Beam Deflection Chart, Quick Reference

Maximum allowable deflection equals beam span divided by the deflection ratio. Convert span to inches first, then divide by the ratio (180, 240, 360, or 480) to get allowable deflection in inches.

What Is Beam Deflection?

Deflection is a serviceability consideration, meaning it addresses comfort, appearance, and finish performance rather than the immediate risk of structural failure, though it is checked as a required part of a complete beam design.

Beam Deflection vs Beam Strength

These are two separate structural checks, and a beam must satisfy both.

Beam Deflection Limits Chart

Reference building code tables commonly list allowable deflection as L/360 for floors and plastered construction, L/180 for rafters with a slope greater than 3/12 and no finished ceiling attached, and L/240 for most other structural members, always subject to the applicable adopted code edition.

L/180 Deflection Chart

L/180 is a less restrictive ratio sometimes applied to members such as rafters without an attached finished ceiling. Always verify the applicable code and design condition before applying this ratio to a specific member.

L/240 Deflection Chart

This ratio is commonly referenced for general structural members and for certain total load conditions, subject to the applicable code.

L/360 Deflection Chart

L/360 is one of the most frequently referenced deflection ratios, particularly for floor systems.

L/480 Deflection Chart

Beam Deflection by Span

Allowable deflection scales directly with span, while actual deflection under a real load scales much faster, with span raised to the third or fourth power depending on load type. This range from 6 to 32 feet reflects common residential and light commercial wood beam spans.

The relative actual deflection trend illustrates the span to the fourth power relationship for a uniformly loaded simply supported beam with unchanged size, material, and load per foot, holding all other variables constant for illustration only.

Beam Deflection by Beam Size

Beam size alone does not determine deflection. It also depends on span, load, modulus of elasticity, moment of inertia, and support conditions.

Beam Deflection by Load

Numerical deflection values are only meaningful under clearly stated assumptions of span, size, material, and support condition. The categories below are organizational reference points, not universal deflection values.

Uniform Load vs Point Load Deflection

Two beams carrying the same total load can experience very different maximum deflection depending on how that load is distributed.

For a simply supported beam with the same total load, the uniform load case and the center point load case produce different deflection coefficients, 5/384 versus 1/48, and different span exponents in the general form, so they should never be assumed equal.

Beam Deflection Under Center Point Load

For a simply supported beam with a single concentrated load at midspan, maximum deflection occurs directly under the load, at the center of the span, and is calculated using the point load formula shown in the formula section below.

Beam Deflection Under Uniform Load

For a simply supported beam with a uniformly distributed load along its full length, maximum deflection occurs at the center of the span, calculated using the uniform load formula shown below.

Beam Deflection for Multiple Point Loads

Beams supporting multiple posts, walls, trusses, or other beams experience combined deflection effects from each individual point load and its specific location. These cases require superposition of individual load effects or direct structural analysis software rather than a single simplified formula, since the location and magnitude of each load changes the result.

Beam Deflection Formula

Deflection depends on load, span, modulus of elasticity, moment of inertia, and support condition together. Different support and load combinations require their own specific formula rather than one universal equation for every case.

These are the standard closed form deflection equations for common support and load cases from mechanics of materials references. w is uniform load per unit length, P is a concentrated point load, L is span, E is modulus of elasticity, and I is moment of inertia.

Beam Deflection Formula for Uniform Load

For the standard simply supported beam case under a uniformly distributed load, delta is deflection, w is the uniform load per unit length, L is span, E is modulus of elasticity, and I is moment of inertia. Deflection increases with the fourth power of span, making span the single most sensitive variable.

Beam Deflection Formula for Point Load

For a simply supported beam with a center point load, delta is deflection, P is the point load, L is span, E is modulus of elasticity, and I is moment of inertia. Deflection increases with the third power of span for this load case.

Maximum Beam Deflection

Maximum deflection occurs at midspan for a symmetrically loaded, simply supported beam, whether the load is uniform or a centered point load. For asymmetrical loading, such as an off-center point load or multiple unevenly spaced loads, maximum deflection can occur away from midspan and generally requires either superposition of standard cases or direct structural analysis.

Beam Deflection and Modulus of Elasticity (E)

Modulus of elasticity, E, is a direct measure of material stiffness.

Different materials and even different species and grades of the same material have different E values, which is why identical beam dimensions under identical loads can deflect differently depending on what the beam is made of.

Beam Deflection and Moment of Inertia (I)

Moment of inertia describes how a cross section’s shape resists bending, and it depends heavily on cross sectional geometry.

Because moment of inertia scales with the cube of depth (h) but only linearly with width (b), increasing beam depth has a dramatically larger effect on stiffness and deflection resistance than increasing width by the same amount.

Beam Depth vs Deflection

Because moment of inertia for a rectangular section increases with the cube of depth, a modest increase in beam depth can substantially reduce deflection under the same span and load, generally making depth the most efficient variable to adjust when deflection governs a design.

Beam Width vs Deflection

Increasing beam width also increases moment of inertia, but only linearly rather than cubically. Doubling width roughly doubles moment of inertia at a given depth, while doubling depth increases moment of inertia roughly eightfold, so width increases are a far less efficient way to control deflection compared with depth increases.

Beam Span vs Deflection

Deflection becomes increasingly important as span increases because of the strong nonlinear relationship between span and deflection.

For a uniformly loaded simply supported beam, deflection is proportional to span to the fourth power, meaning a doubling of span, with everything else held constant, increases deflection by a factor of sixteen. This relationship connects directly to sizing decisions covered in the Beam Size Chart and the LVL Span Chart, where longer spans commonly require deeper or stiffer members specifically to control deflection.

