Formula
Round the perpendicular deck dimension ÷ maximum spacing up to whole spaces. Joists = spaces + 1.
Deck Joist Calculator
Estimate joist count and actual spacing for an early deck framing layout.
How this deck joist estimate works
This is a framing layout aid, not a structural design. It shows how many equal bays fit within a maximum spacing and keeps the tighter actual spacing visible.
Worked example
A 15 ft dimension at 16 inches on center requires the spacing to round up to whole bays; the result includes a joist at each edge.
Before you buy
Confirm the framing plan, span tables, species, grade, hangers, beams, and blocking before ordering lumber. Use the optional span only for a linear-foot planning view.
Assumptions
Defaults are starting points. Replace coverage, density, package yield, spacing, and allowance values with the product or plan you are actually using.
Limitations
This tool does not verify span, load, deflection, beam sizing, ledger attachment, cantilever, or code compliance.