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Structural Calculators

Sizing a structural member is usually a table lookup, and the whole job is knowing which table applies, or whether one exists at all.

The calculators

• Beam Load and Deflection CalculatorIRC Table R301.7 deflection limits, and the honest answer on sizing
• Deck Beam CalculatorIRC Table R507.5(1) · NDS design values
• Deck Ledger CalculatorIRC Table R507.9.1.3(1), all 84 cells, read from the code
• Gauge Size Chart15 U.S.C. 206 (1893), NBS Circulars 391 and 31, IRC 2021
• Header and Girder Size CalculatorIRC Table R602.7(1), read cell by cell from the code
• Joist CalculatorIRC Table R507.6 · cantilever rule R507.6.1
• Occupant Load CalculatorIBC Table 1004.5, every row, with gross and net marked
• Rafter CalculatorIRC R802.4.1: the span is the horizontal projection, not the rafter
• Roof Truss CalculatorIRC R802.10 and ANSI/TPI 1: a truss is a sealed engineered design
• Spiral Staircase Calculator2015 IRC R311.7.10.1 spiral stairways; prices from HomeAdvisor's June 2026 guide
• Stair Construction CalculatorIRC R311.7 · riser R311.7.5.1 · tread R311.7.5.2
• Steel I-Beam CalculatorExact geometry for an I-shape. The IRC has no steel beam table at all
• Steel Weight Calculator490 lb per cu ft carbon steel (AASHTO via FHWA); stainless and aluminum from mill datasheets
• Structural Load CalculatorIRC Table R301.5, and the rule that snow and live load are not added

The International Residential Code carries several span tables that look interchangeable and are not. A deck joist falls under Table R507.6, at 40 psf live load and wet service. A floor joist inside the building envelope falls under R502.3.1, at 30 or 40 psf depending on whether the room is a sleeping area, and dry service: which yields longer spans for the identical piece of lumber. Headers and girders are R602.7(1), rafters R802.4.1, and the ledger connection has a table of its own at R507.9.1.3(1). Pick the wrong one and you get an answer that is confidently wrong by feet.

That is the thing most calculators will not tell you. A tool that reports a span without naming its table has hidden the only decision that mattered. Every page here names the table, prints it in full, and states the load assumptions it was built on.

Three of these pages exist because no table does

A truss cannot be sized from a table by anyone. R802.10 sends it to ANSI/TPI 1 as a delegated engineered design, and R802.10.1 requires a sealed drawing carrying thirteen specified items, approved by the building official before installation. Any calculator that outputs a truss member size is producing a number nobody will seal.

The IRC has no steel beam table for dwellings at all. A rolled section across a basement is an engineered member under AISC 360. What is computable is the geometry, exactly, and that is what the steel page gives you.

There is no general wood beam table either. There are several specific ones for specific load cases, and reading one for another member is meaningless rather than conservative.

Where the tables stop

Prescriptive tables cover ordinary conditions and nothing else. The ledger table stops dead at an 18 ft joist span and its own footnote forbids extrapolating past it. The header table stops at a 36 ft building width and prints dashes where a member is simply not permitted. A hot tub, a planter wall, a species not in the list, a load above 40 psf or a second story all put you outside the table and into engineered design. Each calculator says where its edge is rather than extrapolating past it: a span table that silently interpolates is more dangerous than one that refuses.

Local amendments are the other limit. Jurisdictions modify these sections routinely, and Washington's move to a 60 psf deck live load forced its own code council to publish replacement span tables because the R507 tables are computed at 40 and do not scale. Adoption of a given IRC edition is state by state and county by county, so treat every number here as a planning figure to confirm with your building department.

One page here is not IRC at all. Occupant load is an IBC calculation and does not apply to a house; it is here because the question is asked constantly by people fitting out shops and restaurants.