Beam Load vs Deflection

Increasing load generally increases deflection, since load appears directly in the numerator of every standard deflection formula. The exact relationship, whether linear with total load or scaled differently for distributed versus concentrated loading, depends on the specific structural configuration and load case involved.

Beam Deflection and Support Conditions

Support conditions significantly affect both the magnitude and location of maximum deflection, so the correct formula must always match the actual support condition, not be assumed from a simply supported case.

Simply Supported Beam Deflection Chart

The simply supported beam, with a pin support at one end and a roller support at the other, is the primary calculation model used for the simplest deflection examples on this page, including the uniform load and center point load formulas shown above. It is also the most common condition encountered in typical residential floor and roof beam applications.

Cantilever Beam Deflection Chart

For a simple reference case only, a cantilever with a point load at the free end deflects according to PL^3 / (3EI), while a cantilever with a uniform load along its length deflects according to a related but different formula. Both require the exact length, load, and fixity condition to apply correctly.

Continuous Beam Deflection

A continuous beam spans over three or more supports and develops negative moments over the interior supports, which generally reduces deflection compared with a series of independent simple spans carrying the same load. The exact reduction depends on the number of spans, their relative lengths, and the specific loading pattern, so a universal value cannot be given without defining the exact support and loading arrangement.

Beam Deflection by Material

Wood Beam Deflection Chart

Wood beam deflection depends on species, grade, the resulting E value, beam size, span, and load. Reference design values for sawn lumber, glulam, structural composite lumber, and other wood structural products are published in current design specification documents, and species and grade selection directly changes the E value used in every deflection calculation.

LVL Beam Deflection Chart

LVL deflection depends on depth, width, E value, span, load, and number of plies.

Glulam Beam Deflection Chart

Glulam stiffness depends on the specific combination symbol and manufacturer design values, since glulam beams are manufactured from laminated sawn lumber with properties that vary by layup. Glulam is frequently selected for long span applications specifically because of its high and predictable stiffness relative to comparable solid sawn sections.

Steel Beam Deflection Chart

Steel beam deflection follows the same standard formulas as wood, but with a much higher and more consistent E value.

See the Steel Beam Size Chart for steel section specific information relevant to deflection calculations.

Concrete Beam Deflection Chart

Beam Deflection, Live Load vs Total Load

These represent two different deflection checks that may use different allowable ratios.

The applicable criterion and ratio depend on the structural design and code context, and general span table guidance distinguishes strength loading from deflection considerations, noting that the governing load combination depends on the specific application rather than one universal rule.

Beam Deflection and Dead Load

Dead load contributing to deflection includes the beam’s own self weight, flooring, roofing, wall weight, ceiling finishes, and other permanent fixtures. Because dead load is present continuously, it contributes to both immediate and long term deflection, and pre-camber is sometimes used specifically to offset anticipated dead load deflection.

Beam Deflection and Live Load

Live load deflection results from variable, non-permanent loads such as occupants, furniture, and storage. Because live load is not always present at its maximum design value, live load deflection is often checked against a separate, sometimes more restrictive ratio than total load deflection.

Beam Deflection and Long Term Creep

Deflection is not always a single, fixed value measured immediately after loading.

Beam Deflection and Camber

Camber is an intentional upward curve built into a beam before it carries any load.

Some engineered beams, particularly longer span members, are manufactured with camber so that the beam approaches a level position once dead load deflection occurs in service. Camber offsets a portion of expected dead load deflection but does not eliminate the need for a full structural deflection design check under all applicable loads, since live load deflection still occurs on top of the cambered starting position.

Beam Deflection and Floor Vibration

Floor stiffness affects walking vibration and overall floor comfort, particularly for longer spans. A floor beam that satisfies a standard deflection limit can still feel bouncy or springy under foot traffic if vibration performance was not separately considered, since vibration serviceability and static deflection limits address related but distinct comfort concerns.

Beam Deflection and Cracking

Excessive beam movement can directly damage attached building finishes.

Movement beyond the finish material’s tolerance can contribute to drywall cracking, plaster cracking, tile cracking or debonding, flooring problems, and doors or windows binding or failing to operate properly. Deflection limits in building codes are specifically intended, in part, to help control this kind of finish related cracking and damage by limiting how much the supporting structure is allowed to move.

Beam Deflection and Building Finishes

Beam Deflection vs Beam Span

Two beams with the same span can have very different deflection depending on their other properties. For example, Beam A with a smaller depth, lower E value, and higher load will deflect considerably more than Beam B with greater depth, higher E value, and the same span, even though the span itself is identical for both.

Beam Deflection vs Beam Size

Increasing beam depth is often the most effective single way to reduce deflection, because moment of inertia scales with the cube of depth for a rectangular section. See the Beam Size Chart for depth options and how depth interacts with span, load, and material selection.

Beam Deflection vs LVL Size

Deeper LVL members can provide significantly greater stiffness than shallower members of the same width, following the same depth cubed relationship as solid sawn lumber, combined with LVL’s typically higher and more consistent E value. See the LVL Span Chart for manufacturer specific depth and design value information.

Beam Deflection vs Steel Beam Size

Steel beam deflection depends on the interaction of moment of inertia, the steel modulus of elasticity, section shape, span, and load. Because steel’s E value is far higher than wood or most engineered wood products, a steel section can often achieve lower deflection at a shallower depth than an equivalent wood beam. See the Steel I Beam Chart for section specific properties.

Visual Beam Deflection Guide

How to Read a Beam Deflection Chart

How to Calculate Allowable Beam Deflection

How to Calculate Actual Beam Deflection

Beam Deflection Worked Examples

Beam Deflection Conversion Chart

Beam Deflection Chart for Common Spans

A compact contractor reference combining all four common ratios in one table, matching the quick reference chart at the top of this page.

Common Beam Deflection Mistakes

Frequently Asked Questions

Related Calculators and Charts