Floor & Deck Joist Calculator
Enter frame length and joist spacing to get how many joists you need at 12, 16, or 24 inches on center — for floors, decks, and platforms.
Floor / deck frame
Enter the length of the frame being spanned. Joist length = the span across the width (e.g. deck depth). Add rim joists and blocking separately.
You'll need
Add rim joists (2 × frame length) and blocking rows. Sizing (2×6, 2×8, 2×10) depends on span and load — always check span tables or engineering.
How to calculate joists
Joist count = (frame length in inches ÷ spacing) + 1 end joist. Add two rim joists (each the frame length) plus blocking rows at mid-span for longer runs. Joist size is governed by span tables — this calculator gives count only.
Frame takeoffs from a LiDAR scan
ProBuildCalc turns your iPhone's LiDAR scanner into a job-site measuring kit — walk the space and it captures the square footage, material takeoff, and a blueprint automatically. No tape measure, no graph paper.
Joist calculator FAQ
- How many floor joists do I need?
- Divide the frame length in inches by the on-center spacing and add one for the end joist. A 16-ft frame at 16-inch spacing needs 13 joists. Add rim joists and blocking separately.
- What spacing should floor joists be?
- Sixteen inches on center is the standard residential floor joist spacing for most subfloor and floor finishes. Twelve inches provides a stiffer floor; 24 inches is used in some engineered systems. Deck joists are typically 16 inches OC.
- How do I size a floor joist?
- Joist size depends on span length, spacing, and load. A 2×8 at 16 OC can typically span about 12 feet; 2×10 spans about 15 feet; 2×12 about 18 feet. Always check the span tables in your local code or consult an engineer.
- What is a rim joist?
- A rim joist (or band joist) runs along the perimeter of the frame, perpendicular to the regular joists. It caps the joist ends and ties the frame together. Add two rim joists — one on each end — each equal to the frame length.
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See also: estimating guides · ProBuildCalc by trade