Rebar Diameter & Spacing Calculator for Concrete Beams

Last updated: 2026-09-26

Rebar Diameter & Spacing Calculator for Concrete Beams — Calculate the required rebar diameter and spacing for a reinforced concrete beam based on span, load, and concrete class.
Inputs
m
kN/m
m
m
m
Result
Enter values and press Calculate

How to Use This Calculator

This rebar diameter and spacing calculator is designed for structural engineers, architects, and construction professionals who need to quickly determine the reinforcement requirements for reinforced concrete beams. To use it, simply enter the beam span (in meters), the uniform load (in kN/m), select the concrete class and steel grade, and provide the beam dimensions and cover. The calculator will then compute the required steel area and recommend a suitable rebar diameter and spacing. It also returns the recommended stirrup diameter and spacing for shear reinforcement.

Formula and Methodology

The calculator uses standard reinforced concrete design principles based on Eurocode 2. The design bending moment for a simply supported beam under uniform load is M = wL²/8, where w is the load per unit length and L is the span. The required area of steel is As = M / (fyd × z), where fyd is the design yield strength of the steel and z is the lever arm. The lever arm is calculated as z = d(0.5 + √(0.25 - K/0.9)), with K = M / (b d² fcd). The effective depth d is the beam height minus cover and half the bar diameter. Minimum reinforcement is also considered. The calculator then iterates through standard bar diameters to find one that provides sufficient area with a reasonable number of bars and spacing. Shear reinforcement is based on the shear force and concrete shear capacity, with stirrup spacing limited to 0.75d or 300 mm.

Practical Examples

For example, consider a simply supported beam with a span of 6 meters, a uniform load of 20 kN/m, concrete class C25/30, beam width 300 mm, height 500 mm, and cover 25 mm. The design moment is M = 20 × 6² / 8 = 90 kNm. Using fcd = 25/1.5 = 16.67 MPa and fyd = 500/1.15 = 435 MPa, the required steel area is approximately 5.5 cm². The calculator might select 3 bars of 16 mm diameter (area = 6.03 cm²) with a spacing of about 120 mm. For shear, it might suggest 8 mm stirrups at 150 mm spacing. Another example: a smaller beam with a 3-meter span and 10 kN/m load would require less steel, perhaps 2 bars of 12 mm.

Tips and Best Practices

Always double-check that the selected rebar diameter and spacing comply with local building codes. Ensure that the concrete cover is appropriate for the exposure conditions. Remember that the calculator provides a preliminary design; a detailed analysis may be required for complex structures. When using the terminology, note that in Spanish-speaking countries, beams are called 'vigas' and footings 'sapatas', while in Portuguese, 'viga' and 'sapatas' are also used. This helps avoid confusion when searching for design aids. Finally, consider constructability: very small spacing or large diameters may be difficult to place. Use the results as a starting point for your design.

Related Calculators

FAQ

What is the difference between Spanish and Portuguese terms for beams and footings?

In Spanish, 'viga' means beam and 'sapatas' means footings. In Portuguese, 'viga' also means beam, while 'sapatas' refers to footings. The calculator includes both to help users searching in either language.

How do I choose the right rebar diameter?

The calculator selects the smallest diameter that satisfies the required steel area with a practical number of bars (up to 8) and adequate spacing. Typically, diameters from 10 mm to 20 mm are used for beams.

What is the maximum spacing for stirrups?

Stirrup spacing should not exceed 0.75 times the effective depth or 300 mm, whichever is smaller, as per common design codes.

Can I use this for footings (sapatas)?

The calculator is primarily for beams (vigas), but the reinforcement principles are similar for footings. For footings, the design is based on bending and shear due to soil pressure; you can use the span as the footing width and load as the soil pressure.