
Standard for cars and light trucks
Residential driveway
#4
12 in O.C.
Use #5 if heavy truck traffic is expected
Commercial driveway / parking lot
#5
12 in O.C.
Heavy vehicle loads; verify with local code
Commercial floor slab
#5–#6
8–12 in O.C.
Point loads from racking or equipment require structural review
Foundation wall
#5
12 in O.C. (vertical + horizontal)
ACI 318 requires both vertical and horizontal reinforcement
Structural beam / column
#6–#8+
Per engineer
Always requires a licensed structural engineer
When your structural drawings specify a rebar size and spacing, use exactly that. Do not substitute a larger bar at wider spacing or a smaller bar at tighter spacing — the two are not equivalent. The combination of size and spacing governs both the reinforcement ratio and the crack control geometry. One change affects both.
Common Rebar Estimation Mistakes
⚠️
Using inside clear span only — ignoring the cover dimension.
The first and last bar are placed at the cover distance from the slab edge, not at the spacing distance. Ignoring this means you'll calculate one fewer bar per direction than is actually needed. On a 20-foot slab with 2-inch cover and 12-inch spacing, that's exactly 20 bars — not 19. This calculator handles cover correctly; doing the math in your head often doesn't.
📐
Calculating bars in one direction only.
Rebar grids are bidirectional. Both the bars running length-wise and the bars running width-wise need to be accounted for. Forgetting one direction cuts your order — and your reinforcement — in half. This is surprisingly common among homeowners who think of rebar as going "one way across" the slab.
📏
Not accounting for bar lap splices.
When your slab is longer than the stock bar length (typically 20 or 40 feet), bars must be lapped and tied. ACI 318 minimum lap splice length for #4 rebar in normal-weight concrete is typically 24–36 bar diameters — roughly 1 to 1.5 feet. This calculator shows the net rebar grid requirement; add lap splice length separately based on how many joints your layout requires.
🔩
Ordering to the nearest linear foot instead of the nearest full bar.
Rebar is sold in fixed lengths — you can't buy 47.3 linear feet. Once you have your linear foot total, divide by your stock bar length (20 ft or 40 ft) and round up to the nearest whole bar. Every partial bar still requires a full bar to be purchased. Estimating to the LF and rounding down is how contractors end up short on delivery day.
📦
Confusing rebar size numbers with diameter in inches.
A #4 rebar is NOT 4 inches in diameter. The designation number corresponds to the bar's diameter in eighths of an inch — so #4 is 4/8 = ½ inch diameter, #6 is 6/8 = ¾ inch. Misidentifying the bar size leads to the wrong weight calculation and can result in ordering the wrong material from the supplier.
Frequently Asked Questions
The number of bars in one direction equals the floor of (slab dimension − 2 × edge cover) divided by spacing, plus one. For example, a 10-foot slab with 2-inch cover and 12-inch O.C. spacing: (120 − 4) / 12 = 9.67, floor is 9, plus 1 = 10 bars. Do the same calculation for the perpendicular direction. Each bar spans the full perpendicular dimension of the slab. This calculator does all of that automatically for both directions simultaneously.
The standard for a residential concrete driveway is #4 rebar (½-inch diameter) at 12 inches on center in both directions. In freeze-thaw climates or where delivery trucks are expected, #5 rebar at 12 inches O.C. provides better crack resistance. Some local codes and contractors use 18-inch spacing with #4 for lighter residential applications — check your local building department requirements before ordering. Wire mesh (6×6 W1.4/W1.4 welded wire fabric) is an alternative for driveways but provides significantly less tensile strength than properly placed rebar.
On center (abbreviated O.C.) means the distance is measured from the centerline of one bar to the centerline of the next bar. A 12-inch O.C. spacing means each bar's center is exactly 12 inches from the next bar's center. This is the universal standard in structural drawings. It is not the gap between bars — that gap equals the O.C. spacing minus the bar diameter. For #4 rebar (½-inch diameter) at 12 inches O.C., the clear gap between adjacent bars is 11.5 inches. Always use center-to-center measurements when laying out rebar.
Standard ASTM A615 rebar weights per linear foot are: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft. These are the published CRSI values used throughout the industry. Knowing the total weight helps you verify the supplier's delivery ticket — a bundle of #4 bars labeled as 40 bars × 20 ft should weigh 40 × 20 × 0.668 = 534.4 lbs. If it's significantly off, something is wrong with the count or the bar designation.
Wire mesh (welded wire fabric, or WWF) is acceptable for lightly loaded slabs — patios, walkways, and some thin residential applications — where its primary role is crack control rather than structural reinforcement. Rebar provides far more tensile strength, better control over cover depth (since it sits on chairs), and more reliable lap splices. For driveways, garage floors, and any slab carrying vehicle loads, properly placed rebar is the correct choice. Wire mesh is also notorious for ending up on the ground mid-pour rather than staying elevated in the concrete matrix — a problem that eliminates its effectiveness entirely.
ACI 318-19 Table 20.6.1.3 specifies minimum cover for non-prestressed reinforcement. For concrete cast against and permanently in contact with earth, the minimum is 3 inches. For concrete exposed to weather (exterior slabs, driveways), the minimum is 2 inches for #6 bar and larger, and 1.5 inches for #5 bar and smaller. For concrete not exposed to weather or in contact with ground (interior slabs), the minimum is 1.5 inches for #5 bar and smaller and 1.5 inches for #6 through #11. Always use chairs or spacers to maintain consistent cover — never estimate it by eye.
Rebar chairs (also called supports or spacers) are plastic or metal devices placed under the rebar grid to hold the bars at the correct height above the subgrade during the pour. Yes, they are essential — not optional. Without them, workers walking on the rebar mat during the pour will push it to the ground, eliminating the concrete cover below the steel. Rebar sitting on the ground provides zero structural benefit and will rust through within years because the concrete offers no protection. Use chairs sized for your specified cover — 1.5-inch or 2-inch chairs are standard for residential slabs. Space them no more than 4 feet apart on every bar.
Technically, a 4-inch residential patio used only for foot traffic can be built without rebar — ACI 360R (Guide to Design of Slabs-on-Ground) permits unreinforced slabs in some residential conditions. However, adding #3 or #4 rebar at 18 inches O.C. costs relatively little, significantly improves crack resistance, and limits the width of any cracks that do form. If the patio is in a freeze-thaw climate, on expansive soils, or will have any wheeled load on it (lawn tractor, grill on wheels, delivery dolly), rebar is a no-brainer. Most experienced concrete contractors will refuse to pour a patio without some form of steel reinforcement.
ACI 318 does not specify a mandatory tie frequency for slab-on-grade reinforcement — ties are for construction stability, not structural function. In practice, tying every other intersection (a checkerboard pattern) is standard for residential slabs and is sufficient to hold the grid together during a pour. Tying every intersection is required for heavily reinforced structural elements where bar displacement under concrete weight is a concern. Use 16-gauge tie wire and a tie wire twister tool to speed up the work; for a 20×20 slab with 100 intersections, tying alternating intersections takes under an hour.
A lap splice is required whenever a bar doesn't reach from one end of the slab to the other in a single length. Since rebar is typically sold in 20-foot or 40-foot stock lengths, any slab dimension exceeding your bar length requires a splice. ACI 318 specifies the minimum lap splice length based on bar size, concrete strength, and cover. For #4 rebar in 3,000 PSI concrete with standard cover, the Class B tension lap splice length is approximately 24–30 inches. Overlap the bars by at least this amount and tie at both ends and the middle. Add the total splice length to your linear foot calculation — this calculator provides the net grid length, not splice-adjusted quantities.
