Material calculator
Deck joist calculator — joist count
Count deck joists at a selected on-center spacing. This deliberately does not size joists; span, species, loads, and ledger attachment are structural/code questions.
You'll need
10 joists
Field joists
What this assumes
- Joists = floor(12 ft × 12 ÷ 16 in) + 1.
- This is a count only. It does not choose 2×6, 2×8, 2×10, or any structural size.
Coverage rates & sources
Every number this calculator uses is a published engineering constant — not an estimate we made up. Here is exactly what it assumes and where each value comes from.
- Joist count formula: (width_in / spacing) + 1Source: Decks.com — `(width×12)/16 + 1`
- Standard joist spacing: 16 in o.c.Source: Standard residential deck framing
- Reference spans (16 in o.c.): 2×6->9 ft, 2×8->12 ft, 2×10->16 ft, 2×12->18 ftSource: Decks.com span reference (display ONLY)
Before you buy
- This counts joists at a given spacing; it does not size them.
- Joist size, species, span, snow load, hot tubs, and other loads are structural and governed by IRC/local code.
- Ledger-board attachment is a major deck-collapse risk and must be bolted per code, never nailed; add rim joists and beams separately.
This is a planning estimate, not a substitute for a pro's on-site measurement. For load-bearing, structural, or code-regulated work, confirm quantities with a licensed contractor.
Related calculators
- Fence calculator — posts, panels & concreteEstimate fence posts, equal sections, panel or picket counts, and concrete from the run length and post-hole dimensions. Concrete uses annular volume: hole volume minus buried post volume.
- Deck board calculator — boards & screwsEstimate decking board count, total decking linear feet, and exposed screw count from deck dimensions, board width/gap, joist spacing, and board length.
- Stair & stringer calculator (IRC)Enter your total height (finished floor to finished floor, or ground to deck) and this lays out a code-compliant stair: number of risers, exact riser height, number of treads, total run, and stringer length. Riser and tread limits come from the International Residential Code.
- Wheelchair ramp slope calculator (ADA)Enter the height the ramp has to climb and this returns the minimum ramp length at the ADA's 1:12 maximum slope, plus how many runs and level landings the rise requires. Built from the 2010 ADA Standards.