How much rebar do I need for a slab?

Multiply the bar count each direction by the length it runs, then add both directions. At 18 in. on-centre spacing, a 10×10 ft slab needs 7 bars each way (10 ÷ 1.5, plus one for the edge) for 140 total linear feet, or 8 sticks of 20-ft rebar with 10% waste. Enter your own length, width, spacing and bar size above for the exact count.

Is it better to use rebar or wire mesh?

Rebar is tied into a precise grid and lap-spliced at specific points — used for slabs-on-grade, footings, walls and any structural member. Welded wire mesh is a pre-fabricated grid, faster to place, typically used for lighter-duty reinforcement like sidewalks, patios and thin toppings where placement speed matters more than exact bar spacing. This calculator computes rebar bar counts and lengths, not wire mesh sheet counts.

How much rebar do I need for a footing?

Footing rebar is specified by a structural drawing or local code, not a rule of thumb — bar size, count and cover vary with footing width, depth and soil conditions. Enter the footing’s length and the number of continuous bars it calls for to get the linear-feet and weight totals; a 40 ft footing with 4 continuous bars needs 160 ft of bar. This page does not recommend a bar size or count on its own.

What size rebar do I need for a driveway?

A driveway slab follows the same grid method as any slab — #3 or #4 bar at 18–24 in. on-centre is common unengineered residential practice, and ACI 318 caps primary spacing at the lesser of 3× the slab thickness or 18 in. Enter the poured area and your chosen spacing above to get the bar count and total length; thinner driveways are also commonly reinforced with welded wire mesh instead.

What is a rebar lap splice?

A lap splice overlaps two rebar ends so they act as one continuous bar. Concrete’s code minimum is the greater of a flat 12-inch floor or a class multiple (1.0× for Class A, 1.3× for Class B) of the development length your structural drawing specifies — never the floor alone if your development length would fall short. Tension lap splices are not permitted at all for #14 or #18 bars.

What size rebar do I need?

Bar size is set by a structural drawing or local code for anything load-bearing — #3 or #4 is common for residential slabs, larger sizes for footings, walls and structural members. Sizing follows eighths of an inch for #3–#8 (a #4 bar is 4/8 in. = 0.500 in. diameter), but breaks that pattern at #9 and up, which are sized to match an older square-bar’s cross-sectional area instead.

What is the difference between grade 40 and grade 60 rebar?

Grade refers to yield strength, not size — Grade 40 yields at 40,000 psi, Grade 60 at 60,000 psi. Grade 60 is the common default for residential and commercial work today; Grade 40 is more common in older construction and some lighter applications. This calculator’s bar count and weight totals are identical regardless of grade — grade changes strength, not the quantity of bar needed.

How much does rebar weigh per foot?

Weight rises with bar size: 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5, up to 13.60 lb/ft for #18 — the full table is in the methodology section below. This calculator multiplies your entered length by the chosen size’s weight automatically; weight is always based on the in-place length, never the purchase length including waste.

Rebar Calculator — Quantity, Weight & Lap Splice

Using these numbers

Sticks are what you buy; in-place feet are what ends up in the work — the stick count is rounded UP to a whole 20-ft length — you cannot buy 7.7 sticks. The in-place and purchase figures stay separate so you can see the waste allowance, not just the final number you pay for.
Weight is based on in-place length, never purchase length — you weigh what is tied into the work, not what you bought including offcuts and waste. If you need a delivered-weight figure for a supplier order, use the purchase length instead.
A spacing outside 6–36 inches clamps, it doesn’t reject — the calculator moves an out-of-range spacing to the nearest bound and says so, then computes at the clamped value — it never silently substitutes a different number without telling you.
Lap splice states which constraint actually governs — the 12-inch code floor and your development length are combined with a maximum, never treated as alternatives — the result names whichever one is larger, including the case where they land on the exact same number.
A bundle changes the splice length, and now says so on the page — selecting a 3- or 4-bar bundle adds the CRSI-sourced increase on top of whichever class governed — shown as its own line, not folded silently into the main figure.
What this page doesn’t compute stays visible, not blank — the development length itself, the ACI spacing warning, a footing’s transverse ties, and tie-wire length all have no sourced way to compute them here — each is named as an absence with its reason, never a zero or a missing row.

Methodology

This page computes a bar count and a total length, then converts that to whole sticks and a weight. Every formula below is the same one the calculator runs on every keystroke — grid and continuous modes share one shared function each, and lap splice is a separate, simpler combination. No dollar figure appears here; what rebar costs is the sibling calculator’s subject.

