Floor joist calculator
Floor length, joist span and on-center spacing: you get the number of joists, their length with bearing, linear footage, rim joist and hangers.
Purchase quantity
14 unit
Net measurement: 14 unit · Waste 10 %
- Rafters / joists14 unit
- Length of each joist13.78 ft
- Joist linear length212.205 ft
- Rim joist36.089 ft
- Joist hangers28 unit
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How it's calculated (visible formula)
joists = ceil(length ÷ spacing) + 1; piece = span + 2 × bearing; linear = joists × piece × (1 + waste)
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure floor length (the direction joists are spaced along).
- Measure clear span between supports and add bearing at each end.
- Choose on-center spacing to suit the subfloor (16 or 24 in).
- Count joists: length ÷ spacing + 1.
- Add rim joist, hangers and waste to the order.
Worked example: 16 ft floor, 13 ft span, 16 in on center
- 16 ft ÷ 16 in = 12 → 12 + 1 = 13 joists (14 at metric spacing)
- Piece: 13 ft + 2 × 4 in ≈ 13.7 ft
- Order 14 ft lumber
→ About 14 joists of 14 ft, 32 ft of rim joist and 28 hangers.
Common mistakes
- Forgetting the end joist (the +1).
- Not adding bearing length at each wall.
- Choosing joist size without span tables — deflection governs long spans.
Pro tips
- Double joists under parallel partitions or point loads.
- Add blocking or bridging at mid-span on spans over 8–10 ft.
- Install every joist crown-up.
- Ventilate crawlspaces under floors to prevent rot.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- What joist spacing should I use?
- 16 in on center is standard; 12 in for tile floors or long spans, 24 in with thick subfloor.
- How much bearing does a joist need?
- At least 1½ in on wood or metal and 3 in on masonry, or a hanger.
- What size joist for a 13 ft span?
- Check your local span tables: often 2×10 at 16 in for residential loads.
- What lumber for floor joists?
- Graded dimensional lumber (No. 2 SPF or SYP) or engineered I-joists.
- How much deflection is allowed?
- Residential floors typically L/360 for live load.
- Solid joists or I-joists?
- I-joists are lighter, straighter and span farther.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.