Construction

Concrete Beam Sizing Calculator (h & b from Span)

Q: Is the L/12 rule approved by ACI 318?

ACI 318-19 Table 9.3.1.1 specifies a *minimum* h of L/16 for simply supported non-prestressed beams. The L/12 rule is more conservative — about 33 % deeper — so it always exceeds the code minimum for deflection control. Many engineers prefer it because the extra depth handles heavier loads, vibration, and future renovations without triggering explicit deflection calculations.

Q: Can I use this for T-beams or L-beams in a slab-beam floor system?

Yes, as a first pass. T-beams have an effective flange that increases moment capacity, so the required web depth is often 10–20 % less than a rectangular beam. ACI 318-19 Section 6.3 defines effective flange widths. After using this calculator for initial sizing, a T-beam analysis will typically allow you to reduce h somewhat, saving concrete and dead load.

Q: What is the minimum beam width allowed by ACI 318?

ACI 318-19 does not impose a universal minimum width, but Section 26.10.1 ties minimum cover and bar-spacing requirements to the width. In practice, widths below 20 cm (8 in) rarely accommodate two layers of bars plus stirrups and required concrete cover (typically 4 cm / 1.5 in for interior exposure per ACI 318-19 Table 20.6.1.3.1). Most engineers use 20 cm as an absolute practical minimum.

Q: How does span length affect required reinforcement?

As span grows, bending moment increases with the square of the span (M = wL²/8 for uniform load). For a given cross-section depth, doubling the span quadruples the required moment capacity, pushing the reinforcement ratio up. ACI 318-19 caps the maximum tension steel ratio at ≈ 0.75ρ_balanced (roughly 2–3 % for Grade 60 bars in f'c = 28 MPa concrete) to ensure ductile failure. Beyond that limit, depth or width must increase.

Q: How much does a 6 m concrete beam weigh?

Using h = 50 cm and b = 25 cm, section area = 1,250 cm² = 0.125 m². For a 6 m beam: volume = 0.125 × 6 = 0.75 m³. Normal-weight concrete weighs 2,400 kg/m³, so the beam weighs roughly 0.75 × 2,400 = **1,800 kg (≈ 1.8 metric tons)**. Adding Grade 60 rebar (typically 80–120 kg for a beam this size) brings the total to ≈ 1,900 kg. This dead load must be included in slab and column design.

Q: Does the L/12 rule apply to post-tensioned or prestressed concrete beams?

No. Prestressed and post-tensioned beams can span significantly farther with shallower depths because the prestress force counteracts dead-load deflection. ACI 318-19 Chapter 9 and PCI Design Handbook (7th Ed.) provide separate depth ratios — typically L/24 to L/36 for simply supported PT beams under normal loads. Using L/12 for a PT beam would oversize the section considerably.

Q: When must I hire a licensed structural engineer instead of using this calculator?

This tool provides preliminary proportions only — not a structural design. A licensed Professional Engineer (PE) is legally required by IBC Section 1604.1 for all structural elements of buildings subject to permits. You must engage a PE whenever actual loads, seismic zone (ASCE 7-22), soil conditions, or code compliance (ACI 318, AISC, etc.) must be formally verified. Treat the L/12 output as a starting point for your engineer, not a permit-ready specification.

Find the right concrete beam height and width for your span in seconds. Uses the L/12 depth rule (ACI 318-19) with a reference table for spans 3–12 m. Free, no sign-up.

Data verified · June 2026
Edited by Martín Rodríguez
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Sizing a concrete beam starts with two simple ratios used by structural engineers worldwide. This calculator applies the L/12 depth rule: beam height h = span ÷ 12 (in cm when span is in metres × 100), and beam width b = h ÷ 2. Enter your clear span and get the preliminary cross-section instantly — plus a reference table for the most common spans.

For a 6 m span: h = 50 cm, b = 25 cm. For a 9 m parking structure beam: h = 75 cm, b = 38 cm. These proportions are conservative relative to the ACI 318-19 minimum (L/16 for simply supported beams) and satisfy typical deflection limits before any reinforcement calculation.

When to use this calculator

Preliminary sizing of simply supported floor beams in a reinforced concrete building before full structural analysis begins.
Estimating formwork lumber quantities and concrete volume for a residential or light commercial beam during early cost budgeting.
Checking whether an existing beam's recorded dimensions are proportionally reasonable against the L/12 rule before commissioning a detailed structural audit.
Classroom or exam problem verification—confirming hand-calculated beam depths for spans ranging from 3 m to 12 m without manual arithmetic.
Pre-design coordination between architect and structural engineer to set floor-to-floor heights and slab soffit elevations in a multi-story RC frame.

