Methodology & code sources

Every number on DeckCalc HQ comes from the published building code — not an estimate or a model we invented. The free calculator is a faithful, transparent reader of the IRC R507 prescriptive deck provisions, so a result here is exactly what a plans examiner checks against.

The tables we use

Joist and beam spans are transcribed verbatim from the IRC span tables for No. 2 grade lumber at the standard residential deck load of 40 psf live + 10 psf dead, with the wet-service factor applied and deflection limited to L/360. Footings are computed from each post's tributary load divided by your soil's presumptive bearing value; depth follows the local frost line.

• IRC Table R507.6Deck joist maximum spans by size, spacing and species
• IRC Table R507.5Deck beam maximum spans by supported joist span and species
• IRC R507.3 / R507.3.1Footing size from tributary load over presumptive soil bearing
• IRC Table R507.9.1.3Ledger fastener (lag screw / through-bolt) spacing
• IRC R507.4Deck post sizing and connections
• IRC R311.7Stair riser, tread, run and landing requirements
• IRC R312Guard height where the walking surface is over 30" above grade
• IRC R403.1.4Footing depth below the frost line
• IRC Table R401.4.1Presumptive load-bearing values of soils
• AWC DCA6Prescriptive Residential Wood Deck Construction Guide

What this tool does not do

• Engineered designs — spans beyond the prescriptive tables (joists over ~18 ft, heavy snow load, unusual geometry) need a licensed engineer.
• Local amendments — many jurisdictions modify footing depth, guard height (some require 42\") or fastener rules. Always confirm locally.
• Cantilevers, multi-level decks, and free-standing layouts beyond the standard ledger-to-beam single span.
• Cost figures are planning ranges from typical 2026 US prices, not quotes.