Concrete · Reinforcement

Rebar spacing calculator

How many bars, running which way, and how much they weigh. Plus the two numbers most rebar calculators skip: the ties at every grid crossing and the chairs that hold the mat off the ground. Imperial or metric.

Detail A — Two-way mat

Mat — raw counts

Bars total—

Bars along length—

Bars along width—

Grid intersections—

To order (with waste, rounded up)

— Total bar — lin ft

20 ft sticks—

Steel weight—

Ties & supports

Ties (alternate)—

Tie wire—

Chairs / supports—

01 How the math works

A two-way mat is bars running one direction crossed by bars running the other, on a grid. The count in each direction is the span the bars cross divided by the on-center spacing, plus one for the bar at the far edge. Every bar is held back from each edge by the cover distance, so each is shorter than the slab by twice that clearance.

Bars across a span = floor((span − 2 × edge clearance) ÷ spacing) + 1
Bar length = slab dimension − 2 × edge clearance
Total steel = Σ (bars × bar length), both directions
Grid crossings = bars one way × bars the other way

Bar size is picked by number. The number is the diameter in eighths of an inch, so a #4 is 4/8 = 1/2 in and weighs 0.668 lb/ft under ASTM A615 — the one piece of rebar trivia worth keeping. Multiply the total linear feet by that per-foot weight for the tonnage the yard bills you for.

02 Worked example

Take the backyard patio slab: 10 ft × 12 ft × 4 in (3.05 m × 3.66 m × 100 mm). Reinforce it with a #4 mat at 18 in (457 mm) on center each way, bars held 3 in (76 mm) back from every edge — the same 3 in as the ACI 318 cover for concrete cast against ground.

Bars running the 12 ft direction, spaced across the 10 ft width: floor((9.5 ÷ 1.5)) + 1 = 7 bars, each 11.5 ft (3.51 m). The other way: floor((11.5 ÷ 1.5)) + 1 = 8 bars, each 9.5 ft (2.90 m). That is 15 bars, 156.5 lin ft (47.70 m), or 104.5 lb (47.4 kg) of #4.

Grid crossings: 7 × 8 = 56. Tie the perimeter plus every other interior crossing and you tie 41 of them, not 56. On 3 ft (0.9 m) support spacing the mat rides on 5 × 4 = 20 chairs.

03 When this calculator is wrong

The linear-foot total is honest about the steel in the slab and quiet about three things that decide what you actually carry home and how the mat behaves in the pour.

Stock length wastes steel the foot count hides. Rebar ships in 20 ft (6.10 m) sticks. An 11.5 ft bar leaves an 8.5 ft offcut — too short for a second 11.5 ft bar. The 9.5 ft bars nest two to a stick (19 ft); the 11.5 ft bars don't. Cut nesting turns the example mat into 11 sticks, not the 9 the linear-foot math with 10% waste suggests. Order by bar length, not by total feet.
The mat has to sit up on chairs, or the steel does nothing. Reinforcement laid on the subgrade and hooked up during the pour ends up near the bottom, out of position. Chairs at 3–4 ft (0.9–1.2 m) on center hold it at the 3 in (76 mm) cover ACI 318 requires against ground. This count assumes a flat grid; thin #3 sags between supports and soft ground wants a row more.
Ties are not structural, so don't tie all 56. Ties hold the grid steady while concrete goes in — nothing more. The perimeter-plus-alternate pattern (41 here) is enough for a mat that isn't heavily walked. Move toward tying every crossing only if the steel gets trafficked before the pour.
Lap splices are not in the total. Bars longer than the stock length splice with an overlap ACI sets from the bar size and mix strength. This calculator sizes bars to fit the slab and adds no laps, so a mat with bars over 20 ft needs the lap length added per splice.

04 What to do with the result

Buy the bars by length, not by the foot: count the 11.5 ft and 9.5 ft bars, then work out sticks from how they nest into 20 ft stock. Add a roll of tie wire and the chair count with a few spare, because chairs get stepped on. Round the whole order up — the cost of returning unused steel is small next to a stopped tie-off waiting on one more stick.

One editorial position worth stating: if you are reaching for rebar because you are pouring a thicker slab, put the steel in — don't just add concrete. Going from a 4 in slab to 6 in without reinforcement roughly doubles the material cost and adds almost no structural benefit; a thicker plain slab is a thinner slab that weighs more. The exception is a driveway taking heavy vehicles, where 5–6 in with a proper mat is the right call. Thickness and reinforcement go together, not one instead of the other.

05 Common questions

How much rebar do I need for a concrete slab?: Set the slab size and the bar spacing, and the count follows: bars each way are the span divided by the spacing, plus one. The 10 × 12 ft example with #4 at 18 in on center takes 15 bars — 156.5 lin ft (47.70 m), about 104.5 lb (47.4 kg) of steel.
What size rebar goes in a 4 inch slab?: #3 and #4 are the common residential choices for a slab on grade. The size and spacing are a structural decision set by the load and the local code, not a fixed rule — for anything carrying real weight, confirm the mat with an engineer or the building department.
How far apart should rebar be spaced?: Spacing is a design input, not a constant. Residential slab mats commonly run 12–18 in (305–457 mm) on center; tighter spacing means more steel and more crack control. The code or the engineer sets the figure — the calculator takes whatever you enter.
How many rebar ties do I need?: One per crossing you choose to tie. The full grid is bars-one-way × bars-the-other; you rarely tie all of them. The perimeter-plus-alternate pattern is standard field practice and comes to roughly half — 41 of the 56 crossings on the example mat.
Do I need chairs under the rebar, and how high?: Yes — chairs hold the mat at cover so the steel lands at the design depth instead of the subgrade. For a slab cast against ground, ACI 318 wants 3 in (76 mm) of cover, so the chair height matches the depth you want the bars to sit at. Space supports 3–4 ft (0.9–1.2 m) on center.