What size beam do I need for a 12 foot span?

That depends on the load case, and the honest answer is that no general table exists. A girder in an exterior bearing wall uses Table R602.7(1), a deck beam uses R507.5, a rafter uses R802.4.1. This page gives the load, reactions and moment those methods start from.

What is the allowable deflection for a beam?

From IRC Table R301.7, chosen by what the member carries: L/360 under plaster or stucco, L/240 under gypsum board and for all other structural members, L/180 for a rafter over 3:12 with no ceiling attached, and L/600 for a lintel supporting masonry veneer.

How much can a 12 foot beam sag?

At L/240 it is 0.60 in, at L/360 it is 0.40 in and at L/180 it is 0.80 in. The limit is set by the finish rather than the structure, because deflection is about cracking and bounce rather than collapse.

How do I calculate the load on a beam?

Live plus dead load in psf, times the tributary width, which is half the span of whatever frames into it on each side. That gives pounds per linear foot. Multiply by the span for the total, halve it for each end reaction.

What is the maximum moment on a beam?

For a simply supported beam under a uniform load it is wL squared over 8, at midspan. A 400 plf load over 12 ft gives 7,200 ft-lb. That is exact and is where any sizing method begins.

Does the IRC have a table for steel beams?

No. There is no prescriptive steel beam table for dwellings in the IRC. A steel beam in a house is an engineered member designed under AISC 360, and the reaction and moment figures here are what an engineer starts from.

How is deflection checked on a cantilever?

Footnote b to Table R301.7 says L shall be taken as twice the length of the cantilever. A 4 ft cantilever is therefore checked as an 8 ft span, which is much stricter than treating it as 4 ft.

Are LVL beams sized from the code?

No. LVL, PSL and glulam are proprietary products sized from the manufacturer's own published span tables and design values, and those differ between brands for the same nominal size. Use the table for the product you are actually buying.

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Beam Load and Deflection Calculator

IRC Table R301.7 deflection limits, and the honest answer on sizing

There is no general beam table in the residential code. There are several specific ones, for girders in bearing walls, for deck beams, for rafters, and they are not interchangeable because each carries a different load case. This gives you the statics exactly, the deflection limit from Table R301.7, and a straight answer about which table your member belongs in.

Beam load and deflection

Clear span

Member

What it carries

Tributary width (half the span each side)

Live load

psf

Dead load

psf

Load, reactions and moment are exact for a simply supported beam. Member sizing is not offered, and the notes say why.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

The same span at three deflection limits, to scale

Worked example

A 12 ft beam carrying 8 ft of tributary floor at 40 psf live and 10 psf dead, under a gypsum ceiling.

Load. 50 psf across 8 ft of tributary is 400 lb per linear foot.
Reactions. 2,400 lb at each end, which is what the post and footing under it have to carry.
Moment. wL²/8 = 7,200 ft-lb at midspan.
Deflection. At L/240 the beam may sag 0.60 in. At L/360 it is 0.40 in, and at L/180 it is 0.80 in.
Size. Not here. The load case decides which table, and this one is a floor girder, so it is R602.7(1).

Two thirds of a millimetre separates a floor you would notice from one you would not, which is why the deflection limit is picked by the finish rather than by the structure.

The formula

Statics for a simply supported beam under a uniform load, which is exact:

w = (live + dead) × trib R = wL/2 M = wL²/8 Δ_allow = 12L / limit

w: uniform load, lb per linear ft
trib: tributary width, ft: half the span on each side
R: reaction at each end, lb. This is the load on the post below
M: maximum moment at midspan, ft-lb
limit: 360, 240 or 180 from Table R301.7, chosen by what the member carries

The reaction is the number people skip and it is the one that causes trouble: 2,400 lb landing on a post that lands on a rim joist that lands on nothing is a very common defect, and no beam table will catch it for you.

Why nobody can give you a general beam table

A beam's capacity depends on species, grade, moisture content, how long the load is applied, whether the compression edge is laterally braced, and whether it is notched. The National Design Specification handles all of that with adjustment factors, and a prescriptive table can only exist by fixing every one of those variables.

That is exactly what the IRC's tables do, which is why there are several rather than one. R602.7(1) fixes the load case as a girder or header in an exterior bearing wall. R507.5 fixes it as a deck beam in wet service. R802.4.1 fixes it as a rafter. Reading one table for another member is not conservative, it is meaningless.

What is genuinely general is the statics. The load, the reactions and the moment on a simply supported beam under a uniform load are exact and this page gives them, because they are what any sizing method starts from and what an engineer will ask you for.

Deflection is the other general thing, and Table R301.7 is short enough to know by heart. It is chosen by finish, not by structure: a plaster ceiling gets L/360 because plaster cracks, and a bare rafter gets L/180 because nothing is attached to it.

