Lumber & framing calculator
Floor joist calculator
Preliminary joist count and total linear footage from framing run, span and stated spacing.
Calculator result and formula
Joists to plan: 18 joists
Formula: (floor(framing width ÷ spacing) + 1) × (1 + waste)
Formula ID: LUM-JOIST-001 · Version 1.0.0.
Inputs
- Joist span: Clear preliminary joist run; not a structural span approval Default: 12.
- Framing run width: Perpendicular dimension across which joists are spaced Default: 20.
- Joist spacing: On-center spacing; verify design requirements Default: 16.
- Waste factor: Allowance for cuts, blocking and site variation Default: 10.
How this calculator works
This calculator turns a measured framing width and stated on-center spacing into a preliminary joist count and total linear footage.
- Enter the joist run and perpendicular framing width.
- Use the specified on-center spacing from the approved design.
- Verify joist size, grade, span, hangers and blocking independently before purchasing.
Worked example: Example: a 20 ft framing run at 16 in spacing requires 16 baseline joists. At a 12 ft preliminary run plus 10% allowance, the tool shows 16 joists and 211.2 linear feet to plan.
Assumptions and limitations
- Count uses floor(framing width ÷ stated spacing) + 1.
- This is a material-quantity calculation only; it does not select joist size, grade, span, hangers, blocking or structural connections.
- Waste is applied to count before total linear footage is displayed.
Common questions
- Does this select a joist size?
- No. Span, loading, species, grade and local requirements must be determined by the responsible design process.
- Does it include blocking?
- No. The allowance is visible, but detailed blocking and bridging must be separately taken off.
Reference notes
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