ACI 318-19 vs. L/12 Rule: Minimum Beam Depth by Support Condition

Support Condition	ACI 318-19 Min. h (Table 9.3.1.1)	L/12 Rule h	Safety Margin vs. ACI Min.
Simply supported	L/16	L/12	+33% deeper
One end continuous	L/18.5	L/12	+54% deeper
Both ends continuous	L/21	L/12	+75% deeper
Cantilever	L/8	L/12	−33% (no aplicar L/12)

Fuente: ACI 318-19 Table 9.3.1.1 — American Concrete Institute (2019). L/12 es regla preliminar; diseño final requiere verificación por carga según ACI 318-19 Capítulo 9.

How it works

How Concrete Beam Sizing Works

The calculator uses two chained empirical formulas derived from decades of reinforced-concrete practice and codified in ACI 318-19:

Step 1 — Beam Height (depth):
  h (cm) = L (m) × 100 ÷ 12
  → rounds to nearest whole centimetre

Step 2 — Beam Width:
  b (cm) = h ÷ 2
  → rounds to nearest whole centimetre

Why L/12?
ACI 318-19 Table 9.3.1.1 provides minimum depths for non-prestressed beams to avoid explicit deflection calculations:

Support Condition	ACI 318-19 Minimum h	L/12 Depth	Safety Margin
Simply supported	L/16	L/12	+33 % deeper
One end continuous	L/18.5	L/12	+54 % deeper
Both ends continuous	L/21	L/12	+75 % deeper
Cantilever	L/8	L/12	–33 % (do NOT use L/12 for cantilevers)

Using L/12 adds a comfortable margin for heavier loads, vibration control, and crack-width management — making it the standard rule of thumb for preliminary design.

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Quick Reference Table: Beam Sizes by Span

Clear Span (m)	h = L/12 (cm)	b = h/2 (cm)	Section Area (cm²)	Concrete Volume/m (m³)
3.0	25	13	325	0.033
4.0	33	17	561	0.056
5.0	42	21	882	0.088
6.0	50	25	1,250	0.125
7.0	58	29	1,682	0.168
8.0	67	33	2,211	0.221
9.0	75	38	2,850	0.285
10.0	83	42	3,486	0.349
12.0	100	50	5,000	0.500

> Concrete volume per linear metre = h (m) × b (m) × 1 m length.

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Worked Cases

Case 1 — Residential Floor Beam, 6 m Span

Clear span L = 6.00 m

h = 600 ÷ 12 = 50 cm

b = 50 ÷ 2 = 25 cm

Section area = 1,250 cm²; concrete per metre ≈ 0.125 m³ (≈ 300 kg concrete + ≈ 90 kg rebar)

Typical orientation: 4 × Ø16 mm bottom bars + Ø8 stirrups @ 20 cm

Case 2 — Parking Structure Beam, 9 m Span

Clear span L = 9.00 m

h = 900 ÷ 12 = 75 cm

b = 75 ÷ 2 = 37.5 cm → 38 cm

Section area = 2,850 cm²; concrete per metre ≈ 285 L

At this depth a T-beam (with integral slab flange) is often more efficient; the rectangular section is a conservative envelope.

Case 3 — Bridge Approach Slab Beam, 12 m Span

Clear span L = 12.00 m

h = 1,200 ÷ 12 = 100 cm (1.0 m depth)

b = 100 ÷ 2 = 50 cm

At this scale, prestressed or post-tensioned design is strongly recommended; the L/12 rule flags the depth requirement but full design must follow AASHTO LRFD.

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Common Mistakes to Avoid

1. Confusing total depth with effective depth (d). The formula gives total height h. Effective depth d = h − cover − stirrup − ½ main bar (typically d ≈ h − 6 to 8 cm). Using h instead of d in moment-capacity formulas overstates strength.

2. Applying L/12 to cantilevers. Cantilevers require minimum h = L/8 per ACI 318-19; using L/12 undersizes them by up to 33 %.

3. Forgetting minimum fire-rating widths. ACI 318-19 Section 26.10 and IBC Table 722.5.2(5) require minimum beam widths (often 200 mm / 20 cm) for 2-hour fire resistance.

4. Skipping load verification. The L/12 rule assumes typical superimposed live loads of 2.0–4.8 kPa (40–100 psf). Warehouse floors or transfer beams can carry 10–20 kPa; full design per ACI 318-19 Chapter 9 is mandatory.

5. Using centreline-to-centreline instead of clear span. The rule uses clear span (face-to-face of supports), not the centreline distance.