Table R301.7, allowable deflection, in full

Nine rows, and worth reading once:

IRC Table R301.7, allowable deflection of structural members
Structural member	Allowable deflection
Rafters having slopes greater than 3:12 with no finished ceiling attached to rafters	L/180
Interior walls and partitions	H/180
Floors and ceilings with plaster or stucco finish	L/360
Ceilings with brittle finishes (plaster, stucco)	L/360
Ceilings with flexible finishes (gypsum board)	L/240
All other structural members	L/240
Exterior walls, wind loads, with plaster or stucco finish	H/360
Exterior walls with other brittle finishes	H/240
Exterior walls with flexible finishes	H/120
Lintels supporting masonry veneer walls	L/600

Two footnotes change answers. For a cantilever, L is taken as twice the length of the cantilever, so a 4 ft cantilever is checked as an 8 ft span. And exterior walls with an interior gypsum board finish are limited to H/180 rather than H/120.

The strictest row in the table is the lintel over masonry veneer at L/600, because brick is unforgiving and a sagging lintel cracks the wall above it.

Which table your member belongs in

Girder or header in an exterior bearing wall: Table R602.7(1). The header calculator carries all 549 cells of it.

Deck beam: Table R507.5, wet service, and the deck beam calculator has it. The same timber spans less outdoors than in.

Rafter: Tables R802.4.1(1) through (8), with the span measured along the horizontal projection. See the rafter calculator.

Floor joist: Table R502.3.1, and for a deck R507.6.

LVL, PSL, glulam: the manufacturer's published tables. These vary between brands for the same nominal size and the code has nothing to say about them beyond requiring they be used per their listing.

Steel: nothing prescriptive in the IRC. An engineered member under AISC 360.

Deflection is a serviceability limit, not a safety one

A beam that exceeds L/240 has not failed. It is carrying its load with the same margin it always had. What it is doing is sagging enough that people notice: doors bind, tile grout cracks, a marble rolls across the floor.

That is why the limit is chosen by the finish rather than the structure. Plaster gets L/360 because plaster cracks at less movement than gypsum board does, and a rafter with nothing attached gets L/180 because there is nothing up there to crack.

The practical consequence is that on longer spans deflection usually governs before bending does. A beam picked for strength alone on a 20 ft span will very often be bouncy, which is why span tables are frequently deflection-driven at the long end.

Frequently asked questions

What size beam do I need for a 12 foot span?: That depends on the load case, and the honest answer is that no general table exists. A girder in an exterior bearing wall uses Table R602.7(1), a deck beam uses R507.5, a rafter uses R802.4.1. This page gives the load, reactions and moment those methods start from.
What is the allowable deflection for a beam?: From IRC Table R301.7, chosen by what the member carries: L/360 under plaster or stucco, L/240 under gypsum board and for all other structural members, L/180 for a rafter over 3:12 with no ceiling attached, and L/600 for a lintel supporting masonry veneer.
How much can a 12 foot beam sag?: At L/240 it is 0.60 in, at L/360 it is 0.40 in and at L/180 it is 0.80 in. The limit is set by the finish rather than the structure, because deflection is about cracking and bounce rather than collapse.
How do I calculate the load on a beam?: Live plus dead load in psf, times the tributary width, which is half the span of whatever frames into it on each side. That gives pounds per linear foot. Multiply by the span for the total, halve it for each end reaction.
What is the maximum moment on a beam?: For a simply supported beam under a uniform load it is wL squared over 8, at midspan. A 400 plf load over 12 ft gives 7,200 ft-lb. That is exact and is where any sizing method begins.
Does the IRC have a table for steel beams?: No. There is no prescriptive steel beam table for dwellings in the IRC. A steel beam in a house is an engineered member designed under AISC 360, and the reaction and moment figures here are what an engineer starts from.
How is deflection checked on a cantilever?: Footnote b to Table R301.7 says L shall be taken as twice the length of the cantilever. A 4 ft cantilever is therefore checked as an 8 ft span, which is much stricter than treating it as 4 ft.
Are LVL beams sized from the code?: No. LVL, PSL and glulam are proprietary products sized from the manufacturer's own published span tables and design values, and those differ between brands for the same nominal size. Use the table for the product you are actually buying.

Check these numbers yourself

IRC Table R301.7, allowable deflection of structural membersInternational Code Council, Building Code Action Committee code change proposal reproducing Table R301.7 with its footnotes, dated 14 December 2012. Confirm the values against the edition your jurisdiction has adopted.
AWC Maximum Span Calculator for Wood Joists and RaftersAmerican Wood Council. The design path for species, grades and load cases the prescriptive tables do not cover.
AWC National Design Specification for Wood ConstructionAmerican Wood Council. The standard behind every wood span table, and the adjustment factors a prescriptive table hides.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

This page does not size beams. Load, reactions, moment and allowable deflection are exact for a simply supported beam under a uniform load. Capacity depends on species, grade, moisture, load duration and lateral support, and lives in the NDS or the manufacturer's tables. Deflection values are from an ICC document reproducing Table R301.7; confirm against your adopted edition.