Rebar Calculator — Linear Feet & Bar Count

Count the linear feet of rebar in a slab grid from your on-center spacing, including a lap allowance and the bar count each way.

Confirm yield against your product’s bag/spec sheet and order a little extra (5–10%) for spillage, uneven subgrade and over-excavation. Bag yields and coverage vary by mix and brand.

Calculator

ft
ft
in
On center, e.g. 12–18"
Decimal: 0.10 = 10%
Total rebar (with 10% lap)215.6 linear ft
Bars running the length (12 ft each)8 bars
Bars running the width (10 ft each)10 bars
Grid subtotal (no lap)196.0 linear ft

A 12 × 10 ft grid at 16" on center is 8 + 10 bars — about 215.6 linear ft with lap. This is a quantity at your spacing, not a structural design.

Rebar for a flat slab is laid in a grid, and the quantity is pure geometry: how many bars fit each way at your spacing, times their length, plus a bit for the laps where bars overlap. This calculator does that and reports both the total linear feet to buy and the bar count running each direction, so you can order 20-foot sticks with confidence. It is a material quantity at the spacing you choose — not a structural design; a licensed engineer sets the bar size and spacing for load-bearing work.

Formula

Bars fit across the opposite dimension, then run the full length of their own:

bars along length = floor(width ÷ spacing_ft) + 1 (each length ft)

bars along width = floor(length ÷ spacing_ft) + 1 (each width ft)

linear ft = (bars_L × length + bars_W × width) × (1 + lap%)

Worked example

A 12 × 10 ft slab at 16 in on center (1.333 ft), 10% lap:

  • Bars along length = floor(10 ÷ 1.333) + 1 = 8, each 12 ft = 96 lf
  • Bars along width = floor(12 ÷ 1.333) + 1 = 10, each 10 ft = 100 lf
  • Subtotal = 196 lf × 1.10 = 215.6 linear ft

Spacing, laps & bar sizes

Residential flatwork commonly uses #3 or #4 bar at 12–18 in on center, or welded wire mesh as a lighter alternative — all labeled planning typicals, not an engineered spec. The lap allowance matters because bars are spliced end to end where the grid is longer than a stick; 10% covers a typical lap on a modest slab. Buy in standard 20-foot lengths, round the linear feet up, and add a few tie-wire rolls and chairs to hold the grid mid-slab. For anything load-bearing, defer the bar size, spacing and cover to a licensed engineer and your local code. See the rebar spacing reference.

Reference table

Labeled residential flatwork rebar typicals (NOT a structural design):

ItemTypical
Bar size#3 or #4
Spacing (o.c.)12–18 in
Lap / overlap10%

Frequently asked questions

How much rebar do I need for a 12x10 slab?
At 16 in on center with a 10% lap, about 216 linear feet — 8 bars running the length and 10 running the width. Change the spacing above to match your plan.
What spacing should rebar be in a slab?
Residential flatwork typically uses 12–18 in on center with #3 or #4 bar. That is a labeled planning typical; a licensed engineer sizes the bar and spacing for load-bearing slabs and footings.
Why add a lap or overlap percentage?
Rebar comes in fixed lengths (commonly 20 ft) and must overlap where bars splice end to end. The lap allowance — around 10% on a modest slab — covers that extra material so you don’t come up short.
Does this design the reinforcement?
No. It is a linear-foot quantity at the spacing you enter, not a structural design. Bar size, spacing, cover and lap length for load-bearing concrete are an engineer’s call under local code.

Related calculators

Concrete slab calculatorConcrete slab calculator for rectangular and L-shaped slabs: add two rectangles, pick a thickness and get cubic yards and bags with the formula shown.Gravel / sub-base calculatorGravel sub-base calculator: get cubic yards and tons for the compacted base under a slab or driveway, with a compaction allowance and the density shown.Driveway thickness & rebar referenceTypical concrete driveway thickness and rebar planning values — about 4 in for cars, 5–6 in for heavy vehicles, with #3–#4 rebar at 12–18 in on center.