The grid formula: fence-post counting in two directions

A slab, driveway, or wall reinforced with a tied rebar grid gets bars running both directions, each counted the same way: divide the span a bar has to cross by the spacing, floor that, then add one for the bar at the far edge — the same "fence-post" logic as counting fence posts along a run, not the gaps between them. A 10 ft span at 18 in (1.5 ft) spacing gives 10 ÷ 1.5 = 6.667, floored to 6, plus the edge bar is 7 bars.

Each of those 7 bars runs the full length of the OTHER dimension, so the direction’s total length is bar count × that span. A 10 ft × 10 ft slab at 18 in o.c. gets 7 bars running one way (each 10 ft long) and 7 bars running the other way (each 10 ft long) — 140 ft combined, before waste.

A wall uses the identical formula with two independent spacings instead of one shared spacing: a vertical spacing that governs how many horizontal bars there are, and a horizontal spacing that governs how many vertical bars there are — doubled again if the wall calls for two layers, one per face. A driveway or patio uses the exact same slab formula; "driveway" and "slab" are two labels on one calculation, not two formulas that could quietly drift apart.

barsAlongA  = floor(spanB ÷ spacingB) + 1
totalAlongA = barsAlongA × spanA
barsAlongB  = floor(spanA ÷ spacingA) + 1
totalAlongB = barsAlongB × spanB
inPlace     = (totalAlongA + totalAlongB) × layers

  10 ft × 10 ft slab, 18 in o.c., 1 layer:
  10 ÷ 1.5 = 6.667 → floor 6, +1 = 7 bars each way
  7 × 10 + 7 × 10 = 140 ft in place

Continuous bars: footings, piers, columns and beams

A footing’s continuous reinforcement, or a pier or column’s vertical bars, is not a spacing calculation — it is a bar count you already know from a structural drawing, multiplied by the length each bar runs. A 40 ft footing with 4 continuous bars called for is 4 × 40 = 160 ft, full stop. This page does not recommend a bar count for a given footing; that is an engineering decision, not a geometry one.

Footing steel is conventionally ordered close to length with little allowance for offcuts, so the waste default for this mode is 0%, not the grid mode’s 10% — adjustable either way.

inPlace = barCount × span

  4 continuous bars × 40 ft span = 160 ft in place

Purchase length, whole sticks, and weight

Purchase length is the in-place total times one plus the waste percentage. Sticks come standard in 20 ft lengths, so the purchase total is divided by 20 and rounded UP to a whole stick — you cannot buy 7.7 sticks. That ceiling is the one place this page rounds mid-calculation on purpose: it is the true number of sticks to buy, not a display rounding that gets fed back into another formula.

Weight is always keyed off the IN-PLACE length, never the purchase length — you weigh what ends up in the work, not what you bought including the offcuts.

purchase = inPlace × (1 + waste%)
sticks   = ceil(purchase ÷ 20)
weight   = inPlace × weightPerFoot[barSize]

  140 ft in place, 10% waste, #4 (0.668 lb/ft):
  purchase = 140 × 1.10 = 154 ft → 154 ÷ 20 = 7.7 → 8 sticks
  weight   = 140 × 0.668 = 93.52 lb

Lap splice: the 12-inch floor, Class A/B, and what’s excluded

A tension lap splice length is the GREATER of a flat 12-inch floor or a class multiple of the development length (ld) a structural drawing supplies — never the floor alone, and never ld alone if ld would fall short of the floor. Class A uses a 1.0× multiplier, Class B uses 1.3×. If ld is left blank, this page shows only the bare 12-inch floor, labeled explicitly as not the real answer — a genuine result needs ld from a structural calculation, and this page does not compute ld itself (see "What this page does not compute" below).

A bundled lap splice needs more length again: +20% for a 3-bar bundle, +33% for a 4-bar bundle, applied to whichever of the floor or the class-multiplied ld governed. Tension lap splices are not permitted for #14 or #18 bars at all — not a clamped range, a code prohibition — confirmed by two independent sources across two ACI 318 editions.

