Rebar Calculator

Bar sizes and weights

Bar	Diameter	Weight (lb/ft)	20 ft bar	Typical use
#3	3/8 in	0.376	7.5 lb	Slab mesh replacement, curbs
#4	1/2 in	0.668	13.4 lb	Driveways, footings, slabs
#5	5/8 in	1.043	20.9 lb	Structural slabs, walls
#6	3/4 in	1.502	30.0 lb	Foundations, heavy loads

Choosing spacing

Application	Typical grid	Notes
Patio / shed slab (4 in)	#3 @ 18 in OC	Mesh is often used instead
Driveway (5–6 in)	#4 @ 12–18 in OC	Two mats for heavy vehicles
Foundation slab	#4 @ 12 in OC	Per engineered plan
Strip footing	2 × #4 continuous	3 in cover, lapped at corners

How the count works

Lines in one direction = floor(width × 12 ÷ spacing) + 1, and the same for the other direction. Each line runs the full length of the slab, so total linear feet is the sum of both directions, plus waste for cuts and splices.

Splices are where most estimates go wrong: when a run is longer than the 20 ft bar, each joint needs an overlap — commonly 40 bar diameters, about 20 inches for #4. The waste field covers those laps and offcuts.

Placement rules that matter

Cover: 3 inches of concrete over rebar in ground contact, 1½–2 inches for interior slabs.
Chairs: one chair per 4 ft each way keeps the mat at mid-depth; rebar lying on the ground does nothing.
Laps: stagger splices rather than stacking them in one line.
Tie wire: 16-gauge annealed wire, tied at every other intersection.

Why 40 bar diameters is a rule, not a code value

Splice length is not a fixed multiple of bar diameter. It comes from the development length, which depends on bar size, concrete strength, cover, spacing, and whether the bar is coated or confined. ACI 318 then defines two splice classes: Class A at 1.0 times the development length and Class B at 1.3 times, and most residential work needs Class B because the steel area provided rarely meets the conditions for Class A.

Forty bar diameters is a field approximation that lands conservatively for #3 to #5 bars in 3,000 to 4,000 psi concrete at normal cover. It gets you a safe number quickly, which is why it is repeated everywhere. It is not a minimum you can rely on for #6 and larger, for low-strength mixes, or for epoxy-coated bar in thin sections. Where there is an engineered plan, the plan wins.

Where the linear footage actually goes

A 30 × 30 slab with #4 at 18 inches each way looks like 1,260 linear feet: 21 lines per direction, 30 feet long, two directions. That is 63 twenty-foot sticks. Order that and you will run out.

Every 30-foot line is longer than a 20-foot bar, so each one needs two bars and a lap. With a 20-inch lap, each line consumes 40 feet of bar to cover 31.7 feet, and the 8.3 feet left over is an offcut you cannot count on using. The real count is 42 lines × 2 bars, or 84 sticks — a third more than the naive figure.

That gap is the entire reason reinforcement estimates miss. The number of bars is arithmetic; the number of laps is the estimate. On a 30-foot run with 20-foot stock you pay a lap per line. On a 40-foot run you pay two.

Weight decides how you get it home

#4 weighs 0.668 lb per foot, so those 84 sticks are 1,680 feet and about 1,122 lb. That is a trailer load, not a trunk load, and it is the point at which people discover that rebar is sold in bundles and delivered for a reason.

Chairs are the other item that gets forgotten. At one chair every 4 feet each way on a 900 square foot slab, that is roughly 56 supports — enough that buying a box of 100 is cheaper than two trips.

Five things an inspector checks

Cover. Three inches where concrete is cast against earth, less where it is not.
Position. The mat sits at mid-depth on chairs. Rebar lying on the ground or the vapor barrier contributes nothing.
Laps. Staggered rather than stacked in one line across the slab.
Size and spacing. Matching the plan, measured rather than assumed.
Clearance from forms. Bar touching formwork leaves no cover and rusts through at the edge first.

Frequently asked questions

What spacing should I use?

#3 or #4 at 12–18 inches on center for residential slabs; 12 inches for driveways and structural work. Use the engineered plan when there is one.

How much does #4 rebar weigh?

0.668 lb per foot — about 13.4 lb for a 20 ft stick.

How long should splices be?

40 bar diameters is the common rule: 20 inches for #4, 25 inches for #5. Codes can require more based on concrete strength.

Rebar or wire mesh?

Mesh is cheaper and fine for light slabs on good base; rebar is required where loads are heavier or the slab is structural. Mesh often ends up at the bottom of the pour, which is where it helps least.

How much rebar do I need for a 30 x 30 slab?

About 84 twenty-foot sticks of #4 at 18 inches each way. The slab is 42 lines of 30 feet, and each line takes two bars because 30 feet exceeds the 20-foot stock length.

Why does the bar count come out higher than length divided by 20?

Because of laps. Every run longer than the stock length needs a splice, commonly 40 bar diameters, and the offcut at the end of each line usually cannot be reused.

How many chairs do I need?

One every 4 feet each way. On a 900 square foot slab that is roughly 56 supports, which is why buying a box of 100 beats making two trips.

Can I use 40 bar diameters for #6 bar?

Treat it as a rule of thumb only. Splice length comes from development length, which depends on concrete strength, cover, and confinement; for #6 and larger or low-strength mixes, use the engineered plan.

Note: Quantities are estimates. Structural reinforcement design belongs to a licensed engineer; this tool only does the arithmetic.

Last reviewed: October 30, 2026 · Sources: CRSI bar data; ACI 318 cover and splice provisions

Rebar quantity calculator