Steel Joist Calculator — Span, Weight & Load for Cold-Formed Steel Floor & Ceiling Joists

Weight per foot equals cross-sectional area in square inches multiplied by 3.4. That constant comes from steel's density: 490 lb per cubic foot ÷ 12 in/ft ≈ 3.4 lb per ft for every in² of area. A 6″ × 16 ga C-channel with a 1-5/8″ flange has roughly 0.52 in² of area, so it weighs about 1.8 lb/ft. Multiply by the joist length and count for a whole-floor steel weight takeoff — a 20 × 14 ft room at 16″ o.c. carries about 18 joist runs ≈ 250 ft of track ≈ 450 lb.

In the long-tail sense of 'calculate steel joist stiffness k', the value that matters is bending stiffness EI — the modulus of elasticity (29,500,000 psi for steel) times the moment of inertia of the section. A 6″ 16 ga C-section has I ≈ 2.7 in⁴, so EI ≈ 79 ×10⁶ lb·in². The spring stiffness at mid-span of a simply supported member under a point load is k = 48·EI/L³, and under uniform load the center deflection is 5·w·L⁴/(384·EI). Steel's E is about 1.6× spruce's, but cold-formed sections have thin webs, so depth — not material — decides who wins on stiffness.

Cold-formed C-channels are solid rolled sheets bent into a C or sigma shape, typically spanning 6–20 ft in residential floors and ceilings. Open-web steel joists (K-series, LH-series) are shop-welded trusses of hot-rolled angles and round bars that span 8–60 ft and carry commercial roof and floor loads through top and bottom chords. You cannot read an OWSJ load table onto a C-channel or vice versa: OWSJ capacity is listed per series/spacing designation (like 14K3 @ 2'-0" o.c.), while CFS joists use per-section bending and shear checks under AISI S240.

With the simplified model in the dashboard above, a 6″ 16 ga 50 ksi C-section at 16″ o.c. under 40/10 psf with L/360 reaches roughly the low-teens in feet — comparable to a 2×10 sawn joist — while a 10″ or 12″ section extends well past 18 ft. Real spans vary by manufacturer and section family (S162, S250, sigma shapes), so treat every number here as a sizing indicator and confirm against the supplier's APA-style catalog before ordering.

Per linear foot, a 6″ 16 ga C-channel (≈1.8 lb/ft) tracks lumber-market prices closely and can beat 2×10 lumber when steel is soft. The hidden costs move the other way: steel needs screw guns with self-drilling screws (clutch-adjusted to avoid strip-out), clip angles and web stiffeners at bearings, thermal breaks at the envelope, and trade familiarity. Whole-floor, steel usually wins on consistency and straightness (no crown hunting, no shrinkage callbacks) and loses 5–15% on installed cost in single-family work.

Corrosion: interior dry locations ship with G60 galvanizing (0.60 oz/ft² total both sides); humid or coastal jobs specify G90, and exposed applications use painted or ZAM-coated sheet. Fire: cold-formed steel is non-combustible, which helps, but thin sections lose strength quickly in fire — assemblies rely on the membrane (gypsum board) rating rather than member coating, and multifamily work references UL-listed floor/ceiling assemblies exactly like wood framing does.