This figure previously cited a masonry-code rule (a 40×-bar-diameter minimum from IBC’s masonry chapter) that does not apply to concrete rebar at all — removed, not narrowed, once the error was found. Concrete’s own lap-splice floor has no bar-diameter term whatsoever; the 12-inch minimum is the same regardless of bar size.

classA = max(12in, 1.0 × ld)
classB = max(12in, 1.3 × ld)
bundled = classA_or_B × (1.20 for 3-bar, 1.33 for 4-bar)

  ld = 24 in:  Class A = max(12, 24) = 24 in — ld governs
  ld = 8 in:   Class A = max(12, 8)  = 12 in — the code floor governs

Rebar sizes, grades and weight

US rebar #3 through #8 is sized by its diameter in eighths of an inch — a #4 bar is 4/8 in. = 0.500 in. diameter. That rule stops holding exactly at #9: sizes #9, #10, #11, #14, and #18 are the diameter of a round bar with the same cross-sectional area as an older square-bar series, not a simple eighths count. #9 is 1.128 in., not 9/8 = 1.125 in.; #11 is 1.410 in., not 1.375 in.; #18 is 2.257 in., not 2.25 in. — confirmed against the ASTM standard’s own table, not derived from the eighths shortcut.

Weight per foot rises with size, from 0.376 lb/ft (#3) to 13.60 lb/ft (#18) — the full table is in the constants register below. Grade (40 or 60) refers to yield strength in psi, not size; Grade 60 is the common residential and commercial default. Coating (black, epoxy, or galvanized) changes cost and corrosion resistance, never the bar count or weight this page computes — the cost calculator has current price ranges by coating.

What this calculator does not compute, and why

The ACI 318 development length (ld) itself is not computed — the formula needs concrete strength, bar yield strength, spacing/cover, and transverse-reinforcement inputs this page does not collect. Silently assuming values for those would be worse than asking for ld from a structural drawing, so ld stays a required input for lap-splice mode, not a derived one.

The ACI 318 maximum slab spacing (the lesser of 3× the slab thickness or 450 mm / 18 in.) is a real code limit, but this page’s grid mode has no thickness input to check it against — not implemented, not silently assumed.

A footing’s transverse-tie length is not computed — it needs the footing’s width, which is not one of this mode’s inputs; only the continuous-bar component is computed. Tie-wire length is not computed either — the number of bar intersections is computable from the grid geometry, but no sourced length-of-wire-per-intersection figure exists to convert that count into a length.

Lap-splice consumption for a continuous run longer than one 20 ft stock length is not modeled in the purchase-quantity rule — whether the waste allowance already covers the extra length two spliced sticks need is not stated anywhere in the sourcing for this page, so nothing is assumed.

Welded wire mesh and fiber mesh are named as alternatives to a tied rebar grid — mesh is priced and cut by sheet or roll coverage, and fiber mesh is a mix-design dosage from a ready-mix supplier — but neither is computed by this calculator. This page computes rebar bar counts and lengths only.

Constants register

Constant	Value	Source	As of
Rebar weight, #3–#18	#3 0.376 · #4 0.668 · #5 1.043 · #6 1.502 · #7 2.044 · #8 2.670 · #9 3.400 · #10 4.303 · #11 5.313 · #14 7.650 · #18 13.60 (lb/ft)	harrissupplysolutions.com, "Steel Rebar Sizes & Stock", cross-checked against southernrebar.com — identical to 3 decimal places	2026-09-11
Rebar diameter, #3–#18	#3 0.375 · #4 0.500 · #5 0.625 · #6 0.750 · #7 0.875 · #8 1.000 · #9 1.128 · #10 1.270 · #11 1.410 · #14 1.693 · #18 2.257 (in)	ASTM A615/A615M, Table 1, corroborated via materialsmath.com and turn2engineering.com — two independent, non-supplier tabulations of the standard’s own table	2026-09-15
Eighths-of-an-inch sizing rule, scope	Exact for #3–#8 only (size ÷ 8 = diameter). Does NOT hold for #9–#11, #14, #18.	ASTM A615/A615M, Table 1 — those larger sizes are round-bar diameters of equal area to an older square-bar series, not an eighths count	2026-09-15
ACI 318 max slab spacing	Lesser of 3× slab thickness or 450 mm (18 in.)	ACI 318, via firgelliauto.com, "Rebar Spacing Calculator"	2026-03-08
Common unengineered residential spacing	18–24 in. on-centre, both directions	rakenapp.com (undated) + procore.com (dated 2024-07-11, stale per this page’s own 365-day freshness rule)	2024-07-11 (older source)
ACI 318 lap-splice floor	12 in., flat — no bar-diameter term	ACI 318 §25.5.1, via CRSI Technical Note ETN-D-1-15	2015
ACI 318 splice classes	Class A = 1.0× ld · Class B = 1.3× ld	ACI 318 §25.5.1 / Table 25.5.2.1, via CRSI Technical Note ETN-D-1-15	2015
Bundled-bar lap-splice increase	+20% (3-bar bundle) · +33% (4-bar bundle)	crsi.org, "Lap Splices"	2026-09-11
Tension lap splice exclusion	Not permitted for #14 or #18 bars	CRSI Technical Note ETN-D-1-15; independently confirmed for ACI 318-19 by curiouscivileng.com	2015 / 2026-09-16
Standard rebar stock length	20 ft (30/40/60 ft also exist, not used as the default)	harrissupplysolutions.com	2026-09-11

