Deck Joist Calculator
Sizing deck joists correctly prevents bouncy, sagging, or dangerous decks. This deck joist span calculator uses the IRC 2021 R507.6 span tables for decks built at 40 psf live load, the residential minimum. Enter your deck dimensions, joist size, species, and spacing to get joist count, maximum allowable span, and a lumber list.
Deck Joist Span Chart (IRC 2021 R507.6)
Maximum deck joist spans at 40 psf live load + 10 psf dead load, Southern Pine (or pressure-treated Southern Pine, the most common deck framing lumber):
| Joist Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2x6 | 9'-11" | 9'-0" | 7'-7" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" |
| 2x10 | 16'-6" | 15'-0" | 12'-1" |
| 2x12 | 18'-0" | 17'-1" | 14'-1" |
For Douglas Fir-Larch, reduce spans by about 5%. For Hem-Fir or Spruce-Pine-Fir, reduce by 12-15%. Deck joist span pressure treated values match Southern Pine because nearly all pressure-treated deck lumber in the US is Southern Pine.
Deck Joist Spacing Chart
Spacing determines both the maximum span and the stiffness you feel when walking on the deck:
- 12" OC: stiffest deck, needed for some composite decking specs and maximum spans. About 30% more lumber than 16" OC.
- 16" OC: the standard spacing for wood decking and most composite products. Best cost-to-stiffness ratio.
- 24" OC: allowed by code for solid wood decking but feels bouncy. Most composite decking manufacturers require 12" or 16" OC.
How to Use This Deck Joist Spacing Calculator
- Enter deck length (parallel to the house / ledger).
- Enter deck width (the joist span from house to beam).
- Pick joist size. 2x8 is the most common; 2x10 or 2x12 for wider decks.
- Pick spacing. Check your decking spec: composite usually requires 16" OC or tighter.
- Pick species. Most pressure-treated deck lumber is Southern Pine.
The deck joist span calculator compares your planned span to the IRC allowable max. If it exceeds, size up the joists, tighten the spacing, or add a mid-span beam.
2x6 Deck Joist Spacing and Span
A 2x6 pressure-treated deck joist at 16" OC can span up to 9 feet. At 24" OC, the same joist maxes out at about 7 feet 7 inches. 2x6 is only appropriate for small decks (8 feet deep or less); most residential decks use 2x8 minimum.
2x8 Deck Joist Span Chart
A 2x8 pressure-treated joist at 16" OC spans 11 feet 10 inches. At 12" OC, that goes up to 13 feet 1 inch. For a 12-foot-deep deck, 2x8 at 16" OC is at the edge; many builders use 2x10 for peace of mind.
Cantilever Limits
IRC R507.6 limits deck joist cantilever (the overhang past the beam) to 1/4 of the backspan. For a 12-foot joist span, the max cantilever is 3 feet. Cantilevers beyond this can cause uplift at the ledger and dangerous deflection at the free end.