Carpentry / Free field reference
Floor Joist Span Calculator
Joist span depends on species, grade, size, spacing, and loads. This calculator compares your clear span with a published sleeping-area table for No. 2 lumber at 16-inch centers and a 30 psf live load.
Scope: Limited table lookup: residential sleeping areas, No. 2 lumber, 16 in on center, 30 psf live load, 10 or 20 psf dead load, and L/360 live-load deflection. Not for living areas, decks, roofs, engineered joists, or concentrated loads.
Quick reference
| Species | 2 × 6 | 2 × 8 | 2 × 10 | 2 × 12 |
|---|---|---|---|---|
| Douglas Fir-Larch | 10 ft 9 in | 14 ft 1 in | 17 ft 2 in | 19 ft 11 in |
| Hem-Fir | 10 ft 0 in | 13 ft 2 in | 16 ft 10 in | 19 ft 8 in |
| Southern Pine | 10 ft 3 in | 13 ft 3 in | 15 ft 8 in | 18 ft 6 in |
| Spruce-Pine-Fir | 10 ft 3 in | 13 ft 6 in | 17 ft 2 in | 19 ft 11 in |
Before you use the result
- Measure clear span between support faces. The calculator compares your measurement with the table; it does not interpolate or extend the table to other spacing or loads.
- Bearing, bracing, connections, holes, notches, member condition, and the building’s eligibility for prescriptive construction still need checking. A span comparison alone does not approve a structure.
- For other grades, spacing, uses, or loads, use the complete AWC tables or obtain a project-specific structural design.
Sources & edition notes
This is an independent HandySuite reference. The linked publisher’s code or equipment documentation governs the work.
Common questions
How far can a 2 × 8 joist span?
In this specific sleeping-area table, a No. 2 Douglas Fir-Larch 2 × 8 at 16-inch centers spans up to 14 ft 1 in with a 30 psf live load and 10 psf dead load. Other species, loads, spacing, and grades produce different limits.
Can I use this calculator for a deck or living room?
No. The initial table covers the stated sleeping-area conditions only. Use a table or design appropriate to the actual use and loads.