Rebar Calculator

By Michael Woo · Updated June 2026

Pro tips

Subtract edge clearance before dividing by spacing

Rebar is never placed flush against the form edge. ACI 318 requires a minimum of 1.5 inches (for cast-in-place concrete exposed to soil) to 3 inches (for concrete cast directly against earth) of clear cover from the edge of the concrete to the nearest rebar surface. On a 10-foot (120-inch) slab with 2-inch edge cover on each side, the available grid width is 116 inches, not 120 inches. With 12-inch on-center spacing, you get 10 spaces (11 bars), not 10 bars. That single extra bar per direction does not sound like much, but on a 40-foot by 60-foot slab with 12-inch spacing, the edge-clearance correction adds 2 bars in each direction — 4 extra bars total — each 20 feet long. That is 80 linear feet of rebar (roughly 53 pounds of #4 bar) that a naive calculator misses. For #5 bars, those 80 feet weigh 83 pounds and cost an extra $40–$60 at current pricing of $0.50–$0.75 per foot.

Factor in lap splice length when ordering stock bar lengths

Rebar is sold in standard lengths of 20 feet (most common at retail) and 30 or 40 feet (from rebar suppliers). When a bar must extend longer than the stock length, bars are overlapped (lapped) by a minimum splice length that depends on bar size and concrete strength. For #4 rebar in 3,000 psi concrete, the ACI lap splice length is 24 inches (2 feet). For #5 rebar, it increases to 30 inches. On a 35-foot slab span using 20-foot stock bars, each bar needs one splice, consuming 2 feet of overlap per bar. If you have 30 bars spanning that direction, you lose 30 × 2 feet = 60 linear feet to splices — an extra three 20-foot bars worth of material. That is 15% more rebar than the simple length × count formula yields. Commercial projects with 40-foot or 60-foot spans may have 2–3 splices per bar, compounding the effect. Always add splice lengths to your total linear footage before ordering.

Convert total linear footage to weight for accurate ordering

Rebar suppliers price and inventory by the ton, not by the foot. You need to convert your calculated linear footage to pounds using the per-foot weight of each bar size: #3 rebar weighs 0.376 lb/ft, #4 weighs 0.668 lb/ft, #5 weighs 1.043 lb/ft, #6 weighs 1.502 lb/ft, and #7 weighs 2.044 lb/ft. A residential driveway requiring 1,200 linear feet of #4 rebar weighs 1,200 × 0.668 = 802 pounds (0.40 tons). A commercial slab needing 8,500 linear feet of #5 bar weighs 8,500 × 1.043 = 8,866 pounds (4.43 tons). Suppliers sell in full-ton or half-ton increments with a 5–10% cutting waste allowance built in for standard orders. If your calculated weight is 4.43 tons, order 5 tons to cover splices, cutting waste, and bent bars at column footings. Under-ordering by even 200 pounds on a time-sensitive pour can cause a costly project delay while waiting for a supplemental delivery.

Rookie mistakes

Counting grid spaces instead of bars

The classic fence-post error: a 10-foot span divided by 12-inch spacing yields 10 spaces but requires 11 bars to bound those spaces. On a 20-foot by 20-foot slab with 12-inch spacing, the correct count is 21 bars in each direction (42 total), not 20 bars in each direction (40 total). Those 2 missing bars — each 20 feet long — represent 40 linear feet and 26.7 pounds of #4 rebar. Scale that error to a 50-foot by 100-foot commercial pad with 12-inch spacing and you are short by 2 bars in the 50-foot direction and 2 bars in the 100-foot direction: 300 linear feet, 200 pounds, and roughly $150 of material. The structural consequence is worse than the cost: the outermost 12 inches of slab on two sides has no reinforcement, making those edges susceptible to shrinkage cracking in the first 28 days of curing. Always count number-of-spaces plus one to get the bar count.

