When choosing steel reinforcement for concrete construction, rebar size plays a critical role in determining structural strength and durability. Among all standard rebar sizes, #4 rebar is one of the most commonly used worldwide. Many buyers, contractors, and engineers ask the same question: what size is #4 rebar?

This straightforward size guide covers all essential details about #4 rebar, such as its diameter, weight, metric equivalent, grades, and practical uses in real-world scenarios.

What Size Is #4 Rebar?

#4 rebar has a nominal diameter of 1/2 inch (12.7 mm).

In the U.S. rebar sizing system, the number refers to the diameter measured in eighths of an inch:

#4 rebar = 4/8 inch

4/8 inch = 1/2 inch (12.7 mm)

This measurement system is standardized and commonly utilized across North America and numerous global markets.

What Size Is #4 Rebar

#4 Rebar Dimensions and Weight

Below are the standard physical specifications of #4 rebar:

Specification

Value

Nominal Diameter

1/2 inch (12.7 mm)

Cross-Sectional Area

0.20 in² (129 mm²)

Weight per Foot

0.668 lb/ft

Weight per Meter

0.994 kg/m

Standard Lengths

20 ft, 40 ft

Surface Type

Deformed (ribbed)

Metric Equivalent of #4 Rebar

In metric-based countries, #4 rebar is approximately equivalent to 12mm–13mm rebar, commonly labeled as D13.

Approximate comparison:

#4 rebar ≈ 12 mm / 13 mm rebar

Used extensively in Asia, Africa, and the Middle East

Because of slight differences between standards, it is always recommended to confirm project specifications before substitution.

Common Grades of #4 Rebar

#4 rebar is available in several strength grades depending on the standard:

Common Grades

Grade 40 – Yield strength 40,000 psi

Grade 60 – Yield strength 60,000 psi (most common)

Seismic Grade (A706) – Improved ductility for earthquake zones

International Standards

ASTM A615 / A706

BS 4449

AS/NZS 4671

JIS / GB standards

Grade 60 #4 rebar is predominantly utilized in both residential and commercial building projects because of its outstanding strength-to-cost efficiency.

Typical Applications of #4 Rebar

Thanks to its versatility, #4 rebar is suitable for a wide range of construction projects.

Common uses include:

Concrete slabs and floors

Residential foundations

Driveways and sidewalks

Beams and columns (light to medium load)

Retaining walls

Small bridges and infrastructure projects

Its moderate diameter makes it easy to cut, bend, and install on-site.

Applications of #4 Rebar

Why #4 Rebar Is So Popular

There are several reasons why #4 rebar is one of the most frequently specified sizes:

Meets most residential building codes

Cost-effective compared to larger rebar sizes

Strong enough for many structural applications

Widely available from global rebar suppliers

Easy to handle and fabricate

For many contractors, #4 rebar provides the ideal balance between performance and efficiency.

#4 Rebar vs Other Common Sizes

Rebar Size

Diameter

Typical Use

#3 Rebar

3/8 in (9.5 mm)

Light slabs, patios

#4 Rebar

1/2 in (12.7 mm)

Slabs, foundations

#5 Rebar

5/8 in (15.9 mm)

Beams, columns

#6 Rebar

3/4 in (19.1 mm)

Heavy structural work

#4 rebar is often selected when #3 is too light and #5 is unnecessary or too costly.

Buying #4 Rebar: What to Check

When sourcing #4 rebar, buyers should confirm:

Diameter tolerance

Steel grade and standard

Length and bundling

Surface deformation quality

Mill test certificate (MTC)

Reliable suppliers will provide full documentation and consistent quality.


So, what size is #4 rebar?

In simple terms, #4 rebar measures 1/2 inch (12.7 mm) in diameter, making it one of the most practical and widely used reinforcement bars in concrete construction.

From slabs and foundations to beams and walls, #4 rebar delivers dependable strength, easy installation, and excellent value—making it a top choice for contractors and builders around the world.