Steel Beam Size Calculator

What the steel beam size calculator does

A beam must be strong enough not to yield and stiff enough not to sag. Strength depends on the section modulus Sx, stiffness on the moment of inertia Ix. The steel beam size calculator finds the bending moment from your span and load, divides by the allowable stress for the required Sx, works out the Ix that keeps deflection within your limit, and picks the lightest W-shape in its table that has both. It is a first estimate for planning and budgeting, not a stamped design.

How to size a steel beam: the formulas

For a simply supported beam:

E is 29,000 ksi for steel, Fb the allowable bending stress, and n the deflection limit such as 360. The beam must meet both Sx and Ix.

Steel beam size calculator example

A 20 ft span carrying 500 lb per foot, Fb = 30 ksi, deflection limit L/360:

W10x12 is strong enough (Sx 10.9) but too flexible (Ix 53.8), so the steel beam size calculator picks W12x16 (Sx 17.1, Ix 103). A 16 ft beam with a 6,000 lb point load at midspan needs Sx 9.6 and Ix 57.2, met by a W12x14.

How to use the steel beam size calculator

Enter the clear span in feet. Choose a uniform load in pounds per foot, such as a floor load spread along the beam, or a single point load at midspan in pounds. Keep the allowable bending stress at 30 ksi for A992 steel unless your engineer gives another value, and choose the deflection limit: L/360 for floors under live load, L/240 for total load, or L/180 for roofs. Press Calculate.

Reading the steel beam sizing result

The suggested shape is the lightest in the table that meets both requirements, which usually means stiffness controls on longer spans and strength on short heavy ones. The designation W12x16 means a nominal 12 in deep wide flange weighing 16 lb per foot. Add that self weight to your load and recheck if it is a large share. Treat the answer as a budget and planning size; a structural engineer must confirm it, including lateral bracing, connections, bearing, and the posts below.

When to use a steel beam calculator

• Budgeting a beam to open up a load-bearing wall
• Comparing beam depths for headroom
• Checking a quoted beam size before a site visit
• Learning how span changes the required section modulus
• Preparing questions for your structural engineer

Common steel beam size calculator mistakes

• Forgetting dead load. Include the floor, walls, and beam weight, not only live load.
• Ignoring bracing. An unbraced compression flange lowers the capacity.
• Using total span with supports. Enter the span between bearing centers.
• Skipping the posts and footings. The load must reach the ground.

Tips for steel beam sizing

• Collect loads per square foot and multiply by the tributary width.
• Prefer a deeper, lighter shape when headroom allows.
• Bring the result to your engineer as a starting point.

Related structural tools

Beam sizing ties into other checks. The beam deflection calculator finds the sag of a chosen beam, the moment of inertia calculator works out section properties, and the steel weight calculator gives the weight to lift and ship.

Required section modulus and beam strength

Bending stress in a beam equals the moment divided by the section modulus, so the required Sx is simply the moment divided by the stress you allow. For A992 steel with a yield stress of 50 ksi, allowable stress design commonly uses about 30 to 33 ksi for a laterally braced beam, which is why 30 ksi is the default. That allowance only holds if the compression flange cannot twist sideways. A floor beam with joists or decking fastened along its top is braced; a long beam carrying a single point load with nothing attached to its top is not, and its capacity can drop sharply from lateral torsional buckling. Shear rarely controls on these shapes but matters for short heavy spans. Look up full properties of any shape with the moment of inertia calculator.

Steel beam calculator deflection limits

Deflection limits come from the building code: L/360 under live load for floors keeps plaster and tile from cracking, L/240 under total load is common, and L/180 suits roofs without brittle finishes. A 20 ft floor beam at L/360 may sag no more than 0.67 inches. Because deflection grows with the span to the fourth power under uniform load, doubling the span needs about eight times the moment of inertia at the same load per foot. Once you have a candidate, the beam deflection calculator checks its actual sag.

W shape beam chart

Section properties of common light W-shapes used by the steel beam size calculator:

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