How to use this calculator
- Enter Length along ledger in ft.
- Enter Spacing in in.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
Joists = (supported length in inches ÷ spacing) + 1, plus doubled rims and headers. Span capability depends on size, species and load — a 2×8 SPF spans about 11 ft at 16 in OC for decks. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Where this fits in the project
Framing quantities chain together: footings support posts, joists set the deck frame, and surface boards follow. Price hardwood and specialty stock in board feet, sheet goods by the sheet.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the joist calculator:
- Lumber Calculator — Joists are horizontal lumber members.
- Deck Calculator — Deck framing uses joists for support.
- Plywood Calculator — Subfloor plywood is laid over joists.
- Stud Calculator — Studs are vertical; joists are horizontal.
- Stair Calculator — Stairs connect between floor joist levels.
- Flooring Calculator — Flooring is installed over the joist-subfloor system.
Frequently asked questions
How far can a 2×8 joist span?
Roughly 10–12 ft at 16 in OC depending on species and load — verify in the IRC span tables for your case.
Why 12 in OC for some decks?
Diagonal decking and some composites require tighter spacing — composite boards often mandate 12 in OC on angles.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.