Worked Example — 6 m Residential Floor Beam

Clear span: L = 6 m

h = 6 m × 100 ÷ 12 = 600 ÷ 12 = 50 cm

b = 50 ÷ 2 = 25 cm

Cross-section: 50 × 25 cm (area = 1,250 cm²)

Concrete volume per linear metre: 0.50 × 0.25 = 0.125 m³

50 × 25 cm beam — ACI 318-19 compatible preliminary section

Frequently asked questions

What does the L/12 rule mean for concrete beam sizing?

The L/12 rule means the beam height h (in metres) equals the clear span L divided by 12. So for a 6 m span, h = 0.50 m (50 cm). The width b is typically set at h/2. This rule is a conservative preliminary-design guide — more stringent than ACI 318-19's minimum of L/16 for simply supported beams — providing a safety margin for heavier loads and deflection control.

Is the L/12 rule approved by ACI 318?

ACI 318-19 Table 9.3.1.1 specifies a minimum h of L/16 for simply supported non-prestressed beams. The L/12 rule is more conservative — about 33 % deeper — so it always exceeds the code minimum for deflection control. Many engineers prefer it because the extra depth handles heavier loads, vibration, and future renovations without triggering explicit deflection calculations.

What concrete strength (f'c) does the L/12 rule assume?

ACI 318-19 Table 9.3.1.1 span-to-depth ratios are calibrated for normal-weight concrete (≈ 2,400 kg/m³ / 150 pcf) and Grade 60 reinforcement (fy = 420 MPa / 60,000 psi). For lightweight concrete (density < 1,840 kg/m³), ACI 318-19 requires multiplying the minimum depth by a factor of 1.09–1.65, so the L/12 starting depth would need upward adjustment.

Can I use this for T-beams or L-beams in a slab-beam floor system?

Yes, as a first pass. T-beams have an effective flange that increases moment capacity, so the required web depth is often 10–20 % less than a rectangular beam. ACI 318-19 Section 6.3 defines effective flange widths. After using this calculator for initial sizing, a T-beam analysis will typically allow you to reduce h somewhat, saving concrete and dead load.

What is the minimum beam width allowed by ACI 318?

ACI 318-19 does not impose a universal minimum width, but Section 26.10.1 ties minimum cover and bar-spacing requirements to the width. In practice, widths below 20 cm (8 in) rarely accommodate two layers of bars plus stirrups and required concrete cover (typically 4 cm / 1.5 in for interior exposure per ACI 318-19 Table 20.6.1.3.1). Most engineers use 20 cm as an absolute practical minimum.

How does span length affect required reinforcement?

As span grows, bending moment increases with the square of the span (M = wL²/8 for uniform load). For a given cross-section depth, doubling the span quadruples the required moment capacity, pushing the reinforcement ratio up. ACI 318-19 caps the maximum tension steel ratio at ≈ 0.75ρ_balanced (roughly 2–3 % for Grade 60 bars in f'c = 28 MPa concrete) to ensure ductile failure. Beyond that limit, depth or width must increase.

How much does a 6 m concrete beam weigh?

Using h = 50 cm and b = 25 cm, section area = 1,250 cm² = 0.125 m². For a 6 m beam: volume = 0.125 × 6 = 0.75 m³. Normal-weight concrete weighs 2,400 kg/m³, so the beam weighs roughly 0.75 × 2,400 = 1,800 kg (≈ 1.8 metric tons). Adding Grade 60 rebar (typically 80–120 kg for a beam this size) brings the total to ≈ 1,900 kg. This dead load must be included in slab and column design.

Does the L/12 rule apply to post-tensioned or prestressed concrete beams?

No. Prestressed and post-tensioned beams can span significantly farther with shallower depths because the prestress force counteracts dead-load deflection. ACI 318-19 Chapter 9 and PCI Design Handbook (7th Ed.) provide separate depth ratios — typically L/24 to L/36 for simply supported PT beams under normal loads. Using L/12 for a PT beam would oversize the section considerably.

When must I hire a licensed structural engineer instead of using this calculator?

This tool provides preliminary proportions only — not a structural design. A licensed Professional Engineer (PE) is legally required by IBC Section 1604.1 for all structural elements of buildings subject to permits. You must engage a PE whenever actual loads, seismic zone (ASCE 7-22), soil conditions, or code compliance (ACI 318, AISC, etc.) must be formally verified. Treat the L/12 output as a starting point for your engineer, not a permit-ready specification.

Sources & references

Methodology & trust

Editorial

Construction calculator with its formula verified automatically against ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute, per our editorial policy and methodology.

Updates

Updated: June 2026. Parameters are verified periodically against the cited sources.

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Indicative results. For critical decisions, consult a professional.

📌 How to cite this calculator

Rodríguez, M. (2026). Concrete Beam Sizing Calculator (h & b from Span). Hacé Cuentas. https://hacecuentas.com/en/concrete-beam-sizing-dimensions

Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.

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