Rounding rule

In-place bar count is (span ÷ spacing) + 1 per direction — the fence-post rule — floored before the +1.

Purchase count, in standard 20-foot sticks, is the in-place total linear feet × (1 + waste%), ceiled to whole sticks — you buy whole sticks, and the breakage allowance rounds up, not down.

Weight keys off the in-place linear feet only, never the purchase total — you weigh what’s in the work, not what you bought including offcuts.

Carry full precision through the whole chain; round once, at the render boundary (L1). A whole-stick count is the one exception — it is the true purchasable quantity, not a display rounding fed back into another calculation.

FAQ

How much rebar do I need for a slab?: Multiply the bar count each direction by the length it runs, then add both directions. At 18 in. on-centre spacing, a 10×10 ft slab needs 7 bars each way (10 ÷ 1.5, plus one for the edge) for 140 total linear feet, or 8 sticks of 20-ft rebar with 10% waste. Enter your own length, width, spacing and bar size above for the exact count.
Is it better to use rebar or wire mesh?: Rebar is tied into a precise grid and lap-spliced at specific points — used for slabs-on-grade, footings, walls and any structural member. Welded wire mesh is a pre-fabricated grid, faster to place, typically used for lighter-duty reinforcement like sidewalks, patios and thin toppings where placement speed matters more than exact bar spacing. This calculator computes rebar bar counts and lengths, not wire mesh sheet counts.
How much rebar do I need for a footing?: Footing rebar is specified by a structural drawing or local code, not a rule of thumb — bar size, count and cover vary with footing width, depth and soil conditions. Enter the footing’s length and the number of continuous bars it calls for to get the linear-feet and weight totals; a 40 ft footing with 4 continuous bars needs 160 ft of bar. This page does not recommend a bar size or count on its own.
What size rebar do I need for a driveway?: A driveway slab follows the same grid method as any slab — #3 or #4 bar at 18–24 in. on-centre is common unengineered residential practice, and ACI 318 caps primary spacing at the lesser of 3× the slab thickness or 18 in. Enter the poured area and your chosen spacing above to get the bar count and total length; thinner driveways are also commonly reinforced with welded wire mesh instead.
What is a rebar lap splice?: A lap splice overlaps two rebar ends so they act as one continuous bar. Concrete’s code minimum is the greater of a flat 12-inch floor or a class multiple (1.0× for Class A, 1.3× for Class B) of the development length your structural drawing specifies — never the floor alone if your development length would fall short. Tension lap splices are not permitted at all for #14 or #18 bars.
What size rebar do I need?: Bar size is set by a structural drawing or local code for anything load-bearing — #3 or #4 is common for residential slabs, larger sizes for footings, walls and structural members. Sizing follows eighths of an inch for #3–#8 (a #4 bar is 4/8 in. = 0.500 in. diameter), but breaks that pattern at #9 and up, which are sized to match an older square-bar’s cross-sectional area instead.
What is the difference between grade 40 and grade 60 rebar?: Grade refers to yield strength, not size — Grade 40 yields at 40,000 psi, Grade 60 at 60,000 psi. Grade 60 is the common default for residential and commercial work today; Grade 40 is more common in older construction and some lighter applications. This calculator’s bar count and weight totals are identical regardless of grade — grade changes strength, not the quantity of bar needed.
How much does rebar weigh per foot?: Weight rises with bar size: 0.376 lb/ft for #3, 0.668 lb/ft for #4, 1.043 lb/ft for #5, up to 13.60 lb/ft for #18 — the full table is in the methodology section below. This calculator multiplies your entered length by the chosen size’s weight automatically; weight is always based on the in-place length, never the purchase length including waste.