Question 1

How much rebar do I need for a slab?

Accepted Answer

Divide each span by the grid spacing and add one to get the bar count in that direction, then multiply each count by the length it runs. A 20 × 12 ft slab on a 12-inch grid needs 13 bars at 20 ft plus 21 bars at 12 ft — about 512 linear feet.

Question 2

What size rebar for a concrete slab?

Accepted Answer

#3 or #4 covers most residential slabs, patios, and driveways. #4 (half-inch) is the usual default. Footings and structural work often call for #5 and up, and that is an engineering decision rather than a preference.

Question 3

What does the rebar number mean?

Accepted Answer

The number is the diameter in eighths of an inch. A #4 bar is 4/8 — half an inch. A #8 is a full inch. Weight rises with the square of diameter, so a #8 weighs four times a #4 per foot.

Question 4

How far apart should rebar be in a slab?

Accepted Answer

Twelve to eighteen inches on centre in each direction is typical for residential work. Closer spacing distributes cracking more finely rather than preventing it. Your local code or engineer sets the actual figure.

Question 5

How much should rebar overlap?

Accepted Answer

ACI 318 requires a lap splice of at least 40 bar diameters for most conditions — 20 inches for a #4 bar, 25 for a #5. Tie the overlapping bars together; do not simply lay them alongside each other.

Question 6

Where in the slab should the rebar sit?

Accepted Answer

In the middle third of the thickness for a slab in pure bending, which for a 4-inch slab means roughly 2 inches up. Rebar lying on the sub-grade does nothing — use chairs or dobies. This is the most common rebar mistake in DIY pours.

Question 7

Do I need rebar or is wire mesh enough?

Accepted Answer

Mesh controls shrinkage cracking in a lightly loaded slab; rebar carries structural load. A patio may be fine with mesh, a driveway that will see vehicles usually wants rebar, and anything carrying a wall needs engineered reinforcement.

Question 8

How much does rebar weigh?

Accepted Answer

#3 is 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044, and #8 is 2.67. A 20-foot #4 bar weighs about 13.4 lb, which is what makes a large slab order a delivery rather than a pickup-truck errand.

Rebar Calculator

Inputs

Result

Formula

References

Related tools & materials

Calculators

Don't forget

Also useful

Frequently asked questions

Cite this page

Embed on your site