Ordering one bar size for both slab and footing reinforcement

Residential projects often have a 4-inch slab with thickened edges (footings) that are 12–18 inches deep. The slab field typically requires #3 or #4 rebar at 18-inch spacing, but the footing requires #4 or #5 bars per the structural engineer's specification — often 2 bars top and 2 bars bottom continuously around the perimeter. A 40-foot by 50-foot slab with a 12-inch-wide by 18-inch-deep thickened edge requires 180 linear feet of perimeter footing bar × 4 bars = 720 linear feet of footing rebar, plus stirrups (vertical ties) every 24 inches — that is 90 stirrups at roughly 4 feet each = 360 linear feet of stirrup bar. Total footing steel: 1,080 linear feet. If you order only the slab grid rebar (calculated at perhaps 2,400 linear feet of #4), you are 1,080 feet short because the footing steel is a separate line item. This mistake leaves you scrambling on pour day, unable to pass footing inspection without the required continuous reinforcement.

Ignoring bend allowances for hooks and corners

Structural drawings often require 90-degree hooks at bar ends (particularly in footings and grade beams) or 180-degree hooks for anchorage. A standard 90-degree hook on a #4 bar extends 12 inches past the bend point per ACI 318 (8 bar diameters minimum for #3–#5, plus a 4.25-inch bend radius). Each hook consumes approximately 18 inches of straight bar length. If your footing has 4 continuous bars around a 180-foot perimeter, each bar needs hooks at its start and end for a total of 8 hooks consuming 8 × 18 inches = 144 inches = 12 feet of additional rebar. At corner locations where two bars overlap with hooks, the overlap plus hook tail adds 30–36 inches at each of the 4 corners — another 10–12 feet total. These 22 feet of hook and corner allowances are invisible in a linear-footage calculator that assumes straight bars only. On a project with 20+ hooked bar ends (common in combined slab-and-footing pours), the missed footage can exceed 30 feet, enough to leave you one stock bar short.

Example project costs

Standard driveway pour

400 sq ft (20×20 ft) · #4 at 12″ each way

Total bar length	880 LF
20-ft bars	44 bars
Total weight	588 lbs
Total	880 LF · 44 bars · 588 lbs

Backyard patio

200 sq ft (16×12.5 ft) · #3 at 18″

Total bar length	293 LF
20-ft bars	15 bars
Total weight	110 lbs
Total	293 LF · 15 bars · 110 lbs

Garage slab-on-grade

1,200 sq ft (40×30 ft) · #4 at 18″

Total bar length	1,760 LF
20-ft bars	88 bars
Total weight	1,176 lbs
Total	1,760 LF · 88 bars · 1,176 lbs

What NOT to build with rebar calculator

Don't use rebar calculator for: Rebar calculated for a concrete slab on expansive clay soil without engineering review

Expansive clay soils exert uplift pressures up to 7,000 PSF when wetted—exceeding the dead load of a residential slab. ACI 360R-10 identifies expansive soil as a special condition requiring a geotechnical assessment before slab design.

Frequently Asked Questions

How much rebar do I need for a 400 sq ft concrete slab?

For a 400 sq ft slab using #4 bar at 12-inch spacing: 2 × 400 / 1 × 1.10 = 880 LF = 44 standard 20-ft bars, weighing approximately 588 lbs. At 18-inch spacing: 587 LF = 30 bars. The calculator handles any area, spacing, and bar size combination.

What size rebar should I use for a residential concrete slab?

#4 (1/2-inch diameter) at 12-inch spacing is the field standard for driveways and garage slabs. #3 at 12–18 inches works for lightly loaded patios and walkways. #5 is specified for slabs subject to heavy equipment loads. ACI 318-19 Section 24.4.3.2 sets the structural minimum, but local code amendments and soil type often govern.

What spacing should I use for rebar in a concrete slab?

12 inches each way is the residential driveway standard. 18 inches is adequate for a residential patio or walkway under light foot traffic. 24 inches is the structural minimum per ACI 318-19 for slabs, but most residential contractors consider it underbuilt. Use the closer spacing if you're on clay soil or in a hard freeze zone.

How do I calculate rebar weight for an order?

Multiply total linear footage by the weight per foot: #3 = 0.376 lbs/LF, #4 = 0.668 lbs/LF, #5 = 1.043 lbs/LF. These are CRSI Handbook values—all suppliers use the same numbers. An 880 LF order of #4 weighs 880 × 0.668 = 588 lbs. Most steel yards price by weight; most home centers price by bar count.

Do I need rebar in every concrete project?

Slabs subject to vehicle loads or spanning any soft soil need rebar. Walkways, garden borders, and decorative slabs on stable, compacted soil can use wire mesh (welded wire fabric) instead—it resists shrinkage cracking at lower cost than rebar. Footings for structural walls always require rebar per IBC and IRC requirements.

How much waste factor should I add when ordering rebar?

10% is the industry standard waste allowance, built into this calculator. It accounts for overlap splices (40× bar diameter per ACI 318-19 Section 26.7.1—20 inches for #4), cuts at slab edges, and bars damaged during placement. For projects with many corners or irregular shapes, bump to 12–15% to avoid a re-order mid-pour.

Rebar size, spacing, and ACI 318 requirements for residential concrete

The three most common rebar sizes for residential and light commercial concrete are #3 (3/8-inch diameter), #4 (1/2-inch), and #5 (5/8-inch). Their weights per linear foot are published in the CRSI Handbook: #3 = 0.376 lbs/ft, #4 = 0.668 lbs/ft, #5 = 1.043 lbs/ft. These numbers are derived from steel density (490 lb/ft cubed) and the bar's cross-sectional area—they don't change by supplier. For residential concrete, ACI 318-19 Section 24.4.3 sets minimum reinforcement for slabs not designed as structural: 0.0018 × b × h gross cross-sectional area. For a 4-inch slab, that works out to #3 at 18-inch spacing or #4 at 24-inch spacing as the structural minimum. In practice, most residential contractors use #4 at 12-inch spacing for driveways and garage slabs because it's a durable field standard—the over-reinforcement relative to the code minimum costs little compared to the concrete and labor. IRC 2021 Table R404.1.2 specifies #4 rebar at 48-inch maximum horizontal spacing for concrete foundation walls, but for slabs the local practice (driven by climate, soil type, and frost depth) matters more than the code minimum. In freeze-thaw climates, tight spacing (12-inch) reduces crack propagation. In arid climates with stable soil, 18-inch spacing is common. The formula: Total LF = 2 × (Area / Spacing) × 1.10 waste factor. The factor of 2 accounts for bars running in both X and Y directions across the slab (a bidirectional grid). The 10% waste covers lapping (bars must overlap a minimum of 40 bar diameters per ACI 318 Section 26.7.1—that's 20 inches for #4 bar) and cut-off ends. Steel rebar pricing tracks BLS PPI WPU101707 (hot-rolled bars and shapes, updated monthly by BLS).

Estimating bar count, ordering in bundles, and placing rebar correctly

Rebar is sold in 20-foot lengths, which is the standard mill cut. For ordering: divide your total linear footage by 20 and round up to the nearest whole bar. A 400 sq ft driveway at #4 and 12-inch spacing requires 2 × 400 / 1 × 1.10 = 880 LF, or 44 bars. At a current market price of $0.80–$1.20/LF for #4 (plus delivery), that's approximately $700–$1,050 in material. Bundles: rebar is sold in 31-bar bundles from most steel suppliers (620 LF of 20-ft #4 bar per bundle, approximately 420 lbs). Buying a full bundle can reduce the per-bar price 5–10% vs. cut quantities at a home center. For projects over 20 bars, call a local rebar yard for mill pricing. Placement sequence matters: set chairs (plastic or concrete rebar chairs) every 4 feet to maintain 2-inch cover from the bottom of the slab as required by ACI 318-19 Section 20.6.1.3. Lay bars in one direction first, then the crossing direction, and tie intersections with 16-gauge wire at every other crossing for a tight grid. Common mistakes: placing rebar directly on the ground (no chairs), which corrodes from soil moisture within 5–10 years; placing it in the top half of the slab where it provides tension capacity in the wrong direction for a ground-bearing slab; and under-lapping (ACI requires 40× bar diameter minimum—24 inches for #3, 32 inches for #4, 40 inches for #5).

How this is calculated

Formula: LF = 2 × area ÷ (spacing ÷ 12) × 1.10 waste; weight = LF × lbs/ft (#3=0.376, #4=0.668, #5=1.043 per CRSI Handbook)

Input	Value	Unit
Slab area	400	sq ft
Bar spacing	12	in
Rebar size	1

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